Patches of crab megalopae in the mouth of Delaware Bay--an analysis of spatial scales.ABSTRACT Recent studies have shown that zoea zo·e·a
n. pl. zo·e·ae or zo·e·as
A larval form of crabs and other decapod crustaceans, characterized by one or more spines on the carapace and rudimentary limbs on the abdomen and thorax. larvae Larvae, in Roman religion
Larvae: see lemures. of brachyuran brach·y·u·ran also brach·y·u·ral or brach·y·u·rous
Of or belonging to the Brachyura, a group of crustaceans including the true crabs, characterized by a short abdomen concealed under the cephalothorax.
n. crabs often occur in patches that are formed at spawning and persist on time scales ranging from days to at least a week. In the present work we investigated the spatial distribution of the megalopal stage of 2 taxa taxa: see taxon. of brachyurans (Callinectes sapidus and Uca spp.) in Delaware Bay Delaware Bay: see Delaware, river.
Inlet of the Atlantic Ocean. Forming part of the New Jersey-Delaware state border, it extends southeast for 52 mi (84 km) from the junction of the Delaware River with Alloway Creek to its entrance on the east coast of North America North America, third largest continent (1990 est. pop. 365,000,000), c.9,400,000 sq mi (24,346,000 sq km), the northern of the two continents of the Western Hemisphere. (ca. 38.6[degrees]N, 75.2[degrees]W). We used a combination of high-frequency plankton plankton: see marine biology.
Marine and freshwater organisms that, because they are unable to move or are too small or too weak to swim against water currents, exist in a drifting, floating state. sampling and data from a moored current meter an instrument for measuring the velocity, force, etc., of currents.
See also: Current to characterize the length scale of patches of megalopae at a single station in the mouth of the bay. The study consisted of 12 separate sampling periods during the late summers of 2001 and 2002. During each period, plankton were collected every 10 rain over a single flood-tidal phase. Current-velocity data were collected simultaneously via an S-4 current meter moored at the station. Subsequent analysis demonstrated the transport of patches of megalopae past the station during five of the sampling periods. The radius of the patches was determined through autocorrelation Autocorrelation
The correlation of a variable with itself over successive time intervals. Sometimes called serial correlation. analysis. The radii ra·di·i
A plural of radius.
a plural of radius of patches of C. sapidus megalopae ranged from approximately 500-2,000 m, whereas the radii for Uca patches was typically smaller with values around 500 m. This patchy distribution has important consequences for transport and settlement of the megalopae in juvenile habitat.
KEY WORDS: Callinectes sapidus, Uca, patches, megalopa, zoea, crab
Larvae of some species of estuarine es·tu·a·rine
1. Of, relating to, or found in an estuary.
2. Geology Formed or deposited in an estuary.
Adj. 1. estuarine - of or relating to or found in estuaries
estuarial crabs are retained within the estuary until settlement, whereas others are flushed to the continental shelf where they may develop through several zoeal stages before returning to the estuary as megalopae (Epifanio 1988). For example, a number of species of mud crabs (family: Xanthidae) produce zoea larvae that are retained in the estuary. These larvae exhibit a pattern of vertical migration that allows them to take advantage of vertical shear in tidal currents, ultimately resulting in retention (Cronin & Forward 1979, Cronin 1982, Dittel & Epifanio 1982, Forward & Tankersley 2001). Conversely, crabs in the families Ocypodidae (e.g., fiddler crabs Uca spp.) and Portunidae (e.g., blue crabs Callinectes sapidus) often have zoea larvae that are passively exported from estuarine systems.
Once in the waters of the continental shell early zoeal stages of Uca are typically distributed near the surface, whereas later stages assume deeper positions in the water column. This allows the larvae to take advantage of gravitational grav·i·ta·tion
a. The natural phenomenon of attraction between physical objects with mass or energy.
b. The act or process of moving under the influence of this attraction.
2. circulation for eventual transport back toward the estuary (Epifanio et al. 1988, Dittel et al. 1991). In contrast, blue crab zoeae remain in surface waters of the continental shelf where they are subject to buoyancy-driven and wind-driven dispersal (for review see Epifanio & Garvine 2001). Eventual delivery of blue crab megalopae back to the estuary is provided by across-shelL downwelling Downwelling is the process of accumulation and sinking of higher density material beneath lower density material, such as cold or saline water beneath warmer or fresher water or cold air beneath warm air. It is the sinking limb of a convection cell. circulation, which along the east coast of North America is coincident with southward south·ward
adv. & adj.
Toward, to, or in the south.
A southward direction, point, or region.
south wind events (Jones & Epifanio 1995, Garvine et al. 1997). On entering the estuary, the megalopae of both taxa undergo a well-documented change in behavior (Epifanio et al. 1984, DeVries et al. 1994). This consists of upward migration during nocturnal flood tides, which allows megalopae to take advantage of circulation at tidal frequency to facilitate further transport up the estuary (Tankersley & Forward 1994, Tankersley et al. 1995, Forward et al. 1997, Welch & Forward 2001).
Regardless of differences in their respective transport agents, the settlement of megalopae of blue crabs and fiddler crabs occurs as discrete temporal pulses. This has been interpreted as circumstantial evidence circumstantial evidence
In law, evidence that is drawn not from direct observation of a fact at issue but from events or circumstances that surround it. If a witness arrives at a crime scene seconds after hearing a gunshot to find someone standing over a corpse and holding a that megalopae are distributed as well defined aggregations in space (Jones & Epifanio 1995, Epifanio 1995, Natunewicz 2000). Indeed, recent work in Delaware Bay and on the adjacent continental shelf has documented the aggregated distribution of zoeal stages of blue crabs and fiddler crabs in patches ranging from fat ellipses Ellipses is the plural form of either of two words in the English language:
In this study, we describe a 2-y investigation of the distribution of C. sapidus and Uca spp. megalopae at a single station in the mouth of Delaware Bay. The study is unique in its application of continuous, high frequency plankton sampling conducted coincident with high-frequency determinations of current velocity. The overall sampling scheme had an Eulerian character that allowed determination of the horizontal distribution of megalopae as they were transported past the station.
MATERIALS AND METHODS
This study was conducted in the southern Middle Atlantic Adj. 1. middle Atlantic - of a region of the United States generally including Delaware; Maryland; Virginia; and usually New York; Pennsylvania; New Jersey; "mid-Atlantic states"
mid-Atlantic Bight bight, broad bend or curve in a coastline, forming a large open bay. The New York bight, for example, is the curve in the coast described by the southern shore of Long Island and the eastern shore of New Jersey. The term bight may also refer to the bay so formed. at the mouth of Delaware Bay just north of Cape Henlopen Cape Henlopen is the southern cape of the Delaware Bay along the Atlantic coast of the United States. It lies in the state of Delaware, near the town of Lewes, Delaware. Off the coast on the bay side are two lighthouses, called the Harbor of Refuge Light and the Delaware (Fig. 1). Tidal currents at the bay mouth may reach velocities >1 m [s.sup.-1], and tidal amplitude reaches nearly 2 m during spring-tide periods (Garvine 1991). At subtidal frequencies, surface circulation in the adjacent coastal ocean is dominated by buoyancy-driven flow associated with the Delaware coastal current and wind-driven flow characterized by oscillating os·cil·late
intr.v. os·cil·lat·ed, os·cil·lat·ing, os·cil·lates
1. To swing back and forth with a steady, uninterrupted rhythm.
2. periods of upwelling up·well·ing
1. The act or an instance of rising up from or as if from a lower source: an upwelling of emotion.
2. and downwelling circulation (Garvine et al. 1997, Epifanio & Garvine 2001).
[FIGURE 1 OMITTED]
Megalopae were collected over 4 cruises that took place during mid to late summer in 2001 and 2002. These periods were chosen to coincide with expected peak density of blue crab and fiddler crab megalopae at the study site (Jones & Epifanio 1995). Each cruise consisted of 3 consecutive nights of sampling during a spring tide period (Table 1). Thus, the total sampling effort consisted of 12 nights of sampling distributed over the 2 y of the investigation. Patches of megalopae were encountered on 5 of those nights.
The duration of each night of sampling varied from 5-6 h depending on weather conditions, and the total number of samples per night varied from 30-37 (see later). Sampling consisted of surface plankton tows from an 8-m vessel. Surface tows provided an adequate picture of megalopal distribution because it has been demonstrated in previous studies that blue crab and fiddler crab megalopae migrate to surface waters during periods of nocturnal flooding tides (Epifanio et al. 1984, Dittel et al. 1991, Tankersley & Forward 1994). Tows were taken every 10 rain with a plankton net (0.5 m diameter; 253-[micro]m mesh) during the flood tide phase. Tow length was always 300 m; distance towed was determined using differential GPS See GPS augmentation system. . Because results of previous work indicated that megalopae should be most common in the water column during nocturnal flood tides (Tankersley & Forward 1994, Tankersley et al. 1995, Forward et al. 1997, Welch & Forward 2001), the ideal starting time Noun 1. starting time - the time at which something is supposed to begin; "they got an early start"; "she knew from the get-go that he was the man for her"
commencement, get-go, offset, outset, showtime, start, kickoff, beginning, first for each series of plankton tows would have coincided with sunset and low slack water slack water
1. A period of cessation in the strong flow of a current of water, especially at high or low tide.
2. An area in a sea or river unaffected by currents; still water.
Noun 1. . However, these events rarely occur at exactly the same time, and some compromise was necessary in determining actual starting times. For example, low slack water occurred more than >1 h before sunset on 2 dates when patches of megalopae were observed in the water column. On the first of these dates (September 16, 2001), sampling was begun approximately 1.5 h before sunset and continued until ~0.5 h before slack high water (Table 2). On the second of these dates (September 18, 2001), the initiation of sampling was delayed by inclement in·clem·ent
1. Stormy: inclement weather.
2. Showing no clemency; unmerciful.
in·clem weather until ~1.5 h after sunset and ~3 h past low slack water. In this case we decided to continue sampling beyond high slack water to achieve the necessary data points for autocorrelation analysis (see below); this represented the only ebb-tide sampling conducted during the investigation. Sampling never extended beyond sunrise in either of the 2 y of the investigation.
The overall sampling scheme had an Eulerian character in that tows were always taken from the same location, in the same compass direction The horizontal direction expressed as an angular distance measured clockwise from compass north. (toward 138[degrees] true North), and over the same distance during the flooding phase of the tide. Essentially, we were sampling at a fixed location, whereas water and megalopae were transported via tidal currents past the sampling site. Samples were preserved in 4% formaldehyde for later analysis. Water temperature and salinity were measured at hourly intervals. The volume of water filtered for each tow was calculated from flowmeters (General Oceanics Model 2030) fixed in the center of the nets. Samples were split with a Folsom plankton splitter (Dittel & Epifanio 1982). The number of splits was determined by the size of the sample. Subsamples were chosen randomly for further analysis. Megalopae in each subsample sub·sam·ple
A sample drawn from a larger sample.
tr.v. sub·sam·pled, sub·sam·pling, sub·sam·ples
To take a subsample from (a larger sample). were enumerated This term is often used in law as equivalent to mentioned specifically, designated, or expressly named or granted; as in speaking of enumerated governmental powers, items of property, or articles in a tariff schedule. and identified through standard microscopy, and densities (number of megalopae [m.sup.-3]) were calculated using relevant flowmeter See flow meter. readings.
Megalopae of C. sapidus and Uca spp. represented the most abundant taxa collected throughout the investigation. We did not distinguish between the 2 likely species of Uca (U. pugnax and P. minax) because examination of morphologic characteristics does not allow identification of fiddler crab megalopae beyond the level of genus (Sandifer 1972).
Physical Data and Determination of Spatial Series
Data from each night of sampling yielded a time series of megalopal density at 10-min intervals over a single flood tide phase. However, the intent of our investigation was to determine the spatial (as opposed to temporal) scale of patches of megalopae at the study site. This required the conversion of each time series to its equivalent spatial series. We did this by using data from an S-4 current meter that was moored ~2 m below the surface at the station. Because our sampling concept was Eulerian, we were able to use current meter data to represent water motion past the station. Because tidal currents are rectilinear rec·ti·lin·e·ar
Moving in, consisting of, bounded by, or characterized by a straight line or lines: following a rectilinear path; rectilinear patterns in wallpaper. at the study site, the direction toward which the current flowed was always 318[degrees] (true North) during the flood tide phase. Thus, we could treat the current as a scalar scalar, quantity or number possessing only sign and magnitude, e.g., the real numbers (see number), in contrast to vectors and tensors; scalars obey the rules of elementary algebra. Many physical quantities have scalar values, e.g. quantity and calculate the displacement of water during each sampling interval as:
D = S x T
where D is displacement (m), S is average current speed for a given time interval (m [s.sup.-1]), and T is duration of each interval (600 s). The total displacement of water on any given night was calculated as the sum of the 10-min displacement values determined over the entire 5-6-h sampling period. This value was then used to derive a spatial series of megalopal density that was based on the original time series (Fig. 2). Coordinate values in this spatial series consisted of megalopal density along the Y-axis and displacement of water along the X-axis. Because of variation in current speed (S) at different times in the flood phase, the displacement values (D) for the respective sampling intervals were not equal. Thus, the coordinate X-Y data points were not evenly distributed along the X-axis. This required further manipulation of the spatial series before statistical analysis could be undertaken (see later).
[FIGURE 2 OMITTED]
Statistical Analysis of Patch Dimension
Analysis of spatial distribution included determination of some threshold density of megalopae that signified the existence a "patch" (Natunewicz & Epifanio 2001). This process was qualitative and involved initial inspection of the data sets to get a sense of the "signal to noise" (i.e., "patch to non-patch") distribution of the larvae. In this study, we chose a threshold value of 7 megalopae [m.sup.-3], which was approximately 5 times the median density of C. sapidus megalopae over the entire duration of the study and l0 times the median density of Uca megalopae. This threshold is about two orders of magnitude lower than the threshold value used by Natunewicz and Epifanio (2001) for zoea larvae of C. sapidus and three to four orders of magnitude lower than maximum densities observed in previous investigations of patches of zoea larvae (Natunewicz et al. 2001, Petrone 2003). Thus, our threshold value reflects the expected decrease in density of megalopae resulting from mortality of zoea larvae. Moreover, this threshold is coherent with reported values for relative densities of zoeae and megalopae in other investigations (Dittel & Epifanio 1982, Epifanio et al. 1988, Olmi 1995). Time series of megalopal densities that did not include at least one value greater than the threshold were considered "patch-less" and were excluded from further analysis.
The next step in the analysis was entirely quantitative and used autocorrelation techniques to determine the characteristic length scale of the patches. In the context of patches of zooplankton zooplankton: see marine biology.
Small floating or weakly swimming animals that drift with water currents and, with phytoplankton, make up the planktonic food supply on which almost all oceanic organisms ultimately depend (see , this value is best visualized as 0.5 x L, where L is one of the linear dimensions of a patch (Natunewicz 2000). Thus, the characteristic length scale of a circular patch would be equivalent to the radius (see below), whereas the length scale of an ellipse ellipse, closed plane curve consisting of all points for which the sum of the distances between a point on the curve and two fixed points (foci) is the same. It is the conic section formed by a plane cutting all the elements of the cone in the same nappe. could be equivalent to half the dimension of either of the axes. In previous studies of zoea larvae (Natunewicz & Epifanio 2001, Petrone 2003), the shape of patches observed in open water ranged from quasi-circles to fat ellipses (i.e., ratio of major-to-minor axis ranged from 1.1-1.8). As a matter of convenience, we will consider the length scales determined in the present study as the radii of the respective patches.
Autocorrelation demands X-Y coordinates that are distributed at equal intervals along the X-axis, and this was not the case in any of the spatial series of megalopal density generated in our study (see above). Thus, each of the spatial series was subjected to additional processing to meet the requirements of the statistical analysis. This consisted of dividing the entire domain of the X-axis (i.e., the entire displacement of water for a given sampling night) into a number of equal intervals determined by the number of samples collected on a given night. Values for the megalopal density corresponding to each of these intervals were interpolated interpolated /in·ter·po·lat·ed/ (in-ter´po-la?ted) inserted between other elements or parts. from the original data using cubic spline In computer graphics, a smooth curve that runs through a series of given points. The term is often used to refer to any curve, because long before computers, a spline was a flat, pliable strip of wood or metal that was bent into a desired shape for drawing curves on paper. See Bezier and B-spline. techniques (Press et al. 1992). This resulted in a set of evenly spaced X-Y values that were used in subsequent autocorrelation analysis. Depending on the number of intervals constituting a series, autocorrelation coefficients were calculated for 10 or 15 lags per series.
The length scale of a given patch was defined as the point in space at which the distribution of megalopal densities in space first becomes de-correlated (Rowe & Epifanio 1994). For purposes of the present analysis, de-correlation was defined as the point at which the correlation coefficient Correlation Coefficient
A measure that determines the degree to which two variable's movements are associated.
The correlation coefficient is calculated as: first assumed a value [greater than or equal to] 0 (Richerson et al. 1978). The length scale was determined by summing the number of lags required to achieve de-correlation and multiplying by the distance represented by each lag. Because autocorrelation analysis can be conducted by lagging the respective data sets in either direction, the point of de-correlation represents only half the overall dimension of the patch. In the case of a circular patch, this is represented by the radius.
Current speeds over the duration of the study ranged from slack water near the times of low and high tides to >50 cm [s.sup.-1] at maximum flood. For each night of sampling, the flooding tide was characterized by a progression of decreasing temperatures and increasing salinities. Temperatures over the course of the study ranged from 21.6[degrees]C to 24.9[degrees]C, and salinities ranged from 28.3 [per thousand] to 32.0 [per thousand].
Characterization of Patches
We used results of autocorrelation analysis to confirm the existence of patches during the various cruises and to determine their spatial scale. A total of 6 patches of megalopae (3 C. sapidus and 3 Uca) were observed over the course of the investigation (Table 1). As expected, the density of megalopae (see below) was considerably less than reported for zoea larvae ([10.sup.2] - [0.sup.4] [m.sup.-3]) in previous investigations (Natunewicz & Epifanio 2001, Jancaitis 2003, Petrone 2003). In our study, the maximum density of Uca megalopae within a patch ranged from approximately 10-70 [m.sup.-3]. Density of C. sapidus megalopae never exceeded 16 individuals [m.sup.-3].
Patches arrived at the sampling site throughout the flood tidal phase, but megalopae were rare in the water column near times of slack water (Table 2, Fig. 3, Fig. 4), before sunset (Fig. 3a), or during the ebb phase (Fig. 3b). The radii of patches of C. sapidus megalopae ranged from approximately 500-2000 m, whereas the radii for Uca patches was typically smaller with values around 500 m (Table 1, Fig. 3, Fig. 4). The ranges for C. sapidus and Uca patches overlapped those reported for zoeae of C. sapidus in earlier work near Delaware Bay (Natunewicz & Epifanio 2001, Natunewicz et al. 2001).
[FIGURES 3-4 OMITTED]
Results of this study show that blue crab and fiddler crab megalopae are distributed in discrete patches as they are transported into the estuary from the adjacent shelf. As expected, maximum density of megalopae within patches was less than that observed for zoea larvae in previous investigations of patch dynamics Definitions
Patch dynamics is a conceptual approach to ecosystem and habitat analysis that emphasizes dynamics of heterogeneity within a system. Diverse patches of habitat created by natural disturbance regimes are seen as critical to maintenance of diversity. (Natunewicz & Epifanio 2001, Natunewicz et al. 2001, Petrone 2003), but was similar to the highest densities reported in earlier, more general studies of megalopal distribution (Dittel & Epifanio 1982, Epifanio et al. 1988, Little & Epifanio 1991, DeVries et al. 1994, Olmi 1995). Because our sampling plan restricted measurements to a single dimension, earlier work has shown consistently that patches of crab larvae assume a quasi-circular to elliptical el·lip·tic or el·lip·ti·cal
1. Of, relating to, or having the shape of an ellipse.
2. Containing or characterized by ellipsis.
a. shape when found in open water (Natunewicz & Epifanio 2001, Natunewicz et al. 2001, Jancaitis 2003, Petrone 2003). Thus, the length scale in the present investigation serves as a surrogate for the approximate radius of a patch. Autocorrelation analysis indicated that the radius of patches of C. sapidus megalopae in our investigation ranged from around 550-2000 m. This implies that typical patches of blue crab megalopae have greater linear dimensions than those reported for conspecific con·spe·cif·ic
Of or belonging to the same species.
An organism belonging to the same species as another.
Noun 1. patches of zoea larvae, which have radii on the order of 250-1,200 m (Natunewicz & Epifanio 2001). In contrast, patches of Uca megalopae in our study had radii (~500 m), well within the range reported for zoea larvae. The reasons for these different results are unclear.
In the current investigation, megalopae were collected at different times during the flood tide phase but were rare in samples collected near the times of slack water, before sunset, or during the ebb phase. This temporal pattern has been observed in earlier studies of megalopal distribution (Epifanio 1988, Little & Epifanio 1991, DeVries et al. 1994) and generally supports the contention that blue crab and fiddler crab megalopae use flood tide transport to facilitate upstream movement within the estuary (Tankersley & Forward 1994, Welch & Forward 2001). Results of previous laboratory studies of these taxa suggest that flood tide transport is based on rhythmic vertical migration of the megalopae (Forward & Tankersley 2001). For example, blue crab megalopae in open waters of the continental shelf have an endogenous circadian rhythm circadian rhythm: see rhythm, biological.
Inherent cycle of approximately 24 hours in length that appears to control or initiate various biological processes, including sleep, wakefulness, and digestive and hormonal activity. in which they are active in the day and inactive at night (Tankersley & Forward 1994). However, in estuarine waters, megalopae also respond to exogenous Exogenous
Describes facts outside the control of the firm. Converse of endogenous. cues and exhibit a negative photoresponse that dominates their endogenous circadian rhythm (Forward & Tankersley 2001). Additional exogenous cues (i.e., increases in salinity and turbulence) act to override the endogenous behavior, resulting in C. sapidus megalopae only being present in the water column during nocturnal, flooding tides (Welch & Forward 2001).
In contrast, fiddler crab megalopae have an endogenous circatidal rhythm where swimming activity is increased during times of flood tides; this, combined with negative geotaxis geotaxis
see geotropism. , results in upward swimming in the water column (Tankersley & Forward 1994). However, Uca megalopae also exhibit negative phototaxis phototaxis /pho·to·tax·is/ (-tak´sis) the movement of cells and microorganisms in response to light.phototac´tic
n. (Tankersley et al. 1995), which has been invoked to explain the general absence of fiddler crab megalopae in the water column during daylight hours, regardless of tidal stage (Dittel et al. 1991, Little & Epifanio 1991, DeVries et al. 1994).
Results from previous investigations of patch dynamics indicate that aggregations of crab larvae are probably formed at the time of hatching and are the result of simultaneous spawning by assemblages of female crabs (Petrone 2003). Moreover, these patches of zoea larvae appear to remain aggregated on a scale of days to weeks, and careful field measurements have documented development through several zoeal stages within a given patch (Natunewicz & Epifanio 2001). Because aggregative physical processes (e.g., Langmuir circulation, frontal boundaries, gyres) are generally considered important in controlling the horizontal distribution of zooplantonic organisms (Epifanio 1987, Clancy & Epifanio 1989, Eggleston et al. 1998), recent observations demonstrate that patches of blue crab larvae maintain a well defined structure, even when advected to areas of the continental shelf where circulation patterns favor dissipation (Natunewicz et al. 2001). The mechanisms for maintaining patch structure under these conditions are not clear and are not inferable from the present investigation.
That aside, the results of our study demonstrate unequivocally that blue crab and fiddler crab megalopae are distributed in well-defined patches as they enter the estuary. However, our results do not answer the question of whether a typical patch of larvae maintains its spatial integrity through the entire sequence of development. For example, an alternative explanation of our results might postulate postulate: see axiom. a scenario where several consecutive episodes of nocturnal migration to surface waters would serve to meld a number of small patches of megalopae into one large patch or at least into a nearly contiguous series of small patches that might be interpreted statistically as one large patch. This could occur if the nightly ascent of a number of patches of megalopae from the bottom were slightly out of phase. In this case it would be possible for a late-ascending patch to migrate upward into an early-ascending patch that was already in surface waters passing above.
Another explanation for the observed patchy distribution might invoke the concentration of otherwise dispersed megalopae in tidal fronts as they enter the estuary (Epifanio 1987, Clancy & Epifanio 1989, Eggleston et al. 1998). However, our sampling program was not designed to investigate frontal dynamics and casts no light on this possibility. Yet another scenario would result in aggregation at preferred locations on the bottom when megalopae sink out of the water column near the time of slack high water (Welch & Forward 2001), (i.e., megalopae could be attracted to areas of structured bottom such as seagrass or algal algal
pertaining to or caused by algae.
is very rare but systemic and udder infections are recorded. See protothecosis.
the algae Prototheca trispora and P. beds). The upward migration of these bottom-aggregated megalopae on the next nocturnal flood tide (Tankersley & Forward 1994, Tankersley et al. 1995) would result in a newly formed patch at the surface. However, the present conceptual model for megalopal transport within the estuary (Forward & Tankersley 2001, Epifanio & Garvine 2001) calls for megalopae to leave the water column near the time of high slack tide Noun 1. slack tide - the occurrence of relatively still water at the turn of the (low) tide
tide - the periodic rise and fall of the sea level under the gravitational pull of the moon when turbulent kinetic energy Turbulent Kinetic Energy (TKE) is the mean kinetic energy per unit mass associated with eddies in turbulent flow. It is a concept used to assess what contribution to buoyancy is brought by turbulence. has fallen below some threshold value (Welch & Forward 2001). Therefore, the location of the eventual landing point on the bottom would seem purely dependent on chance. Thus, it is not at all clear how this mechanism would allow megalopae to aggregate in a particular area of bottom--especially in the deep coastal water (>20 m) immediately offshore of our sampling station where any effects of settlement cues would appear moot.
It seems to us that a more parsimonious par·si·mo·ni·ous
Excessively sparing or frugal.
parsi·mo explanation, and one more consistent with the present conceptual model, would have megalopae already distributed in patches as they approach the estuary. Once the patch enters the estuary, the megalopae within the patch would begin to display a pattern of diel/tidal vertical migration wherein a rise in salinity during nocturnal flood phase would elicit upward swimming, high turbulent kinetic energy during flood would maintain upward swimming, and a decline in turbulence near slack water would cue a cessation of swimming with subsequent sinking to the bottom--all of this tempered by the interactive effect of light (Tankersley & Forward 1994, Tankersley et al. 1995). That all the megalopae in the patch would exhibit these respective behaviors more or less simultaneously seems entirely reasonable. So in this scenario, the expected pattern of vertical migration is not the process that forms the observed patches, but rather the process that controls the diel/tidal periodicity periodicity /pe·ri·o·dic·i·ty/ (per?e-ah-dis´i-te) recurrence at regular intervals of time.
1. of the occurrence of a previously formed patch in surface waters of the estuary.
But these various possibilities aside, our findings provide the first clear evidence that crab megalopae are often distributed in well defined patches and that these patches have radii on the order of hundreds to a few thousand m. Moreover, our results provide support for the hypothesis that the observed pattern of episodic megalopal settlement within many estuaries (e.g., Jones & Epifanio 1995, vanMontfrans et al. 1995) is at least partly a function of this patchy distribution.
TABLE 1. Characteristics of megalopal patches for blue crabs Callinectes sapidus and fiddler crabs Uca spp. C. Sapidus Uca spp. Date Density Radius Density Radius 2001 Sept. 5 -- -- -- -- 2001 Sept. 6 -- -- 67.5 500 2001 Sept. 7 -- -- -- -- 2001 Sept. 16 8.7 1500 -- -- 2001 Sept. 17 -- -- -- -- 2001 Sept. 18 8.0 2000 -- -- 2002 July 28 16.4 550 10.1 600 2002 July 29 -- -- -- -- 2002 July 31 -- -- 9.1 600 2002 Aug. 12 -- -- -- -- 2002 Aug. 13 -- -- -- -- 2002 Aug. 14 -- -- -- -- Density = maximum concentration of megalopae within a patch ([m.sup.-3]). Radius = characteristic length scale of patch (m) as determined by auto-correlation. Dashes indicate that maximum megalopal density did not exceed the threshold value on a particular sampling date (see text). TABLE 2. Details of sampling schedule on dates when patches of megalopae were observed in the water column at the mouth of Delaware Bay. Predicted times of sunset (SS), low slack water (LSW), and high slack water were (HSW) taken from US Naval Observatory tables and NOAA tide tables, respectively. Note that low slack water occurred approximately 3.5 h before sunset on September 16, 2001 and approximately 1.5 h before sunset on September 18. Sampling began approximately 1.5 h before sunset on September 16 and approximately 1.5 h after sunset on September 18 (see text for discussion). All times are EDT. Date SS LSW Start Stop HSW Sampling Sampling 2001 Sept. 06 19:25 19:06 19:35 02:05 01:12 2001 Sept. 16 19:09 15:44 17:35 01:35 22:05 2001 Sept. 18 19:06 17:38 20:35 03:35 23:48 2002 July 28 20:17 19:28 19:30 01:00 01:48 2002 July 31 20:14 21:49 21:50 02:30 04:15
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Cronin, T. W. & R. B. Forward, Jr. 1979. Tidal vertical migration: an endogenous rhythm in estuarine crab larvae. Science 205:1020-1022.
Cronin, T. W. 1982. Estuarine retention of larvae of the crab Rhithropanopeus harrisii. Estuar. Coast. Shelf Sci. 15:207-220.
DeVries, M. C., R. A. Tankersley, R. B. Forward, Jr., W. W. Kirby-Smith & R. A. Luettich. 1994. Abundance of estuarine crab larvae is associated with tidal hydrologic variables. Mar. Biol. 118:403-413.
Dittel, A. I. & C. E. Epifanio. 1982. Seasonal density and vertical distribution of crab larvae in Delaware Bay. Estuaries 5(3): 197-202.
Dittel, A. I., C. E. Epifanio & O. Lizano. 1991. Flux of crab larvae in a mangrove mangrove, large tropical evergreen tree, genus Rhizophora, that grows on muddy tidal flats and along protected ocean shorelines. Mangroves are most abundant in tropical Asia, Africa, and the islands of the SW Pacific. creek in the Gulf of Nicoya The Gulf of Nicoya (Spanish: Golfo de Nicoya) is an inlet of the Pacific Ocean, located at . , Costa Rica Costa Rica (kŏs`tə rē`kə), officially Republic of Costa Rica, republic (2005 est. pop. 4,016,000), 19,575 sq mi (50,700 sq km), Central America. . Estuar. Coast. Shelf Sci. 32:129-140.
Eggleston, D., D. Armstrong, W. Elis & W. Patton. 1998. Estuarine fronts as conduits for larval larval
1. pertaining to larvae.
see cutaneous and visceral larva migrans. transport: hydrodynamics hydrodynamics: see mechanics.
The study of fluids in motion. The study is based upon the physical conservation laws of mass, momentum, and energy. and spatial distribution of megalopae. Mar. Ecol. Prog. Ser. 164:73-82.
Epifanio, C. E. 1987. The role of tidal fronts in maintaining patches of brachyuran zoeae in estuarine waters. J. Crust. Biol. 7:513-517.
Epifanio, C. E. 1988. Transport of invertebrate invertebrate (ĭn'vûr`təbrət, –brāt'), any animal lacking a backbone. The invertebrates include the tunicates and lancelets of phylum Chordata, as well as all animal phyla other than Chordata. larvae between estuaries and the continental shelf. Amer. Fish. Soc. Symp. 3:104-114.
Epifanio, C. E. 1995. Transport of blue crab (Callinectes sapidus) larvae in the waters off Mid-Atlantic States Mid-At·lan·tic States
See Middle Atlantic States.
Noun 1. Mid-Atlantic states - a region of the eastern United States comprising New York and New Jersey and Pennsylvania and Delaware and Maryland
U.S.A. . Bull. Mar. Sci. 57:713-725.
Epifanio, C. E. & R. W. Garvine. 2001. Larval transport on the Atlantic continental shelf of North America: a review. Estuar. Coast. Shelf Sci. 52(1):51-77.
Epifanio, C. E., C. C. Valenti & A. E. Pembroke. 1984. Dispersal and recruitment of blue crab larvae in Delaware Bay, U.S.A. Estuar. Coast. Shelf Sci. 18:1-12.
Epifanio, C. E., K. T. Little & P. M. Rowe. 1988. Dispersal and recruitment of fiddler crab larvae in the Delaware River Delaware River
River in Pennsylvania, Delaware, New Jersey, and New York, U.S. Formed by the junction of its eastern and western branches in southern New York, it flows about 405 mi (650 km) to empty into the Atlantic Ocean at Delaware Bay. Navigable to Trenton, N.J. estuary. Mar. Ecol. Prog. Ser. 43:181-188.
Forward, R. B., Jr., J. Swanson, R. A. Tankersley & J. M. Welch. 1997. Endogenous swimming rhythms of blue crab, Callinectes sapidus, megalopae: effects of offshore and estuarine cues. Mar. Biol. 127:621-628.
Forward, R. B. & R. A. Tankersley. 2001. Selective tidal-stream transport of marine animals. Oceanogr. Mar. Biol. Annu. Rev. 39:305-353.
Garvine, R. W. 1991. Subtidal frequency estuary-shelf interaction: Observations near Delaware Bay. J. Geophys. Res. 96:7049-7064.
Garvine, R. W., C. E. Epifanio, C. C. Epifanio & K. C. Wong. 1997. Transport and recruitment of blue crab larvae: a model with advection ad·vec·tion
1. The transfer of a property of the atmosphere, such as heat, cold, or humidity, by the horizontal movement of an air mass: and mortality. Coast. Est. Shelf Sci. 45:99-111.
Jancaitis, L. B. 2003. Distribution and transport of patches of crab larvae in an estuarine environment. M.Sc. Thesis. University of Delaware  The student body at the University of Delaware is largely an undergraduate population. Delaware students have a great deal of access to work and internship opportunities. , Newark, DE. 205 pp.
Jones, M. B. & C. E. Epifanio. 1995. Settlement of brachyuran megalopae in Delaware Bay: an analysis of time series data. Mar. Ecol. Prog. Ser. 125:67-76.
Little, K. T. & C. E. Epifanio. 1991. Mechanism for the re-invasion of the estuary by two species of brachyuran megalopae. Mar. Ecol. Prog. Ser. 68:235-242.
Natunewicz, C. C. 2000. Patchiness of crab larvae in coastal waters: A multi-scale analysis. Ph.D. Dissertation. University of Delaware, Newark, DE. 311 pp.
Natunewicz, C. C. & C. E. Epifanio. 2001. Spatial and temporal scales In snakes, the temporal scales are those scales on the side of the head between the parietals and the supralabials, and behind the postoculars.
There are two types of temporal scales:
Natunewicz, C. C., C. E. Epifanio & R. W. Garvine. 2001. Transport of crab larval patches in the coastal ocean. Mar. Ecol. Prog. Ser. 222: 143-154.
Olmi, E. J. 1995. Ingress An entrance. Contrast with "egress," which means exit. See ingress traffic. See also Ingres 2006. of blue crab megalopae in the York River York River
An estuary, about 64 km (40 mi) long, of eastern Virginia flowing southeast into Chesapeake Bay. , Virginia, 1987-1989. Bull. Mar. Sci. 57(3):753-780.
Petrone, C. J. 2003. Spatial distribution of fiddler crab larvae. Are patches formed at hatching? M.Sc. Thesis. University of Delaware, Newark, DE. 303 pp.
Press, W. H., S. A. Teukolsky, W. T. Vetterling & B. P. Flannery. 1992. Interpolation interpolation
In mathematics, estimation of a value between two known data points. A simple example is calculating the mean (see mean, median, and mode) of two population counts made 10 years apart to estimate the population in the fifth year. and Extrapolation (mathematics, algorithm) extrapolation - A mathematical procedure which estimates values of a function for certain desired inputs given values for known inputs.
If the desired input is outside the range of the known values this is called extrapolation, if it is inside then . In: W. H. Press, S. A. Teukolsky, W. T. Vetterling & B. P. Flannery, editors. Numerical recipes in C. The art of scientific computing, 2nd ed. Cambridge, UK: Cambridge University Press Cambridge University Press (known colloquially as CUP) is a publisher given a Royal Charter by Henry VIII in 1534, and one of the two privileged presses (the other being Oxford University Press). . pp. 114-116.
Richerson, P. J., T. M. Powell, M. R. Leigh-Abbott & J. A. Coil. 1978. Spatial heterogeneity Environments with a wide variety of habitats such as different topographies, soil types and climates are able to accommodate a greater amount of species. Spatial heterogeneity in closed basins. In: J. H. Steele, editor. Spatial patterns in plankton communities. New York New York, state, United States
New York, Middle Atlantic state of the United States. It is bordered by Vermont, Massachusetts, Connecticut, and the Atlantic Ocean (E), New Jersey and Pennsylvania (S), Lakes Erie and Ontario and the Canadian province of , NY: Plenum Press. pp. 239-276
Rowe, P. M. & C. E. Epifanio. 1994. Tidal stream transport of weakfish weakfish: see croaker.
or sea trout
Any of several species (genus Cynoscion) in the drum family (Sciaenidae), carnivorous bottom-dwelling fishes along warm and tropical seashores. in Delaware Bay, USA. Mar. Ecol. Prog. Ser. 110:105-114.
Sandifer, P. A. 1972. Morphology and ecology of Chesapeake Bay Chesapeake Bay, inlet of the Atlantic Ocean, c.200 mi (320 km) long, from 3 to 30 mi (4.8–48 km) wide, and 3,237 sq mi (8,384 sq km), separating the Delmarva Peninsula from mainland Maryland. and Virginia. decapod decapod (dĕk`əpŏd') (Gr.,=10 feet), name for invertebrate animals of the crustacean order Decapoda (phylum Arthropoda) including the crabs, the lobsters and crayfish, and the true shrimps, all having five pairs of legs. crustacean crustacean (krŭstā`shən), primarily aquatic arthropod of the subphylum Crustacea. Most of the 44,000 crustacean species are marine, but there are many freshwater forms. larvae. Ph.D. Dissertation. University of Virginia, Charlottesville, VA. 531 pp.
Tankersley, R. A. & R. B. Forward, Jr. 1994. Endogenous swimming rhythms in estuarine crab megalopae: implications for flood-tide transport. Mar. Biol. 118:415-423.
Tankersley, R. A., L. M. McKelvey & R. B. Forward, Jr. 1995. Responses of estuarine crab megalopae to pressure, salinity and light: Implications for flood-tide transport. Mar. Biol. 122(3):391-400.
Van Montfrans, J., C. E. Epifanio, D. M. Knott, R. N. Lipcius, D. J. Mense n. 1. Manliness; dignity; comeliness; civility.
v. t. 1. To grace. , K. S. Metcalf, E. J. Olmi III, R. J. Orth, M. H. Posey, E. L. Wenner & T. L. West. 1995. Settlement of blue crab postlarvae in western North Atlantic estuaries. Bull. Mar. Sci. 57:834-854.
Welch, J. M. & R. B. Forward, Jr. 2001. Flood tide transport of blue crab, Callinectes sapidus, postlarvae: behavioral responses to salinity and turbulence. Mar. Biol. 139:911-918.
M. BRANDON JONES Brandon Jones is the name of more than one notable person:
Graduate College of Marine Studies, University of Delaware, 700 Pilottown Road, Lewes, Delaware Lewes (pronounced [ˈluɪs]) is an incorporated city in Sussex County, Delaware, United States. According to 2006 Census Bureau estimates, the population of the city is 3,116. 19958
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