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Egg size as a life history character of marine invertebrates: is it all it's cracked up to be?

Introduction

Egg size is one of the most important and often-studied aspects of the life history of marine organisms, and much attention has focused on the ecological factors that drive changes in egg size (e.g., Thorson, 1950; Vance. 1973a. b; Christiansen and Fenchel. 1979; Strathmann. 1985. 2000; Lessios. 1990; Moran. 2004; Levitan, 2000. 2006). Early interest in egg size evolution was spurred by the observation that egg size is strongly associated with developmental mode; species with small eggs generally have long-lived planktonic larvae that disperse widely and must feed and grow in the plankton prior to reaching metamorphosis (=- planktotrophic). whereas species with large eggs tend to have short-lived, nonfeeding larvae or to lack dispersing planktonic stages altogether (= nonplanktotrophie) (Thorson. 1950. and much subsequent interest). Within and among developmental modes, egg size is also linked to a number of fundamental and adaptive traits such as length of larval development (Thorson. 1950; Vance. 1973a. b; Strathmann. 1985; Hadfield and Miller. 1987; Sinervo and McEdward. 1988; Wray and Raff. 1991). larval form (McEdward. 1986; Strathmann. 2000). length of the facultative feeding period (Miner et al., 2005). size at metamorphosis (Strathmann. 1985; Emlet et al., 1987). juvenile growth and survival (Marshall et al., 2003). resistance to starvation (Anger, 1995; Bridges and Heppell, 1996). and fertilization success (Levitan. 1993. 2000. 2006; Podolsky and Strathmann. 1996; Farley and Levitan, 2001; Lutlikhui-zen et al., 2004).

Despite wide interest in and focus on the selective, or top-down, factors that drive the evolution of egg size, considerably less is known about the bottom-up factors that underlie egg size at the physiological, biochemical, and ecological levels. One reason for this gap in knowledge is that egg size is, in and of itself, a simplistic and perhaps not always appropriate measure of the factors under selection. A large egg is generally considered to reflect increased maternal energy investment; in this scenario, the size of an egg directly affects larval physiological performance and success. However, among planktotrophic species, egg size is a poor predictor of energetic content when all data available from a broad range of echinoderms are considered (McEdward and Carson. 1987; McEdward and Coulter. 1987; McEdward and Morgan 2001). Despite this fact, large eggs are correlated with multiple life-history traits that, intuitively, should be reflections of increased egg energy. In particular, larger eggs produce larger larvae that develop more rapidly and reach metamorphosis sooner than larvae from smaller eggs. This presents an apparent contradiction which may be created in part by the diversity of methods used to measure or estimate "egg energy" (McEdward and Morgan, 2001). and in part by the fact that few studies have examined the biochemical correlates of egg size in a rigorous comparative framework. The identity and amount of different biochemical constituents that compose a "large egg." and the role that those constituents play during development, are poorly understood; thus, it has not yet been possible to draw clear, broad, functional connections between egg size and its many life-history correlates.

Another reason we know relatively little about the bottom-up forces influencing the evolution of egg size in marine invertebrates is that relatively few studies have focused on the genetic, morphological, physiological, or stochastic mechanisms that influence the size of eggs a mother produces. These mechanisms presumably drive much of the variation in egg size among individuals on which natural selection acts; likewise, they underlie the complex interactions between selective forces that act on egg size at different levels. Without a better understanding of these mechanisms, it will probably be difficult to improve or to empirically ground-truth models that have explored the evolution of egg size, or to predict how egg size is likely to play a role in species' adaptation to future environmental change.

A comprehensive review of many important egg-related ideas, "Echinoderm Larval Ecology Viewed from the Egg" (Emlet et al), was published in 1987. Over the intervening two decades, considerable work has been done to advance the field. One such compilation of particular note was by Jaeckle (1995), who investigated correlates among size, biochemical composition, energy content, and developmental mode in invertebrate eggs (largely echinoderms); a few years later, Sewell and Manahan (2001) showed that adding data from 14 additional echinoderm species with a wide range of egg sizes to this dataset did not change the relationships described by Jaeckle (1995). The goal of our paper is to continue this conversation by examining the main trends and findings in egg composition and bottom-up factors regulating egg size, to point out remaining gaps in our understanding of how egg size functionally influences larval ecology and evolution, and to suggest some potentially valuable directions for future research.

We highlight two main types of questions and important directions for future work. First, under Question 1 below, we discuss what constitutes an egg. More specifically, what are the biochemical constituents of an egg. and how is egg composition related to egg size? An egg is arguably the most influential cell in an animal's life history (Jaeckle. 1995). and the consequences of egg "size" for subsequent life-history characters appear to be many and strong. However, even standardized for size, eggs are clearly not all created equal among phyla, between developmental modes, between species, or even among individuals within a species. Biochemical compositions vary at all levels, and without quantifying this variation at as many levels as possible it is difficult to say what role size, as separate from composition, plays in larval ecology and life-history evolution. We review the main techniques that have been used to measure egg energy and biochemical composition, and suggest directions for future comparative research.

Second, from a bottom-up perspective, what determines egg size? What are the mechanisms, either genetic, physiological, or stochastic, that determine the size and composition of eggs that a mother produces under different environmental conditions? To what extent are these mechanisms shared among diverse and distantly related taxa? Under Question 2 we give some examples of physiological and genetic mechanisms that may be at work in driving both innate variation in egg size and variation that occurs in response to environmental change. We also highlight the relative paucity of these data in the literature and provide some suggestions for future research directions.

Question 1: What Constitutes an Egg?

Or, what are the biochemical constituents of an egg, and how is egg composition related to egg size?

Egg size is a highly variable character, both between closely related species and among widely divergent taxa of marine invertebrates. For example, among species of echi-noderms. which are perhaps the best-studied group in terms of egg size, egg volume varies by 5 orders of magnitude across taxa (Emlet et al, 1987). Variation in egg size among females of a single species is common as well, though intraspecific variation tends to occur on a much smaller scale (e.g., McEdward and Carson. 1987; Jones et al. 1996; George, 1999; Marshall el al., 2000). Early and important models such as Vance's (1973a) time-fecundity hypothesis, which inspired a number of related models (e.g., Podolsky and Strathmann. 1996: McEdward and Janies. 1997; Levitan, 2000; Miner et al., 2005; and others reviewed in Strathmann, 1985; Havenhand, 1993), assume that a larger egg is energetically more costly for the parent to produce, and that the increased amount of energy in a larger egg functions to shorten the length of planktonic development by reducing the larva's dependence on exogenous food. The assumption that larger eggs contain more energy is largely borne out across species (e.g., Emlet et al, 1987; Jaeckle, 1995; McEdward and Morgan. 2001). although not always within species (McEdward and Carson, 1987; McEdward and Coulter. 1987). Although egg size appears to be at least a broadly accurate index for maternal investment, it is becoming increasingly clear that changes in size alone cannot account for all physiological, ecological, and evolutionary correlates of egg size. To better understand the complex role of egg size in the life histories of marine invertebrates, we need to address more issues such as changes in biochemical composition of eggs with size; patterns of utilization of egg constituents by embryos, larvae, and juveniles; and energy expenditures and requirements of embryos and larvae.

One important issue for theoretical models that consider maternal investment, life-history evolution, and larval transport and performance in the field is to accurately describe the scaling of energy content with egg size for the given group of interest. Several methods, of varying complexity and difficulty, have been utilized to measure egg energy. Our goal here is to briefly review and differentiate between the more common methods, and to highlight some important considerations when selecting appropriate techniques.

The first methodological options to be considered, and sometimes the only options available, are the simplest and least expensive. One technique that has a very long history and has seen wide use for measuring energetic content of eggs and embryos is ash-free dry weight (AFDW or DOW. dry organic weight). Estimating AFDW is simple, but this method contains a number of potential sources of error. Sample preparation is one; salts are hygroscopic and. if not removed from the exterior of marine organisms prior to drying, can cause samples to gain weight after ashing (Moreno et al., 2001). leading to an underestimate of AFDW, This phenomenon can be minimized by rinsing marine samples with ammonium formate (isotonic with seawater) or in some cases with distilled water to remove surface salts. Disturbingly, though, Moreno et al., (2001) found that some inorganic salts retain water at normal drying temperature (80[degrees]C) but lose it at ashing temperatures (450 [degrees]C); thus, even in well-rinsed samples, water may be retained by intracellular salts in "dried" samples only to be burned away at 450 [degrees]C, leading to an overestimate of AFDW. Finally. AFDW measurements do not capture the differing energetic values of lipids, carbohydrates, and proteins (Podolsky. 2002). so AFDW may not be suitable for comparing samples in which biochemical composition is likely to differ. Despite these issues, the relative simplicity and low cost of this assay make it a valuable tool and have led to its widespread use in a number of systems.

Another method that has been employed by many authors (e.g., McEdward et al., 1988; McEdward and Chia. 1991; Moreno and Hoegh-Guldberg. 1999: Miner et al., 2002: Zigler et al., 2008) is the potassium dichromate wet oxidation method (PDWO) described in Parsons et al., (1984). PDWO is a colorimetric method used to estimate total energetic content. Measurements of total energy via PDWO generally correlate tightly with AFDW measurements when the two methods are compared (Jaeckle. 1995; Moran. 1997). A recent study by Pernet and Jaeckle (2004) found a potentially serious drawback to using the PDWO method for comparative studies among species, though. These authors observed that the method gave consistently low estimates of AFDW-specific energy density in eggs of plank-totrophic echinoderm and annelid species. This result may be due to the fact, pointed out by several other authors previously (e.g., Paine, 1971; Crisp, 1984; Gosselin and Qian. 1999). that the PDWO method does not completely oxidize protein, and oxidizes some types of protein more completely than others. Because protein is a major biochemical constituent in all invertebrate eggs. PDWG will underestimate total energy content of any egg sample. This underestimation is a particular problem for comparing energy content across a broad range of egg sizes and taxa. because planktotrophic eggs tend to contain more protein per unit volume than lecithotrophic eggs (Jaeckle. 1995). Thus, the extent of this error is likely to be greater for species with small eggs and to vary with the type of protein present in different eggs (Pernet and Jaeckle, 2004).

A third option is direct ealorimetry, in which samples are combusted in an enclosed chamber and the total energy contained in the samples is measured as heat production (bomb ealorimetry). Because of the small amount of material generally available to larval biologists, either the Parr semi-microbomb (Paine. 1971) or the Phillipson microbomb (Phillipson. 1964) calorimeters are likely to be the most useful, and these are the most commonly used for the determination of caloric value in marine animals (Beukema. 1997). Both types operate on the same principles and are prone to several types of error. First, carbonate breakdown (an endothermic reaction forming CaO in the bomb chamber) can lead to substantial underestimates of calorie value in many types of marine tissue (Paine. 1964, 1971). The recommended way to avoid this issue is to separate calcium carbonate structures from soft tissues, which is feasible in adult organisms (Beukema. 1997) but substantially more difficult in marine larvae. Second, if energy content on a per-gram basis is desired, independent determinations of both ash content and dry weight must be obtained; errors in either will lead to over- or underestimating energy density of samples (discussed in Beukema. 1997). A third issue is that bomb calorimetry is somewhat exacting, requires fairly large tissue mass (generally >5 mg), and requires expensive and specialized equipment.

The strengths of both AFDW and PDWO are that they are relatively simple and inexpensive to perform (though the issues described above must be kept in mind), so despite potential drawbacks they can be used by a wide range of investigators under a wide range of circumstances, Bomb calorimetry provides direct information about caloric value of eggs, but requires specialized equipment and may not work well for later larval stages that contain difficult-to-remove calcium carbonate skeletons. In addition, investigators often want more information than simple egg energy alone, and neither AFDW. PDWO, nor bomb calorimetry provides any direct information about the biochemical composition of eggs.

So how to accurately measure the energy content of eggs. while gaining more detailed information about energy storage and utilization? A fourth method is to individually measure the amounts of different biochemical constituents in eggs and sum their separate energetic values (e.g., Turner and Lawrence. 1979: McClintock and Pearse, 1986: Moran and Manahan. 2003, 2004; Reitzel et al., 2005: Byrne et al., 2008). The three main constituents of interest in marine invertebrate eggs are protein, lipid, and carbohydrate, which are assayed separately using different suites of techniques. Numerous assays and measurement techniques are available for each type of biochemical constituent, and the choice of assay can be highly relevant. While we do not mean this to be a comprehensive review of the entire literature on proximal content analysis, we discuss below the most frequently utilized ways of measuring protein, lipid, and carbohydrate content of marine invertebrate eggs.

Protein

For measuring protein, many researchers turn to one of two colorimetric assays: the Bradford assay (Bradford. 1976; e.g., Moran and Manahan. 2003: Bryan, 2004; Falkner el al., 2006) or a modified Lowry assay (Lowry et al., 1951: e.g., Schiopu et al., 2006; Prowse et al., 2008). Both assays are commercially available as kits, are relatively simple to use, and do not require highly specialized equipment beyond heating blocks and a spectrophotometer. They are also sensitive to microgram-level concentrations of protein, which can be essential when only a limited amount of tissue is available, as in many situations working with eggs of marine invertebrates. However, there are well-known artifacts associated with both methods (the Bradford in particular) that investigators should be aware of. When direct comparisons of the two assays are made on the same material, the Bradford assay generally gives substantially lower estimates of total protein: in marine invertebrates, this has been demonstrated in eggs and larvae of oysters, in which the difference in protein estimated by the Lowry versus Bradford methods reached almost 400% (Chu and Casey. 1986). The differences in protein detection between the two methods are likely attributable to underestimation of protein by the Bradford assay (Chu and Casey. 1986). The Bradford dye reagent does not detect all proteins equally but reacts primarily with arginine residues, and is less sensitive for basic or acidic proteins; this may cause an underestimation of quantity in complex combinations of proteins (Sto-schek, 1990). In addition, choice of standard inevitably affects estimates of protein, particularly if the standard reacts differently with the dye reagent than with sample proteins. One very commonly used standard is bovine serum albumen (BSA). The Bradford assay is considerably more sensitive to BSA than to "average" proteins (Stoschek. 1990), so using this common standard in assaying complex mixes of egg, embryo, and larval proteins with the Bradford assay may add to underestimates of total protein. Further, if protein composition changes during development from egg to larva to juvenile, the sensitivity of the Bradford assay may change correspondingly.

Ideally, the standard for any protein assay should be purified protein from the samples of interest; this point has been made many times in different invertebrate groups (e.g., Zamer et al., 1989). but is rarely feasible in studies involving invertebrate eggs and larvae. It may be advisable to compare results from multiple methods and standards (such as using both bovine gamma globulin (BGG) and BSA as standards for the Bradford assay; Stoschek. 1990) to detect bias. Of the two assays discussed here, the Lowry method may be preferable to the Bradford for invertebrate egg and larval protein assays because it provides higher (and likely more accurate) estimates of total protein and is not as greatly affected by protein composition or choice of standard (Chiappelli et al., 1979; Chu and Casey, 1986). However, both methods may be insensitive to small peptides and amino acids (Mayer et al., 1986; Grossman et al., 2000). Depending on the concentration of these small molecules in experimental samples, total protein may be underestimated regardless of which method is used.

If strong bias or inaccuracy is suspected, this should provide motivation to change methods, or to proceed to more complex quantitative techniques such as the stoichiometric elemental CHN method (Gnaiger and Bitterlich, 1984; e.g., Chaporro et al., 2006) or quantitative amino acid analysis (e.g., Crossman et al., 2000). These latter methods are considerably more time-consuming, expensive, difficult. and equipment-intensive than colorimetric assays such as the Bradford or Lowry. Appropriate choices of standards and assays can greatly reduce the degree of error in the simpler colorimetric methods and conserve their value as tools for understanding the composition of eggs and energetics of embryonic and larval development.

Lipid

There has been a long discussion in the literature about how best to estimate total lipids in marine samples (e.g., Lovern, 1964; Giese, 1966; Marsh and Weinstein. 1966; Knight et al., 1972; Barnes and Blackstock. 1973; Smedes. 1999; Iverson et al., 2001). Our goal here is not to review this extensive literature but to briefly describe the most commonly used methods for studying the role of lipids in marine invertebrate larval life histories, and to call attention once again to the important caveats that must be considered with each method.

The simplest, and on the surface most straightforward, method for estimating total lipids in a sample is to extract and weigh them. In these gravimetric analyses, lipids are extracted from the total sample, usually with a water-meth-anol-chloroform mixture in a particular ratio (e.g., Folch et al., 1957; Bligh and Dyer. 1959). followed by drying and weighing on a microbalance (e.g., Jones et al. 1996). Gravimetry is relatively simple, but it is only as precise and accurate as the equipment available for weighing, and can be affected by the composition of samples (Inouye and Lotufo, 2006). Overall the gravimetric method appears to be most accurate when large amounts of tissue are available for analysis (>5 g; Honeycutt et al., 1995), which reduces the potential impact of error in either direction. With marine invertebrate eggs and larvae, large masses of tissue may not be available, so more sensitive colorimetrie methods are often preferred.

Two colorimetrie methods that have been used for measuring invertebrate egg and larval lipids are the sulfuric acid charring method of Marsh and Weinstein (1966) (e.g., Holland and Gabbott, 1971; Thiyagarajan and Qian. 2003) and the sulphophosphovanillin method of Chabrol and Charonnat (1937) with subsequent modifications (e.g., Barnes and Blackstock. 1973; Achituv et al., 1980). Both are relatively simple, highly sensitive, and inexpensive to perform; but. as with colorimetric protein assays, the composition of samples and the choice of standard will affect the quantitative results. The sulfuric acid charring method, for example, is slightly more sensitive to saturated than to unsaturated lipids, though this is not generally a major concern (Marsh and Weinstein, 1966).

The sulphophosphovanillin method has been used mostly in the vertebrate and insect literature. It too can be reasonably accurate if the correct standards are used, but the colorimetric reaction in this assay requires a carbon-carbon double bond, so lipid classes that lack these bonds, such as triacylglycerols and saturated fatty acids, do not cause a color change (Knight et al., 1972). If the relative proportions of different lipids in a sample are known, then an appropriate correction factor can be calculated and applied; however, when lipid composition is unknown, the sulphophosphovanillin method will likely provide inaccurate estimates of total lipid. A case in point: triacylglycerol appears to be the primary storage lipid in most marine invertebrate eggs and embryos and in many cases serves as the primary source of energy during morphogenesis (Holland. 1978; Moran and Manahan. 2003. 2004; Byrne et al., 2008). An assay that is not sensitive to fully saturated lipid classes may fail to detect important physiological and evolutionary changes in egg and larval energetics. Another issue with this assay is that its efficacy will vary for animals that are acclimated to different environmental temperatures, because lipid saturation levels in ectotherms change with temperature (Weber et al., 2003). Thus, the sulphophosphovanillin technique should be approached with caution for use with seasonal or latitudinal comparative studies.

While both gravimetric and colorimetric analyses are useful for determining total lipid (with appropriate precautions for each method), they cannot separate and quantify individual lipid classes such as neutral lipids and phospholipids. Because different lipid classes play different biochemical and energetic roles in embryogenesis and larval development, many researchers are interested in quantifying these separate classes in eggs and tracking them over development. Lipid classes and fatty acids can be identified using chromatography (e.g., thin layer chromatography (TLC). gas liquid chromatography (GLC)). Examples from the marine invertebrate egg and larval literature include Villinski et al., (2002) for TLC, and Schiopu et at. (2006) for GLC. Unless a mechanism for quantification is integrated into the chromatography system, however, lipids must still be independently quantified, for example by gravimetry (e.g., Villinski et al., 2002). A system that simultaneously identifies and quantifies separate lipid classes, and one that has been used with increasing frequency in the marine literature (e.g., Moran and Manahan, 2004; Steer et al., 2004; Sewell. 2005; Prowse et al., 2008), is flame ionization detection thin layer chromatography (FID-TLC) (methods summarized in Parrish and Ackman. 1983; Parrish. 1987, 1999). These chromatographic methods require considerably more specialized equipment and expertise than gravimetric or colorimetric methods alone, and are liable to their own types of error. However, they also provide considerably more detail on lipid composition and the roles of different lipid classes in developmental energetics than colorimetric or gravimetric methods for estimating total lipid.

Carbohydrate

Carbohydrate makes up the smallest percentage by mass of the three major biochemical constituents, generally constituting less than 3% of eggs and larvae (Jaeckle. 1995). Because carbohydrate is such a minor component of total energy, many studies focus only on quantifying lipid and protein (e.g., Prowse et al., 2008). When information on total carbohydrate is desired, varied techniques have been employed, including the ferricyanate reduction reaction of Folin and Malmros (1929) as modified by Holland and Gabbott (1971) (e.g., Moran and Manahan. 2003), and the phenol-sulphuric acid procedure of Dubois et al. (1956) (e.g., Chaporro et al., 2006). Many of the same issues discussed for protein and lipid, such as differential sensitivity to specific biochemical-constituent classes and appropriate choice of standards, also apply to colorimetric assays of carbohydrate. Because carbohydrate is generally considered a minor energetic constituent, however, even relatively large errors in calculating total carbohydrate are unlikely to substantially affect calculations of total energy budgets, so we will not go into these in detail.

Extraction issues

The accuracy of all these methods for assessing biochemical composition also hinges, in part, on how effectively the constituent of interest is first extracted and purified. Again. we do not aim to review the extensive biochemical literature on protein, lipid, and carbohydrate extraction and purification, but we would like to call attention to one example of how lipid extraction techniques may affect quantitative estimates of lipid through development. Multiple studies have compared the extraction efficiencies of different solvent concentrations for marine (and other) samples (e.g., Smedes and Thomasen, 1996; Smedes and Askland, 1999; Smedes, 1999; Iverson et at., 2001). The extraction protocols of Bligh and Dyer (1959), often as modified by Holland and Gabbott (1971), are widely used for analyzing samples of marine eggs and larvae (e.g., George, 1999; Morais et al., 2002; Moran and Manahan. 2003; Ritar et at., 2003; Lee et al., 2005). One topic of particular interest to scientists working with marine egg and larval samples was raised by Iverson et at. (2001), who found that the extraction method of Bligh and Dyer (1959) underestimated lipid content in samples containing more than 2% lipid compared to the extraction methods of Folch et al., (1957). The difference was substantial; total lipids extracted via the Bligh and Dyer method were 50% lower than those measured by the Folch et al., method in marine samples containing 26% lipid (the highest lipid percentage tested). This difference was attributed to under-extraction by the Bligh and Dyer method (Iverson et al., 2001).

Because marine invertebrate eggs contain energy necessary for morphogenesis and development, even small eggs of planktotrophic species tend to contain around 20% lipid (e.g., for oysters, Moran and Manahan. 2004). Large eggs of lecithotrophic echinoids can be well over half lipid by weight (e.g., McClintock and Pearse. 1986; Hoegh-Guld-berg and Emlet, 1997; Byrne et al., 2008). If the findings of Iverson et al. (2001) are broadly applicable to marine invertebrate samples, there seems to be potential for substantially underestimating lipid content through using the Bligh and Dyer (1959) method, unmodified, for assaying marine eggs. It is often difficult to determine precise extraction methods from the literature because modifications to either Bligh and Dyer (1959) or Folch et al. (1957), though invoked in methods sections, are often not described in detail (Iverson et a., 2001). Hypothetically. however, this type of systematic underestimation due to poor extraction efficiency could go far toward explaining the average 20% of dry mass that is not recovered by biochemical constituent analysis--the "remainder fraction" of Jaeckle and Manahan (1989) and Jaeckle (1995)--and could potentially confound energetic analyses as well.

General patterns

Overall, the use of biochemical constituent analysis has expanded our understanding of egg composition and energy utilization during development of marine invertebrate larvae. As we have pointed out, the methods utilized in the literature are many and diverse, and prone to various types of error. In many instances, despite established drawbacks, a particular method (e.g., AFDW) may be the most desirable because it allows for direct comparisons with previous work on the same questions or taxa. Ideally, however, selection of a "best" method for extraction and quantification in any project will involve not only comparative considerations and the availability of expertise, supplies, and equipment for a particular method, but also an understanding of the accuracy and limitations of different methods, including--but not limited to--those laid out above.

The previous section was intended to highlight some of the more widely used methods and to summarize their strengths and weaknesses, with the goal of providing a general guide for those considering biochemical analysis. But drawing broad comparative inferences about egg size and composition from the literature is a difficult venture, for several reasons. First, biochemical studies relating egg size to composition are relatively rare; second, the majority of studies have focused on a single group, echinoderms; and third, the techniques that have been employed are varied and numerous. Despite these issues, some patterns have begun to emerge. In his 1995 review. Jaeckle combined information from studies that utilized DOM, PDWO, and biochemical constituent analysis to pull out broad patterns for echinoderms, and found a common scaling pattern of egg energy with size across five orders of magnitude in size and among diverse developmental modes. Eggs of lecithotrophic species had greater mass-specific energy contents than eggs of planktotrophic species (see fig. 5 in Jaeckle, 1995), which was attributed to the proportionally higher lipid content of lecithotrophic eggs.

More recent studies, for example by Sewell and Manahan (2001) and Prowse et al (2008). have tended to uphold these general patterns. In a comparative study of asterinid sea stars. Prowse et al. (2008) found that the evolution of lecithotrophic development was associated with a reduction in protein deposition and an increase in triacylglycerol (an energetic lipid class). The one exception to this pattern was a lecithotrophic species with benthic development, which had a protein/lipid ratio similar to that of planktotrophs (presumably to reduce buoyancy; Prowse et al., 2008). Thus, at least in this group, the evolutionary loss of planktotrophy was apparently accompanied by an increase in egg size that was in most cases largely attributable to vitellogenic processes that increased deposition of triacylglycerol. Comparative data for taxa other than echinoderms are largely lacking, so whether this is a general phenomenon that holds across many invertebrate groups remains to be seen.

While the data indicate that size matters in the sense that lecithotrophic species appear to have eggs that are compositionally different from those of planktotrophs. we have yet to understand how evolutionary changes in egg size that occur within developmental modes (e.g., within planktolrophy) are associated with changes in biochemical composition. Additional comparative biochemical studies of non-echinoderm taxa that contain species with a wide range of egg sizes and are well-understood phylogenetically are necessary; examples of candidate taxa for whom phylogeny. developmental mode, and egg size have already been largely determined are the calyptraeid gastropods (Collin. 2004) and arcid bivalves (Marko and Moran. 2002). Further insights could also be gained from comparative biochemical work on closely related species that inhabit different habitats, such as geminate species pairs in tropical America. In many invertebrate taxa the Pacific member of planktotro-phic species pairs has smaller eggs than its Caribbean geminate species (e.g., Lessios, 1990; Moran. 2004). but it is not known whether these evolutionary changes in egg size are also associated with changes in biochemical composition that might reflect differing energetic strategies in the two oceans.

Finally, one additional avenue that has yet to be explored is comparing egg composition in those rare species that show flexibility in developmental mode, a life-history mode known as poecilogony. One example of a poecilogonotis species is the gastropod Alderia witlowi, which produces either small planktotrophic larvae or large larvae that do not need to feed in order to metamorphose. Nonfeeding larvae develop from eggs that are 105 [+ or -] 5 [micro]m in diameter while planktotrophic larvae develop from much smaller eggs (68 [+ or -] 4 [micro]m diameter), and individuals can vary the developmental mode of offspring they produce (Krug, 2007). Because A. willowi shows such strong and unusual plasticity in egg size and developmental mode, working with this species could shed substantial light on how a change in developmental mode is, or is not. accompanied by changes to egg biochemical composition, larval energetics, and vitel-logenic processes.

Question 2: What Determines Egg Size?

Or, what environmental and genetic factors affect egg size, and what physiological mechanisms underlie egg size variation?

Variation in phenotypic traits such as egg size is generally attributed to one or a combination of three factors: environmental influences (environment), genetic differences among organisms (genotype), and stochastic developmental traits (Vogt et al., 2008). As we have discussed. egg size is a highly variable trait both within and among species of marine invertebrates; however, the relative importance of these three factors in determining egg size, and thus in shaping the evolution of larval life histories, has not, to our knowledge, been thoroughly explored in any marine invertebrate. We have some understanding of how environmental factors affect egg size in a handful of taxa. but these findings are often not consistent across taxa. The genetic-basis for egg size variation has rarely been explored in marine animals, and therefore we have very little understanding of how egg size is inherited or how genotype and environment interact to determine egg size at a particular spawning event. Finally, while stochastic developmental events can play a large role in determining variation of many phenotypic trails (e.g., Lajus and Alekseev. 2004). we know virtually nothing about the extent to which egg size variation in marine invertebrates may be underlain by such "developmental noise." if at all. Our goal here is to highlight cases-in-point from the recent literature on determinants of egg size, and to point to areas where further work would be of great value.

Temperature, .salinity, and toxicants

One of the most frequently noted environmental factors that affects egg size in marine invertebrates is temperature. Temperature has a well-documented negative correlation with egg size in many marine taxa. such that both within and among species, animals that live at colder temperatures often produce larger eggs (Rass. 1941; Clark. 1992; Hoegh-Guldberg and Pearse. 1995; Levitan. 2000; Peck et al., 2007). Some of these temperature-size relationships can be attributed to evolutionary shifts in developmental mode or to different adaptive strategies associated with latitude (e.g., Rass. 1941; Thorson, 1950: Marshall, 1953; Mileykovsky. 1971; Alekseev, 1981; Laplikhovsky, 2006; Lardies et at., 2008). However, a substantial component may also be due to a direct physiological response in which the temperature experienced by a female during oogenesis influences the size of eggs she produces. This phenomenon has been observed for cell (including egg) size (see Azevedo et al., 1997. for review) and for body size as well in numerous ectotherm model systems: thus, when egg size changes inversely with temperature, this can be considered part of the size-temperature rule of Rass (1941: Laplikhovsky. 2006). The physiological bases for this pattern are not understood but may include, for egg size, lower temperature sensitivity of vitellogenesis compared to oocyte production rate and egg maturation time (Steigenga and Fischer, 2007). This hypothesis is based on a biophysical model that assumes different temperature constraints on growth (i.e., vitellogenesis), which depends on the rate of protein synthesis, and on differentiation (i.e., oocyte production). which depends on the rate of DNA replication (van der Have and de Jong. 1996).

A handful of authors have experimentally examined the effects of temperature on egg size in marine invertebrates (Dugan et al, 1991: Simonini and Prevedelli. 2003; Steers al., 2004). Simonini and Prevedelli (2003) reared individuals from two geographically separated populations of the polychaete Dinophilus gyrociliatus at a broad range of temperatures and found that the smallest eggs were produced by individuals reared at the highest temperatures; their analysis revealed a significant effect of temperature on egg size. However, in the case of the cephalopod Euprymna tasmanica (Steer el al., 2004). laboratory rearing temperatures representative of tidal sandflat temperatures in austral summer (18 [degrees]C) and winter (11 [degrees]C) did not appear to affect egg size. Similarly. Dugan et al. (1991) found no correlation between egg size and water temperature among northern and southern populations of a sand crab. Emerita analoga, collected from the California coast. More experimental work is necessary to determine how broadly distributed the temperature-size pattern is in a wider variety of marine invertebrate taxa. and whether the effects of temperature on egg size are underlain by similar mechanisms in disparate groups. These are important questions: if physiologically driven plasticity in egg size is heritable, and if temperature-driven, plastic changes to egg size impact larval life histories in predictable ways, then a change of environmental temperature could represent a tipping point that drives or facilitates an evolutionary change to a different developmental mode (Laptikhovsky. 2006).

Salinity is a second environmental factor that has been linked to changes in egg size, though a clear adaptive function of such salinity-driven changes is lacking. Gime-nez and Anger (2001) demonstrated that estuarine crabs. Chasmagnathus granulata, held at a pre-hatching salinity of 15 ppt laid eggs that were larger than those from crabs held at 32 ppt. A similar result was obtained by Skadsheim (1989) for the amphipod Gammarus salinus; adults reared in lower salinities produced eggs with greater volumes than did individuals reared in higher salinities. This salinity-driven plasticity in egg size is likely to be due simply to osmotic water uptake by eggs in low-salinity water, since the brooded eggs of amphipods osmoconform (Vlasbloom and Boiler, 1971). In free-spawners, however, an osmotically driven increase in egg size might provide an energetically inexpensive strategy for increasing the sperm target size of eggs, though we know of no studies that have examined this question.

A third environmental factor that has been demonstrated to influence egg size is the presence of certain chemicals in the environment during oogenesis. Of particular note is the effect of maternal exposure to natural or anthropogenic toxicants. Lardies el al. (2008) demonstrated an association between maternal exposure to copper mine tailings and significantly lower than expected egg volumes in a Chilean population of the snapping shrimp Betaeus emarginatus. As another example, Cox and Ward (2002) showed that individuals of the broadcast-spawning coral Montipora capitata reared in ammonium-enriched experimental tanks produced eggs that were significantly smaller in diameter than control individuals, whereas total fecundity, egg number, chlorophyll a content, and fertilization success were not affected. In both of these examples, egg size decreased in response to the presence of anthropogenic pollution, likely reflecting an organismal stress response, While no mechanistic explanation was proposed for the decrease in egg size seen in response to ammonia, Lardies el at. (2008) proposed that an oxidative stress condition, which develops in some marine species when exposed to excesses of copper, may have forced individuals of B. emarginatits to redirect energetic investment from activities such as oogenesis towards antioxidant mechanisms. Is there a general stress response that results in the production of smaller, fewer, or lower quality eggs in females that are exposed to toxicants or other stressors? If so, we may be able to make broad predictions about how environmental quality (from the perspective of the female) drives carryover effects that reduce the fitness of offspring across marine taxa. Knowing more about the mechanistic bases for the relationship between particular anthropogenic toxicants and how these toxicants affect egg size in a wider variety of taxa would be valuable information for environmental management.

Maternal age, size, and condition

Environmental factors that are experienced by the mother and that have resultant effects on the phenotype and performance of her offspring are termed maternal effects (see Marshall el at, 2008. for a recent review of this topic). Egg size has been demonstrated to be affected by several maternal effects, including maternal age. maternal size, and maternal nutrition, which we highlight individually below.

Egg size has been demonstrated to be affected by maternal age in a wide range of taxa (Wiklund and Persson, 1983: Cavers and Steel. 1984; Kane and Cavers, 1992; Ruo-homaki et al., 1993; Braby. 1994). In at least some groups of marine invertebrates, egg size decreases with advancing maternal age (Qian and Chia. 1992; Ito. 1997). For example. Qian and Chia (1992) investigated the effects of aging on multiple correlates of reproduction in the marine polychaele Capitella sp. In laboratory-reared individuals, egg size, as well as fecundity, egg energy content, total egg volume, and total energy investment, decreased significantly with advancing maternal age and in successive spawns. The mechanisms underlying a decrease in egg size with maternal age are not known, but this pattern is more prevalent in animals with short life spans, determinate growth, and a single reproductive season and may be attributable to a decrease in maternal condition with age. For example, I to (1997) showed that egg size and the number of eggs spawned by the sea slug Haloa japonico decreased over the course of the breeding season under laboratory conditions. Similar pal-terns have been found in other molluscs (Macginitie, 1934; Thompson, 1958; Gibson and Chia, 1995; Steer et al., 2004). With respect to H. japonica, the decrease in egg size and number may reflect the fact that this species feeds very little after the onset of the reproductive season. If egg size reflects maternal investment per ovum, then females likely have decreasing amounts of organic material to package into eggs with each successive spawn. Similarly, decreases in egg size with successive spawns have also been noted for the nudibranch mollusc Adalaria proximo (Jones et al., 1996), which dies after its single spawning season. In contrast, in a long-lived animal. Holiotis laevigata, the grecnlip abalone. successive spawns did not lead to differences in the average egg diameters, although egg diameter was more variable with each successive spawn in a hatchery system (Graham el ui, 2006). Despite no change in egg size, the density of protein and lipid in eggs of H. laevigata increased throughout the spawning season (Fukuzawa et al., 2005): although eggs were of the same size, later-spawned eggs represented higher maternal investment per offspring. This result may reflect either (1) a differential release of eggs throughout the spawning season, such that some eggs are held for longer periods and subsequently packaged more densely with protein and lipid, or (2) a ramping-up in the efficiency of egg production mechanisms over time, such that later spawned eggs are produced at higher quality than earlier spawned eggs. Comparative studies that examine temporal changes in oogenic mechanisms among wild populations will help us to understand the relationship between egg size and maternal age.

Egg size can in some cases be positively correlated with maternal size, with larger mothers producing larger eggs (George. 1994; Chester. 1996; [to, 1997; Marshall et al, 2000: Bingham et al., 2004). For example. Marshall et al. (2000) demonstrated that large Pyuro stolonifera tunicates produced larger eggs than smaller individuals. This pattern held for tunicates from two different populations, although the relationship between maternal body size and egg size differed between sites--that is, the slopes of the regression of egg to body size was different for the two populations. Egg size may be affected by maternal size for the relatively straightforward reason that maternal size reflects the amount of resources a given mother has available for reproduction (George. 1994); this assumes, however, that egg size is determined by maternal resource availability, which may not always be the case (see Meidel et al, 1999: Diaz et al., 2003).

Egg size and maternal investment per egg can also be affected by the availability and quality of the maternal food supply (George, 1990. 1994; George et al., 1990, 1991; de Jong-Westman et al., 1995; Guisande and Harris, 1995; Pond et al., 1996; Bertram and Strathmann. 1998; Steer et al., 2004). As one example, starved individuals of the es-tuarine nudibranch Teneliia adspersa produced eggs that were as much as 26% smaller in volume and 40% lighter in weight than eggs of well-fed individuals (Chester, 1996). Additionally, seastars from sites of low food quality produced larger eggs when food ration was experimentally increased, whereus stars from sites of high food quality did not (reviewed by George, 1996), suggesting that in the field, mothers were physiologically responding to low food by producing smaller eggs. These changes in egg size likely represent maternal nutritional stress rather than an adaptive alteration in size (Bertram and Strathmann. 1998). though the physiological mechanisms underlying stress-related changes in egg size are not known. Lastly, data collected from the eggs of multiple fish species indicate that the nutritional state of females affects the water content, buoyancy, and hence size of eggs (Craik and Harvey, 1987; Kjesbu et al., 1991; Nissling et al., 1994). It seems possible that similar patterns may exist among marine invertebrates, though this has yet to be tested.

Although these studies demonstrated that egg size can be affected by maternal nutrition, few studies have separated the effects of maternal nutrition on egg size versus on egg composition, and then followed by linking these to larval performance. Some exceptional studies in the aquaculture literature have put all of these pieces together, but the emerging picture does not necessarily indicate that egg size is a highly relevant parameter in predicting larval performance. Buchal et al., (1998) reared adult red abalone (Haliotis rufescens) on two diets and found that on the "superior" diet, mothers produced eggs that had higher dry weight and greater lipid and protein contents than did mothers kept on the "inferior" diet. Larvae from eggs produced by mothers on the superior diet went on to have higher hatching and metamorphic success. However, there was no difference in egg size between maternal food treatments. Thus, in this case at least, egg size was not a good indicator of either egg energy content or the subsequent performance of embryos and larvae.

Clearly the size and age of the mother, her nutritional history, and the number of successive spawning events she experiences in a given season are all interrelated factors that can have some bearing on the size of eggs she can produce. What is unknown is whether there are interactive effects of two or more oi' these factors on egg size. Take, for example, the question of whether egg size is distinctly correlated with the interaction of maternal size and nutritional condition: George (1994) found a positive relationship between maternal size and egg size in the sea star Leptasterias epi-chlora, but speculated that the effects of maternal size in this case may have been confounded with maternal nutrition because the larger mothers were from an environment with a higher food supply. Similarly, Chester (1996) demonstrated a correlation between starvation and a decrease in adult size that was associated with a decrease in egg size. Starved individuals of the nudibranch Tenellia adspersa produced smaller eggs than fed individuals produced. In these two studies it is not clear whether maternal size, maternal nutritional condition, or some combination of the two was the primary determinant of changes in egg size. The interrelatedness of these maternal effects suggests that there may be common mechanistic underpinnings for their effects on egg size. Because maternal size, maternal age. and maternal condition are often confounded both in nature and in experimental work, it may be difficult to tease apart any independent effects these factors have on egg size. What does seem clear is that females that are "stressed," either by limited resources, advancing age, or physical and chemical stressors in the environment, in some cases respond by producing smaller eggs. What is not clear is how egg size is mechanistically affected by these stressors, and to what extent different stressors affect egg size by the same or different mechanisms. For these reasons, future studies that can effectively isolate the effects of multiple maternal factors on egg size in a single system would be valuable.

Population density

In a single instance, egg size amongst females has been shown to differ depending on adult population density. Crean and Marshall (2008) demonstrated in field experiments that eggs produced by Styela plicata females reared at low densities were 5% smaller in area than eggs produced by females reared at high density. However, these smaller eggs had a larger area of accessory structures (i.e., follicle cells), such that the overall "target size" for fertilization by sperm was 9% larger for low-density individuals. This small proportional change may represent an energetic tradeoff between investment in eggs and investment in accessory structures such as follicle cells (Crean and Marshall, 2008), suggesting that in this species, plastic responses that increase sperm target size in sperm-limiting environments may limit the resources available to females for producing large eggs. This phenomenon, while currently noted in only one species of ascidian, might also be found in other taxa that utilize egg accessory structures, such as jelly coats, which can increase the effective target size for sperm.

Genetic factors

Evolution by natural selection requires both variation in a trait and genetic heritability of that variation. As we have discussed above, a considerable amount of variation in egg size can be attributed at least in part to the environment, but the component of variation that is underlain by genetic (and hence heritable) factors has been examined in only a few marine invertebrates. Phenotypic variation ([V.sub.P]) in a character such as egg size can be partitioned into variation due to genetics ([V.sub.G]), environment ([V.sub.E]), and their interaction ([V.sub.GxF]). One component of [V.sub.G] accounts for parent-offspring resemblance and is known as additive genetic variance ([V.sub.A]). Narrow-sense hcritability ([h.sup.2]) is a measure of the proportion of [V.sub.p] that is due to [V.sub.A] where [h.sup.2] = [V.sub.A]/[V.sub.P], and is a predictor of the short-term response to selection (Falconer and Maekay, 1996). A recent study by Miles et al. (2007) showed that egg size of the polychaete Hydroides elegans was a heritable character: in response to artificial selection, egg size increased significantly in as little as four generations in their experiments ([h.sup.2] = 0,58). A half-sib breeding design conducted on the same population showed a similar estimate of narrow-sense heritability ([h.sup.2] = 0.45). Similarly high heritability estimates were obtained by Levin et al. (1991) for egg diameter ([h.sup.2] = 0.75) in the poecilogonous polychaete Streblospio benedicti, and by Arcos et al. (2005) for oocyte diameter ([h.sup.2] = 0.57) in the shrimp Litopenaeus vannamei. Also, a study by Hilbish et al. (1993) found a comparable heritability estimate of [h.sup.2] = 0.58 for the shell length of prodissoconch I in the hard clam Mercenaria mercenaria. Prodissoconch I length is tightly correlated with egg size in M. mercenaria because it is formed early in development directly around the egg yolk mass (Goodsell and Hversole, 1992). Additionally, a recent study by Diz et al. (2009), which utilized proteomic and bioinformatic techniques to assess genetic variation underlying protein expression in eggs of the bivalve Mytilus edulis, found a broad-sense heritability ([H.sup.2] = [V.sub.g]/[V.sub.p]) of gene expression for the egg proteome at about 50% or greater. Although the results of Diz et al. (2009) do not pertain specifically to egg size, their study does highlight the possibility that new suites of techniques may be useful in investigating the bottom-up mechanisms underlying egg size variation on many levels of inquiry. The results from these studies suggest that a significant portion of the variability in egg size can be attributed to genetic differences between individuals, and that egg size may be able to evolve rapidly under the appropriate selective regime. Aside from polychaetes and commercially important shellfish, though, few marine invertebrates are tractable model systems for narrow-sense heritability studies due to long generation times and complex rearing requirements.

Even in the presence of strongly heritable variation in egg size among individuals, the evolution of egg size may be limited by phylogenetic constraints (Derrickson and Rick-lefs, 1988; McKitrick, 1993); these are life-history tradeoffs, physiological characteristics, and selective pressures unique to different taxonomic groups that may result in limitations on the extent of evolutionary changes in egg size (Lessios, 1990). Such phylogenetic effects were invoked by Lessios (1990), who measured egg diameters and calculated egg volume of 24 echinoderm species found in coastal waters off the Pacific and Atlantic (Caribbean) coasts of Panama. Of these species, 14 belong to geminate pairs. In 6 of 7 geminate pairs, the eggs of the Pacific species were smaller than the eggs of its Atlantic counterpart, demonstrating six independent evolutionary changes in egg size in the same direction. Despite the fact that the eggs from Pacific species within any given pairing were smaller than their Atlantic geminates, there was no common egg size for a given ocean; rather, in all cases egg sizes of closely related animals were more similar than those of distantly related ones. This result suggests that the optimal egg size for a given environment is not independent of phylogenetic history. In contrast to the similarity in egg sizes found by Lessios (1990) for closely related geminate pair echinoids, Levitan (2006) noted that, in fact, egg size often varies considerably among closely related species of echinoids.

The degree to which egg size is free to evolve in response to natural selection is likely to depend largely on both the genetic and physiological mechanisms by which eggs are made. Research concerning specific genes or gene networks that affect egg size are limited. We do know that in echinoid echinoderms, oogenesis is largely a two-step process consisting of an initial deposition of yolk protein (vitellogene-sis), followed by deposition of lipids (lipogenesis) and non-vitellogenic proteins (Byrne et al., 1999). Yolk protein is deposited during vitellogenesis by activation of the vitellogenin gene (Shyu et at., 1986; Scott et al., 1990), and the levels of activation appear to be conserved at least between congeneric species of Heliocidaris that vary in egg size and developmental mode (Byrne et al., 1999).

The differences in egg size between Heliocidaris spp. are largely attributable, therefore, to the amount of lipid and non-vitellogenic proteins that are deposited subsequent to vitellogenesis. The specific genes that are activated to provision eggs with these components are unknown; however, the temporal separation between vitellogenesis and lipogenesis suggests that these processes may represent separate developmental modules (Raff, 1996; Byrne et al., 1999). Prowse et al., (2008) suggest that elaboration of this aspect of oogenesis may be associated with the evolution of developmental mode, and that increases in egg size due to hypertrophic lipogenesis may transform a species that develops as a facultative planktotroph into a lecithotroph. Similar comparative approaches using taxa beyond echinoderms would contribute substantially to our understanding both of the similarities and differences among oogenic mechanisms in disparate invertebrate taxa and their relationship to egg size, and of the exlent to which evolutionary changes in developmental mode (and egg size) are underlain by particular aspects of the oogenic pathway. In addition to comparative approaches, examining oogenesis and gene expression in poecilogenous species, such as the polychaete Strehtospio benedicti (Levin et al. 1991) or molluscs in the genus Alderia (Krug, 2007), would also be likely to provide valuable additional insights into the mechanisms underlying egg size evolution as it relates to changes in larval developmental mode.

Conclusions

Egg size is one of the most often-studied aspects of the life history of marine organisms, and considerable comparative and theoretical work has explored its many life-history correlates (see references in Introduction). Egg size is a simple character to measure, has a long history in the marine literature, and is closely related to developmental mode in many phyla. Because of its strong correlation with developmental mode (at least within large taxonomic groupings), at a large scale egg size can be a powerful predictor of dispersal potential, and even of geographic range and geological longevity of species (Hansen, 1978, 1980; Jablonski and Lutz. 1983; Bhaud, 1993; Emlet, 1995; Jeffery and Emlet, 2003). Egg size is also broadly correlated with energy content (Jaeckle, 1995) as well as with many energy-related larval life-history characters such as length of larval development (Thorson, 1950; Vance, 1973a, b; Strathmann, 1985; Hadfield and Miller, 1987; Sinervo and McEdward, 1988; Wray and Raff. 1991), larval form (McEdward, 1986; Strathmann, 2000). length of the facultative feeding period (Miner et al.. 2005), juvenile growth and survival (Marshall et al, 2003), and dependence on exogenous food (Anger, 1995; Bridges and Heppell. 1996). For these important reasons, egg size has long been, and will continue to be, a valuable tool for understanding the life histories of marine taxa.

However, there are two ways in which egg size may not be all it's cracked up to be. First, it is important to keep in mind that among most life-history studies that examine egg size evolution, the character that is measured--egg diameter or volume--is likely not the actual target of selection. In the majority of studies dealing with larval development and energetics, differences in "egg size" serve as a convenient, though coarse (McEdward and Morgan, 2001), surrogate for energy reserves provided by the mother to the embryo. larva, or juvenile. A finer-scale understanding of not just the size but the composition of eggs can provide considerably more insight into the selective forces acting on eggs and early life-history stages. For example, Prowse et al. (2008) found that loss of planktotrophy in asterinid sea stars was associated with a reduction in protein deposition (protein density was greater in planktotrophic than lecithotrophic species) and an increase of energetic lipids in the egg, presumably reflecting selection for rapid, nonfeeding development and reduction in unneeded larval feeding structures. One lecithotrophic species, however, had a protein/lipid ratio similar to those of planktotrophs, and this species was also unique among lecithotrophs in exhibiting demersal development. Since proteins are heavier than lipids and increase the weight/volume ratio of eggs, Prowse et al. (2008) speculated that maternal provisioning had in this case been influenced by selection for reduced buoyancy to retain larvae in their benthic habitat. Without knowledge of the biochemical composition of eggs, adaptive evolutionary scenarios like this one would be missed. Additional comprehensive, comparative studies of egg composition, larval energy utilization, the processes of oogenesis, and the correlation of all of these with egg size on a finer phylogenelic scale are vital to shedding light on the selective forces that have shaped the evolution of oogenic strategies in marine invertebrates.

One conceptual framework under which egg size per se is in fact the focus of natural selection is the argument that larger eggs are favored under some conditions because they make better targets for sperm (e.g., Levitan. 1993, 2000. 2006). In that case, too, we argue that a better understanding of the biochemical composition of eggs, relative to size, can shed light on how egg size evolves. Under sperm-limiting conditions, empirical evidence and theoretical models both suggest natural selection will favor a physical increase in egg size. This selective force, because it acts on sperm target size alone, will likely be independent of selection for increased energy investment. Can mothers who experience selection for increased sperm target size in fact optimize their fitness by increasing egg size without a concurrent increase in per-egg energy and subsequent decrease in fecundity? This result might be achieved by producing eggs that are larger target areas, not by increasing ovum size but by adding relatively inexpensive accessory structures such as jelly coats (Podolsky, 2001, 2004) or follicle cells (Crean and Marshall. 2008) to the outside of eggs. Another solution might be for mothers to increase egg size itself via hydration or through the addition of less energy-rich organic constituents (e.g., protein instead of lipid). Are these oogenic strategies physiologically possible, and if so, has this optimization scenario played out? Without a better grasp of egg biochemistry and mechanisms of oogenesis, these sorts of fundamental questions must remain largely an intellectual exercise.

A second area of egg size biology that lacks resolution is the description of relationships between egg size and egg energy at medium-to-fine scales. While there appears to be a strong correlation between egg size and egg energy when these two characters are examined across the entire range of egg volumes seen in echinoderms (Jaeckle, 1995; Sewell and Manahan. 2001), at finer scales this correlation is much weaker, such that within and even among species egg size can be a very poor predictor of egg energy content (McEd-ward and Carson. 1987; McEdward and Coulter. 1987: McEdward and Morgan. 2001). Egg size and egg energy can even be entirely uncoupled; Buchal et at. (1998) demonstrated that a superior maternal diet in abalone resulted in the production of eggs that had higher amounts of per-egg protein and lipid and developed into more successful larvae, yet egg size was entirely unchanged between the two treatments. When intraspecific. between-female variation in egg size is poorly correlated (or not correlated at all) with egg energy content, how do we explain the broad-scale patterns in egg size that are related to energy content, and that have evolved in multiple phyla?

One factor that complicates the detection of intraspecitic relationships between egg size and egg energy is that researchers have used diverse methods to measure biochemical composition, and these methods do not always produce comparable results. A second issue is that most of these biochemical methods are not sensitive enough to accurately measure the biochemical content of single eggs. Yet another set of difficulties lies in the many environmental, genetic, and stochastic factors that may affect egg size at the organismal level. While these factors appear to be numerous, we have a poor understanding of (1) the mechanisms by which these factors generate variation in egg size and egg energy, and how these characters are evolutionarily coupled; (2) whether and to what extent different factors interact with each other (for example, genotype by environment interactions) in generating this variation; and (3) how broadly applicable patterns and trends seen in a particular taxon are across other groups. These are substantial gaps in our knowledge; without a better understanding of the mechanisms that underlie intraspecitic variation in egg size and egg composition among individuals, it is difficult to delve more deeply into how natural selection has shaped the life histories of marine invertebrates. A stronger focus on identifying egg constituents and more experimental and comparative studies that identify the underlying mechanisms that generate intraspecitic variation in egg size are fundamental to generating new insights into the evolution of egg size and life histories of marine invertebrates.

Acknowledgments

We thank R. Emlet, P. Marko, D. Marshall, and one anonymous reviewer for comments on the manuscript, and R. Emlet for inviting the authors to participate in this issue. This work was supported by Clemson University's Department of Biological Sciences and by the National Science Foundation (ANT-0551969 to A.L.M.)

Literature Cited

Achituv, Y, J. Blackstock, M. Barnes, and H. Barnes. 1980. Some biochemical constituents of stage 1 and stage II nauplii of Balanus balanoides (L) and the effect of anoxia on stage I. J. Exp. Mar. Biol Eeol. 42: 1-12.

Alekseev, F. E. 1981. Rass-Thorson-Marshall rule and biological structure of marine communities. Pp. 4-6 in 4th Congress of All-Union Hydrobiologictil Society. Theses of Reports. Part I. G. G. Vinberg. ed. Naukova Dumka. Kiev.

Anger, K. 1995. Starvation resistance in larvae of a semi terrestrial crab. Sesarma curaeaoense (Decapoda: Grapsidael,.J, Exp, Mar. Biol. Ecol. 187: 161-174.

Arcos, F. G., E. Palacios, A. M. Ibarra, and I. S. Raeotta. 2005. Larval quality in relation to consecutive spawnings in white shrimp Litope-naeus vannamei Boone. Aquae. Res. 36: 890-897.

Azevedo. R. B. R., V. French, and L. Partridge. 1997. Life history consequences of egg size in Drasophila melanogaster. Am. Nat. 150: 250-282.

Barnes. H., and .J. Blackstock. 1973. Estimation of lipids in marine animals and tissues; detailed investigation of the sulphophosphovanillin method for total lipids. J. Exp. Mar. Biol Ecol 12: 103-118.

Bertram. D. F., and R. R. Stralhmann. 1998. Effects of maternal and larval nutrition on growth and form of planktotrophic larvae. Ecology 79: 315-327.

Beukema. J. J. 1997. Calorie values of marine invertebrates with an emphasis on die soft parts of marine bivalves. Pp. 3X7-414 in Oceanography and Marine Biology: an Annual Review. A. D. Ansell. R. N. Gibson, and M. Barnes, eds. Antony Rowe. Chippenham, UK.

Bingham, B. L., K. Giles, and W. B. Jaeckle. 2004. Variability in broods of the seastar LepTasterias aequalis. Can. J. Zool. 82: 457-463.

Bhaud, M. 1993. Relationship between larval type and geographic range in marine species: complementary observations on gastropods. Oceanol. Acta 16: 191-198.

Bligh, E. G., and W. F. Dyer. 1959. A rapid method of total lipid extraction and purification. Can. J. Biochem, Physiol. 37: 911-917.

Braby, M. F. 1994. The significance of egg size variation in butterflies in relation to hostplant quality. Qikos 47: 293-302.

Bradford, M. M. 1976. A rapid and sensitive method for the quantitation of microgram quantitiites of protein utilizing the principle of protein-dye binding. Anal. Biochem. 72: 248-254.

Bridges, T. S., and S. Heppell 1996. Fitness consequences of maternal effects in Streblospio benedicti (Annelida: Polvchaeta). Am. Zool. 36: 132-146.

Bryan, P. J. 2004. Energetic cost of development through metamorphosis for the seastar Medtaster aequalis (Stimpson). Mar. Biol. 145: 293-302.

Buchal, M., J.-E. Levin, and C. I.angdon. 1998. Dulse Palmaria mollis as a settlement substrate and food for the red abalone Haliotis rufe-scens. Aquaculture 165: 243-260.

Byrne. M., J. T. Villinski, P. Cisternas. R. K. Siegel. F. Popodi. and R. A. Raff. 1999. Maternal factors and the evolution of developmental mode: evolution of oogenesis in Heliocidaris erythrogramma. Dev: Genes. Evol. 209: 275-283.

Byrne. M., M. A. Sewell, and T. A. A. Prowse. 2008. Nutritional ecology of sea urchin larvae: influence of endogenous and exogenous nutrition on echinopluteal growth and phenotypic plasticity in Tripneu-Stes gratilla. Fund. Ecol. 22: 643-648.

Cavers. P. B. and M. (J. Steel. 1984. Patterns of change in seed weight over lime on individual plants. Am. Nat 124: 324-335.

Chabrol. F., and R. Charonnnat. 1937. Une nouvelle reaction pour I'etudes des lipides: L'oleidemie. Presse Med. 45: 1713.

Chaporro, O. R., F. R. Navarrele, and R. J. Thompson. 2006. The physiology of the larva of the Chilean oyster Ostrea chilensis and the utilisation of biochemical energy reserves during development: an extreme case of the brooding habit. J. Sea Res. 55: 292-300.

Chester, C. M. 1996. The effect of adult nutrition on the reproduction and development of the estuarine nudibranch. Tenellia adspersa (Nord-mann. 1845). J. Exp. Mar. Biol. Ecol. 198: 113-130.

Chiappelli. F., A. Vasil. and D. F. Haggerty. 1979. The protein concentration of crude cell and tissue extracts as estimated by the method of dye binding: comparison with the Lowry method. Anal. Biochem. 94: 160-165.

Christiansen, F. B., and T. M. Fenchel. 1979. Evolution of marine invertebrate reproductive patterns. Theor. Popul Ecol. 16: 267-282.

Chu F. L. E., and B. B. Casey. 1986. A comparison of protein assays for oyster larval proteins using two different standards. Mar. Chem. 19: 1-8.

Clarke. A. 1992. Reproduction in the cold: Thorson revisited. Invertebr. Reprod. Dev. 22: 175-184.

Collin. R. 2004. Phylogenetic effects, the loss of complex characters, and the evolution of development in calyptraeid gastropods. Evolution 58: 1488-1502.

Cox, E. F., and S. Ward. 2002. Impact of elevated ammonium on reproduction in two Hawaiian seleractinian corals with different life history patterns. Mar. Pollut. Bull. 44: 1230 1235.

Craik, J. C. A., and S. M. Harvey. 1987. The causes of buoyancy in eggs of marine releosts. J. Mat: Biol. Assoc. UK 67: 169-182.

Crean, A. J., and I). J. Marshall. 2008. Gamete plasticity in a broadcast spawning marine invertebrate. Proc. Natl Acad. Set. USA 105: 13508-13513.

Crisp. D. J. 1984. Energy Bow measurements. Pp, 284-372 in Methods for the Study of Marine Benthos, N. A. Holme and A. D. Melntyre. eds. Blackwell. London.

Grossman. D. J., K. D. Clements, and G. J. S. Cooper. 2000. Determination of protein for studies of marine herbivory; a comparison of methods. J. Exp. Mar. Biol. Ecol. 244: 45-65.

de Jong-Westman. M., P.-Y. Qian, B. E. March, and T. H. Carefoot 1995. Artificial diets in sea urchin culture; effects of dietary protein level and other additives on egg quality, larval morphometries, and larval survival in the green sea urchin. Strongylocentrotus droebachiensis-Can. J. Zool. 73: 2080-2090.

Derrickson. E. M.. and R. E. Ricklefs. 1988. Taxon-dependent diversification of life-history traits and the perception of phylogenetic constraints. Funet. Ecol. 2: 417-423.

Diaz, E., U., Cotano, and F. Villate. 2003. Reproductive response of Euterpina acutigrons in two estuaries of the Basque Country (Bay of Biscay) with contrasting nutritional environment. J. Exp. May. Biol, Ecol. 292: 213-230.

Diz., A. P., E. Dudley, B. W. MacDonald. B. Pina. E. L. R. Kenchington. E. Zouros. and D. O. F. Skibinski. 2009. Genetic variation underlying protein expression in eggs of the marine mussel Mytilus edulis. Mol. Cell. Proteomics 8: 132-144.

Dubois. M., K. A. Giles, J. K. Hamilton, P. A. Rubers, and F. Smith. 1956. Colorimetric method of determination of sugars and related substances. Anal. Chem. 28: 350-356.

Dugan, .J. E., A. M. Wenner. and D. M. Hubhard. 1991. Geographic-variation in the reproductive-biology of the sand crab Emerita analoga (Stimpson) on the California coast. J. Exp. Mar. Biol. Ecol. 150: 63-81.

Emlet, R. B. 1995. Developmental mode and species geographic range in regular sea urchins ('Echinodermata: Ecbinoidea). Evolution 49: 476-489.

Emlet, R. B., L. R. McEdward. and R. R. Strathmann. 1987. Echinoderrn larval ecology viewed from the egg- Pp. 55-136 in Echinoderm Studies, Vol. 2. M. Jangoux and .J. M Lawrence, eds. Balkema. Rotterdam.

Falconer, D. S., and T. F. C. Mackay. 1996. Introduction to Quantitative Genetics. 4th ed. Longman Sci. and Tech.. Harlow. UK.

Falkner, I., M. Byrne, and M. A. Sewell. 2006. Maternal provisioning in Ophionereis fascials and (I). schayeri: brittle stars with contrasting modes of development. Biol. Bull. 211: 204-207.

Farley, G. S., and D. R. Levitan. 2001. The role of jelly coals in sperm-egg encounters, fertilization success, and selection on egg size in broadcast spawners. Am. Nat. 157: 626-636.

Folch, J., M. Less, and G. H. Stone Stanley. 1957. A simple method for the isolation and purification of total lipids from animal tissues. .J. Biol Chem. 226: 497-508.

Folin, P., and K. Malmros. 1929. Blood sugar and fermentable blood sugar as determined by different methods./ Biol. Chem. 83: 121-127.

Fukazawa, H., H. Takami, T, Kawamura. and Y. Watanabe. 2005. The effect of egg quality on larval period and postlarval survival of an abalone Haliotis discus hannai. J. Shellfish Res. 24: 1141-1147.

George, S. B. 1990. Population and seasonal differences in egg quality of Arbacia lixula (Echinodermata. Echinoidea). Invertebr. Reprod. Dev. 17: 111-121.

George, S. B. 1994. Population differences in maternal size and offspring quality for Leptasierias epichlora (Brandt) (Echinodermata: Asteroidea). J. Exp. Mar. Biol. Ecol. 175: 121-131.

George, S. B. 1996. Hchinoderm egg and larval quality as a function of adult nutritional state. Oceanol. Acta 19: 297-308.

George, S. B. 1999. Egg quality, larval growth and phenotypic plasticity in a forcipulate seastar. J. Exp. Mar. Biol. Ecol. 237: 203-224.

George, S. B., C. Cellario, and L. Fenaux. 1990. Population differences in egg quality of Arbacia lixula (Echinodermata: Eehinoidea): proximate composition of eggs and larval development. J. Exp. Mar. Biol. Ecol. 141: 107-118.

George, S. B., J. M. Lawrence, and L. Fenaux. 1991. The effect of food ration on quality of eggs ofm Luidia clathrata (Say, Echinodermata: Asteroidea). Invertebr. Reprod. Dev. 20: 237-242.

Gibson, G. D., and F. S. Chia. 1995. Developmental variability in the poecilogonous opisthobraneh Haminaea calUdegenita: life-history traits and effects of environmental parameters. Mar. Ecol. Prog. Ser. 121: 139-155.

Giese, A. C. 1966. Lipids in the economy of marine invertebrates. Physiol. Rev. 46: 244-298.

Gimenez, L., and K. Anger. 2001. Relationships among salinity, egg size, embryonic development, and larval biomass in the estuarine crab Chasmagnathus gramtlata Dana, 1851 .J Exp. Mar. Biol. Ecol. 260: 241-257.

Gnaiger, E., and G. Bitterlich. 1984. Proximate biochemical composition and caloric content calculated from elemental carbon hydrogen nitrogen analysis: a stoichiometric concept Oecologia (Berl.) 62: 289-298.

Goodsell, J. G., and A. G. Eversole. 1992. Prodissoconch 1 and II length in Mercenaria taxa. Nautilus 106: 119-122.

Gosselin, L, A., and P. Y. Qian. 1999. Analysing energy content: a new micro-assay and an assessment of the applicability of acid diehromate assays. Hydrobiologia 390: 141-151.

Graham, F., T. Mackrill, M. Davidson, and S. Dannie. 2006. Influence of conditioning diet and spawning frequency on variation in egg diameter for greenlip abalone. Haliotis laevigata. J. Shellfish Res. 25: 195-200.

Guisande, C., and R. Harris. 1995. Effect of total organic content of eggs on hatching success and naupliar survival in the copepod Calanus helgolandicus. Limnol. Oceanogr. 40: 476-482.

Hadfield, M. G., and S. E. Miller. 1987. On developmental patterns of opisthobranehs. Am. Malacol. Bull. 5: 197-214.

Hansen, T. A. 1978. Larval dispersal and species longevity in Lower Tertiary gastropods. Science 199: 885-887.

Hansen, T. A. 1980. Influence of larval dispersal and geographic distribution on species longevity in neogastropods. Paleobiology 6: 193-207.

Havenhand, J. N. 1993. Egg to juvenile period, generation time, and the evolution of larval type in marine invertebrates. Mar. Ecol. Prog. Ser. 97: 247-260.

Hilbish, T. J., E. P. Winn, and P. 1). Rawson. 1993. Genetic variation and covariation during larval and juvenile growth in Mercenaria mer-cenaria. Mar. Biol. (Berl.) 115: 97-104.

Hoegh-Guldberg, O., and R. B. Emlet. 1997. Energy use during the development of a lecithotrophic and a planktotrophic echinoid. Biol. Bull. 192: 27-40.

Hoegh-Guldberg, O., and J. S. Pearse. 1995. Temperature, food availability, and the development of marine invertebrate larvae. Am. Tool. 35: 415-425.

Holland, D. L. 1978. Lipid reserves and energy metabolism in the larvae of benthic marine invertebrates. Pp. 85-123 in Biochemical and Bio-physical Perspectives in Marine Biology, D. C. Sargent and J. R. Malins, eds. Academic Press, London.

Holland, D. L., and P. A. Gabbott. 1971. A micro-analytical scheme for the determination of protein, carbohydrate, lipid and RNA levels in marine invertebrate larvae. J. Mar. Biol. Assoc. UK 51: 659-668.

Honeycutt, M. E., V. A. McFarland, and D. D. McCant. 1995. Comparison of 3 lipid extraction methods lor fish. Bull. Environ. Contain. Toxicol. 3: 469-472.

Inouye, L. S., and G. R. Lotufo. 2006. Comparison of macro-gravimetric and micro-colorimetric lipid determination methods. Talanta 70: 584-587.

Ito, K. 1997. Egg-size and -number variations related to maternal size and age, and the relationship between egg size and larval characteristics in an annual marine gastropod, Haloa japonica (Opisthobranchia; Cephalaspidea). Mar. Eco. Prog. Ser. 152: 187-195.

Iverson, S. J., S. L. C. Lang, and M. H. Cooper. 2001. Comparison of the Bligh and Dyer and Folch methods for total lipid determination in a broad range of marine tissue. Lipids 36: 1283-1287.

Jablonski, D., and R. A. Lutz. 1983. Larval ecology of marine benthic invertebrates: paleobiological implications. Biol. Rev. 58: 21-89.

Jaeckle, VV. B. 1995. Variation in the size, energy content and biochemical composition of invertebrate eggs: correlates to the mode of larval development. Pp. 49-77 in Ecology of Marine Invertebrate Larvae, L. McEdward, ed. CRC Press, Boca Raton, FL.

Jaecklc W. B., and D. T. Manahan. 1989. Growth and energy imbalance during the development of a lecithotrophic molluscan larva (Haliotis rufescens). Biol. Bull. 177: 237-246.

Jeffery, C. H., and R. B. Emlet. 2003. Maeroevolulionary consequences of developmental mode in temnopleurid echinoids from the Tertiary of southern Australia. Evolution 57: 1031-1048.

Jones, H. L., C. D. Todd, and W. J. Lambert. 1996. Intraspecific variation in embryonic and larval traits of the dorid nudibranch mollusc Adaltiriei proximo (Alder and Hancock) around the northern coasts of the British Isles. J. Exp. Mar. Biol. Ecot. 202: 29-47.

Jones, R., J. A. Bates, D. J. Innes, and R. J. Thompson. 1996. Quantitative genetic analysis of growth in larval scallops (Placopecten magclhinicus). Mar. Biol. 124: 671-677.

Kane, M., and P. B. Cavers. 1992. Patterns of seed weight distribution and germination with time in a weedy bioiype of Proso millet Panicum miliocatm. Can. J. Bot. 70: 562-567.

Kjesbu, O. S., J. Klungsoyr, H. Kryvi, P. R. Witthames, and M. G. Walker. 1991. Fecundity atresia and egg size of captive Atlantic cod Gadits morhua in relation to proximate body composition. Can. J. Fish. Aquat. Set 48: 2333-2343.

Knight J. A., S. Anderson, and J. M. Rawle. 1972. Chemical basis of the sulfo-phospho-vanillin reaction for estimating total serum lipids. Clm. Chem. 18: 199-202.

Krug, P. J. 2007. Poecilogony and larval ecology in the gastropod genus Alderia. Am. Malacol. Bull. 23: 99-111.

Lajus, D. L., and V. R. Alekseev. 2004. Phenotypic variation and developmental instability of life-history traits: a theory and a case study on within-population variation of resting eggs formation in Daphnia. J. Limnol. 63 (Suppl. 1): 37-44.

Laptikhovsky, V. 2006. Latitudinal and balhymetric trends in egg size variation: a new look at Thorson's and Rass's rules. Mar. Ecol. 27: 7-14.

Lardies, M. A., M. H. Medina, and J. A. Correa. 2008. Intraspecific biogeographic pattern breakage in the snapping shrimp Betaeus emarginatus caused by coastal copper mine tailings. Mar. Ecol. Prog. Ser. 358: 203-210.

Lee, R. F., A. Walker, and D. J. Reish. 2005. Characterization of lipovitellin in eggs of the polychaete Neanthes arenaceodentata. Comp. Biochem. Physiol. B140: 381-386.

Lessios, H. A. 1990. Adaptation and phylogeny as determinants of egg size in echinoderms from the two sides of the Isthmus of Panama. Am. Nat. 135: 1-13.

Levin, L. A., J. Zhu, and E. Creed. 1991. The genetic basis of life-history characters in a polychaete exhibiting planktotrophy and lecithotrophy. Evolution 45: 380-397.

Levitan, 0. R. 1993. The importance of sperm limitation to the evolution of egg size in marine invertebrates. Am. Nat. 141: 517 536.

Levitan, D- R. 2000. Optimal egg size in marine invertebrates: theory and phylogenetic analysis of the critical relationship between egg size and development time in echinoids. Am. Nat. 156: 175-192.

Levitan, D. R. 2006. Relationship between egg size and fertilization success in broadcast spawning marine invertebrates. Integr. Comp. Biol. 46: 298-311.

Lovern, J. A. 1964. The lipids of marine organisms. Oceanogr. Mar. Biol. Annu. Rev. 2: 168-191.

Lowry, O. H., N. J. Rosbrough, A. L. Farr, and R. J. Randall. 1951. The determination of protein in biologic samples. J. Biol. Chem. 193: 265-275.

Luttikhuizen, P. C., P. J. C. Honkoop, J. Drent, and J. van der Meer. 2004. A general solution for optimal egg size during external fertilization, extended scope for intermediate optimal egg size and the introduction of Don Ottavio 'tango'. J. Theor. Biol 231; 333-343.

Macginitie, G. E. 1934. The egg-laying activities of the sea hare. Tethys californicus (Cooper). Biol, Bull. 67: 300-303.

Marko, P. B., and A. L. Moran. 2002. Correlated evolutionary divergence of a mitochondrial protein and egg size across the Isthmus of Panama. Evolution 56: 1303-1309.

Marsh, J. B., and D. B. Weinstein. 1966. Simple charring method for determination of lipids. J. Lipid Res. 7: 574-576.

Marshall, D. J., C. A. Styan, and M. J. Keough. 2000. Intraspecific co-variation between egg and body size affects fertilisation kinetics of free-spawning marine invertebrates. Mar. Ecol. Prog. Set: 195: 305-309.

Marshall, D. J., T. F. Bolton, and M. J. Keough. 2003. Offspring size affects the post-metamorphic performance of a colonial marine invertebrate. Ecology 84: 3131-3137.

Marshall, D. J., R. M. Allen, and A. J. Crean. 2008. The ecological and evolutionary importance of maternal effects in the sea. Oceanogr. Mar. Biol. Anna. Rev. 46: 203-250.

Marshall. N. B. 1953. Egg size in Arctic, Antarctic, and deepsea fishes. Evolution 7: 328-341.

Mayer, L. M., L. L. Schick, and F. W. Setchell. 1986. Measurement of protein in nearshorc marine sediments. Mar. Ecol. Prog. Ser. 30: 159-165.

McClintock, J, B., and J. S. Pearse. 1986. Organic and energetic content of eggs and juveniles of Antaretic echinoids and asteroids with lecithotrophie development. Comp. Biochem. Physiol. A 85: 341-345.

McEdward, L, R. 1986. Comparative morphometrics of echinoderm larvae. I. Some relationships between egg size and initial larval form in echinoids. J. Exp. Mar. Biol. Ecol. 96: 251-265.

McEdward, L. R., and S. V, Carson. 1987. Variation in egg organic content and its relationship with egg size in the starfish Solaster stimpsoni. Mar. Ecol. Prog. Ser. 37: 159-169.

McEdward, L. R., and F. S. Chia. 1991. Size and energy content of eggs from echinoderms with pelagic lecithotrophic development. J. Exp. Mar. Biol. Ecol. 147: 95-102.

McEdward, L. R., and L. K. Coulter. 1987. Egg volume and energetic content are not correlated among sibling offspring of starfish: implications for life-history theory. Evolution 41: 914-917.

McEdward, L. R., and D. A. Janies. 1997. Relationships among development, ecology, and morphology in the evolution of echinoderm larvae and life cycles. Biol., J. Linn. Soc. 60: 381-400.

McEdward, L. R., and K. H. Morgan. 2001. Interspecific relationships between egg size and level of parental investment per offspring in echinoderms. Biol Hull. 200: 33-50.

McEdward. L. R., S. F. Carson, and F. S. Chia. 1988. Energetic content of eggs, larvae, and juveniles of Florometra serratissima and the implications for the evolution of crinoids life histories. Int. .J. Invertebr. Reprod. Dev. 13: 9-22.

McKitritk, M. C. 1993. Phylogenetie constraint in evolutionary theory; Has it any explanatory power? Anna. Rev. Ecol. Syst. 24: 307-330.

Meidel, K., R. E. Scheibling, and A. Metaxas. 1999. Relative importance of parental and larval nutrition on the larval development and metamorphosis of the sea urchin Strongylocenlrotus droebachiensis. J. Exp. Mar. Biol Ecol. 240: 161-178.

Miles, C. M., M. G. Hadfield, and M. L. Wayne. 2007. Heritability for egg size in the serpulid polychaete Hydroides elegans. Mar. Ecol. Prog. Ser. 340: 155-162.

Mileykovsky, S. A. 1971. Types of larval development in marine bottom invertebrates, their distribution and ecological significence: a re-evaluation. Mar. Biol. 10: 193-213.

Miner, B. G., J. D. Cowart, and L. R. McEdward. 2002. Egg energetics for the facultative planktotroph Clypeaster rosaceus (Echinoder-mata: Echinoidea). revisited. Biol. Bull. 202: 97-99.

Miner, B. G., L. A. McEdward, and L. R. McEdward. 2005. The relationship between egg size and the duration of the facultative feeding period in marine invertebrate larvae. .J. Exp. Mar. Biol. Ecol. 321: 135-144.

Morais, $., L. Narciso, R. Calado, M. L. Nunes, and R. Rosa. 2002. Lipid dynamics during the embryonic development of Plesionika martia martia (Decapoda; Pandalidae). Palaemon serratu and P. elegans (Decapods; Palaemonidae); relation to metabolic consumption. Mar. Ecol. Prog. Ser. 242: 195-204.

Moran, A. L. 1997. Size, form, and function in the early life histories of the gastropod genera Nucella and Littorina. PhD dissertation. University of Oregon, Eugene.

Moran, A. L. 2004. Egg size evolution in Tropical American arcid bivalves: the comparative method and the fossil record. Evolution 58: 2718-2733.

Moran, A. L., and D. T. Manahan. 2003. Energy metabolism during larval development of green and white abalone. Haliotis fulgens and H. sorenseni. Biol. Bull. 204: 270-277.

Moran, A. L., and D. T. Manahan. 2004. Physiological recovery from prolonged 'starvation' in larvae of the Pacific oyster Crassostrea gigas. J. Exp. Mar. Biol. Ecol. 306: 17-36.

Moreno, G., and O. Hocgh-Guldbcrg. 1999. The energetics of development of three congeneric seastars (Patiriella Verrill. 1913) with different types of development. J. Exp. Mar. Biol. Ecol. 235: 1-20.

Moreno, G., P. Selvaknmaraswamy, M. Byrne, and O. Hoegh-Guldberg. 2001. A test of the ash-free dry weight technique on the developmental stages of Patiriella spp. (Echinodermata: Asteroidea). Limnol. Oceanoger-. 46: 1214-1220.

Nissling, A., H. Kryvi, and L. Vallin. 1994. Variation in egg buoyancy of Baltic cod Gadus morhua and its implications for egg survival in prevailing conditions in the Baltic Sea. Mar. Ecol. Prog. Ser. 110: 67-74.

Paine, R. T. 1964. Ash and calorie determinations of sponge and opisthobranch tissues. Ecology 45: 384-387.

Paine, R. T. 1971. The measurement and application of the calorie to ecological problems. Annu. Rev. Ecol. Syst. 2: 145-164.

Parrish, C. C. 1987. Separation of aquatic lipid classes by chromarod thin-layer chromatography with measurement by latroscan flame ionisation detection. Can. J. Fish. Aquat. Sci. 44: 722-731.

Parrish. C. C. 1999. Determination of total lipid, lipid classes, and fatty acids in aquatic samples. Pp. 4-20 in Lipids in Freshwater Ecosystems, M. T. Arts and B. C. Wainman, eds. Springer, New York.

Parrish, C. C., and R. G. Ackman. 1983. Chromarod separations for the analysis of marine lipid classes by latroscan thin-layer chromatography flame ionization detection. J. Chromatogr. 262: 103-112.

Parsons, T. R., Y. Maita, and C. M. Lalli. 1984. A Manual of Chemical and Biological Methods for Seawater Analysis. Pergamon Press, New York.

Peck, L. S., D. K. Powell, and P. A. Tyler. 2007. Very slow development in two Antarctic bivalve molluscs, the infaunal clam Laternula elliptica and the scallop Adamussitum colbecki. Mar. Biol. 150: 1191-1197.

Pernet, B., and W. B. Jaeckle. 2004. Size and organic content of eggs of marine annelids, and the underestimation of egg energy content by dichromate oxidation. Biol. Bull. 207: 67-71.

Phillipson, J. 1964. A miniature bomb calorimeter for small samples. Oikos 15: 130-139.

Podolsky, R. D. 2001. Evolution of egg target size: an analysis of selection on correlated characters. Evolution 55: 2470-2478.

Podolsky, R. D. 2002. Fertilization ecology of egg coats: physical versus chemical contributions to fertilization success of free-spawned eggs. J. Exp. Biol. 205: 1657-1668.

Podolsky, R. D. 2004. Life history consequences of investment in free-spawned eggs and their accessory coats. Am. Nat. 163: 735-753.

Podolsky. R. D., and R. R. Strathmann. 1996. Evolution of egg size in free-spawners: consequences of the fertilization-fecundity trade-off. Am. Nat. 148: 160-173.

Pond, D., R. Harris, R. Head, and D. Harbour. 1996. Environmental and nutritional factors determining seasonal variability in the fecundity and egg viability of Calanus helgolandicus in coastal waters off Plymouth. UK. Mar. Ecol. Prog. Ser. 143: 45-63.

Prowse, T. A. A., M. A. Sewell, and M. Byrne. 2008. Fuels for development: evolution of maternal provisioning in asterinid sea stars. Mar. Biol. 153: 337-349.

Qian, P. Y., and F. S. Chia. 1992. Effect of aging on reproduction in a marine polychaete, Capitella sp. J. Exp. Mar. Biol. Ecol. 156: 23-38.

Raff, R. A. 1996. The Shape of Life: Genes, Development and the Evolution of Animal Form. University of Chicago Press, Chicago.

Rass, T. S. 1941. Geographic Parallelisms in Morphology and Development of Teleost Fish of Northern Seas. MOIP (Moscow Society of Naturalists). Moscow (In Russian).

Reitzel, A. M., C. M. Miles, A. Heyland, J. D. Cowart, and L. R. McEdward. 2005. The contribution of the facultative feeding period to echinoid larval development and size at metamorphosis: a comparative approach. J. Exp. Mar. Biol. Ecol. 317: 189-201.

Ritar, A. J., G. A. Dunstan, B. J. Creara, and M. R. Brown. 2003. Biochemical composition during growth and starvation of early larval stages of cultured spiny lobster (Janus edwardsii) phyllosoma. Comp. Biochem. Physiol. A 136: 353-370.

Ruohomaki, K., S. Hanhimaki, and E. Haukioja. 1993. Effects of egg size, laying order and larval density on performance of Epirrita autumnata (Lep, Geometridae). Oikos 68: 61-66.

Schiopu, D., S. B. George, and J. Castell. 2006. Ingestion rates and dietary lipids affect growth and fatty acid composition of Dendraster excentricus larvae. J. Exp. Mar. Biol. Ecol. 328: 47-75.

Scott. L, B., P. S. Leahy. G. L. Decker, and W. J. Lennarz. 1990. Loss of yolk platelets and yolk glycoproteins during larval development of the sea urchin embryo. Dev. Biol. 137: 368-377.

Sewell, M. A. 2005. Utilization of lipids during early development of the sea urchin Evechinus chloroticus. Mar. Ecol. Prog. Ser. 304: 133-142.

Sewell, M. A., and D. T. Manahan. 2001. Echinoderm eggs; biochemistry and larval biology. Pp. 55-58 in Echinoderms 2000: Proceedings of the 10th International Conference, Dunedin. CRC Press. Boca Raton. FL.

Shyu, A. B., R. A. Raff, and T. Blumenthal. 1986. Expression of the vitellogenin gene in female and male sea urchin Strongylocentrotus purpuratus. Proc. Natl. Acad. Sci. USA 83: 3865-3869.

Simonini, R., and D. Prevedelli. 2003. Effects of temperature on two Mediterranean populations of Dinophilus gyrociliatus (Polychaeta: Dinophilidae). I. Effects on life history and sex ratio. J. Exp. Mar. Biol. Ecol. 291: 79-93.

Sinervo, B., and L. R. McEdward. 1988. Developmental consequences of an evolutionary change in egg size: an experimental test. Evolution 42: 885-899.

Skadsheim, A. 1989. Regional variation in amphipod life history: effects of temperature and salinity on breeding. J. Exp. Mar. Biol. Ecol. 127: 25-42.

Smedes, F. 1999. Determination of total lipid using non-chlorinated solvents. Analyst 124: 1711-1718.

Smedes, F., and T. K. Askland. 1999, Revisiting the development of the Bligh and Dyer total lipid determination method. Mar. Pollut. Bull. 38: 193-201.

Smedes, F., and T. K. Thomasen. 1996. Evaluation of the Bligh and Dyer lipid determination method, Mar. Pollut. Bull. 32: 681-688.

Steer, M. A., N. A. Moltschaniwskyj, D. S. Nichols, and M. Miller. 2004. The role of temperature and maternal ration in embryo survival: using the dumpling squid Euprymna tasmanica as a model. J. Exp. Mar. Biol. Ecol. 307: 73-89.

Steigenga, M. J., and K. Fischer. 2007. Ovarian dynamics, egg size, and egg number in relation to temperature and mating status in a butterfly. Entomol. Exp. Appl. 125: 195-203.

Stoscheck, C. M. 1990. Quantitation of protein. Methods Enzymol 182: 50-69.

Strathmann, R. R. 1985. Feeding and nonfeeding larval development and life-history evolution in marine invertebrates. Annu. Rev. Ecol Syst. 16: 339-361.

Strathmann, R. R. 2000. Form, function, and embryonic migration in large gelatinous egg masses of arenicolid worms. Invertebr. Biol. 119: 319-328.

Thiyagarajan, V., and P.-Y. Qian. 2003. Effect of temperature, salinity and delayed attachment on development of the solitary ascidian Styela plicata (Lesueur). J. Exp. Mar. Biol. Ecol. 290: 133-146.

Thompson, T. E. 1958. The influence of temperature on spawning in Adalaria proxima (A. & H.) (Gastropoda Nudibranchia). Oikos 9: 246-252.

Thorson, G. 1950. Reproduction and larval ecology of marine bottom invertebrates. Biol. Rev. Camb. Philos. Soc. 25: 1-45.

Turner, R. L., and J. VI. Lawrence. 1979. Volume and composition of echinoderm eggs: implications for the use of egg size in life history models. Pp. 25-40 in Reproductive Ecology of Marine Invertebrates, S. E. Stancyk, ed. University of South Carolina Press, Columbia. SC.

van der Have, T. M., and G. de Jong. 1996. Adult size in ectotherms: temperature effects on growth and differentiation. J. Theor. Biol. 183: 329-340.

Vance, R. R. 1973a. On reproductive strategies in marine benthic invertebrates. Am. Nat. 107: 339-352.

Vance, R. R. 1973b. More on reproductive strategies in marine benthic invertebrates. Am. Nat. 107: 353-361.

Villinski, J. T., J. C. Villinski, M. Byrne, and R. A. Raff. 2002. Convergent maternal provisioning and life-history evolution in echinoderms. Evolution 56: 1764-1775.

Vlasbloom, A. G., and G. Bolier. 1971. Tolerance of embryos of Marinogammarus marinus and Orchestia gammarella (Amphipoda) to lowered salinities, Neth. J. Sea Res. 5: 334-341.

Vogt, G., M. Huber, M. Thiemann, G. van den Boogaart, O. J. Schmitz, and C. D. Shubart. 2008. Production of different phenotypes from the same genotype in the same environment by developmental variation. J. Exp. Biol. 211: 510-523.

Weber, L. P., P. S. Higgins, R. I. Carlson, and D. M. Janz. 2003. Development and validation of methods for measuring multiple biochemical indices of condition in juvenile fishes. J. Fish. Biol. 63: 637-658.

Wiklund, C., and A. Persson. 1983. Fecundity, and the relation of egg weight variation to offspring fitness in the speckled wood butterfly Pararge aegeria, or why don't butterfly females lay more eggs? Qikos 40: 53-63.

Wray, G. A., and K. A. Raff. 1991. The evolution of developmental strategy in marine invertebrates. Trends Ecol. Evol. 6: 45-50.

Zamer, W. E., J. M Shick, and D. W. Tapley. 1989. Protein measurement and energetic considerations--comparisons of biochemical and stoichiometric methods using bovine serum albumen and protein isolated from sea anemones. Limnol. Oceanogr. 34: 256-263.

Zigler, K. S., H. A. Lessios, and R. A. Raff. 2008. Egg energetics, fertilization kinetics, and population structure in echinoids with facultatively feeding larvae. Biol. Bull. 215: 191-199.

AMY L. MORAN * AND JUSTIN S. McALISTER

Department of Biological Sciences, 132 Long Hall, Clemson University, Clemson. South Carolina 29634

Received 13 November 2008; accepted 14 April 2009.

* To whom correspondence should he addressed. [E-mail: moran@clemson.edu
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Author:Moran, Amy L.; McAlister, Justin S.
Publication:The Biological Bulletin
Article Type:Report
Geographic Code:1USA
Date:Jun 1, 2009
Words:15194
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