Biochemical composition, reproductive activity and food availability of the lion's paw scallop Nodipecten subnodosus in the Laguna Ojo de Liebre, Baja California Sur, Mexico.

ABSTRACT The relationships between the nutrient storage cycle, reproductive activity, and available food for the lion's paw scallop Nodipecten subnodosus were investigated during a one-year period in Laguna Ojo de Liebre, Baja California Sur, Mexico. Based on histologic data, this study showed that the reproductive cycle of N. subnodosus could be divided in four periods: (1) period of quiescence from December to March (with a small reproductive activity in February-March), (2) period of initial reproductive activity from April to May, (3) period of maximum ripeness from June to September, and (4) period of spawning from September to November. The seasonal changes in biochemical composition of somatic tissues of N. subnodosus at this locality indicated a clear nutrient storage and use cycle related to reproductive activity. However, the biochemical reserves of adductor muscle, digestive gland, and mantle were used differently over time. Proteins and acylglyceridcs could be transferred from the testis portion to the ovary portion during the ripening process. We showed that gametogenesis in N. subnodosus from Laguna Ojo de Liebre relied on stored reserves, as a consequence of the low food quantity (maximum 1.2 [micro]g [L.sup.-1] chlorophyll a) and the low nutritional quality (maximum todd index 17%) of the food available for suspension-feeders. The spawning of N. subnodosus in Laguna Ojo de Liebre coincided with the chlorophyll a maximum concentration, so it appeared to be triggered more by the phytoplankton abundance than by temperature.

KEY WORDS: food availability, Nodipecten subnodosus, nutrient transference, reproduction, scallop

INTRODUCTION

The reproductive cycle of marine bivalves is strongly related to nutrient storage and use cycles and environmental parameters such as water temperature and food availability (Giese 1969. Gabbott 1975, Bayne 1976), Gametogenesis is a process that requires energy, and the way to obtain energy differs among species (Park et al. 2001). Some species use the recently ingested energy from the seston (opportunistic species); others use the energy of substrates stored in various organs and tissues (digestive gland, adductor muscle, or mantle) through feeding prior to its gametogenesis (conservative species) (for reviews see Gabbott 1975, Bayne 1976, Barber & Blake 1991). In pectinids, the way in which the substrates are used, and the tissue or organ from which the associated energy is obtained, may vary among species (Ansell 1974, Comely 1974, Taylor & Venn 1979, Epp et al. 1988, Barber & Blake 1991, Martinez 1991, Mathieu & Lubet 1993).

Nodipecten subnodosus is a large pectinid species, reaching a maximum length of 218 mm (Felix-Pico et al. 1999). The lion's paw scallop supports a small commercial fishery only on its northern range of distribution because of its scarce presence in other areas of the Baja California peninsula. The lion's paw is an organism that reaches its commercial size in a relatively short time (Gutierrez-Villasenor & Chi-Barragan 1997). Its large adductor muscle is attractive for human consumption, a fact that has important implications for fisheries and aquaculture. However, only a few studies on growth (Carvajal-Rascon 1987, Garcia-Dominguez et al. 1992, Garcia-Pamanes et al. 1994, Barrios 1997, Ortiz-Cuel et al, 1997, Felix-Pico et al. 1999, Racotta et al. 2003) and reproduction (Reinecke-Reyes 1996, Gutierrez-Villasenor & Chi-Barragan 1997, Racotta et al. 2003) have been reported.

This study analyzes the seasonal variations in biochemical composition in a wild population of N. subnodosus and its relation to their reproductive activity and food availability.

MATERIALS AND METHODS

Animals

Samplings were done in the Laguna Ojo de Liebre (27[degrees]55'-27[degrees]35' LN and 114[degrees]20'-113[degrees]50' LW) located in the Mexican Reserve of the Biosphere "El Vizcaino." From December 2000 to November 2001, 10 lion's paw scallops of 13-16 cm in shell height and 12-15 cm in shell length were collected monthly by divers at 7 m of depth. Scallops were transported alive in a cool and wet environment to lower their metabolism.

In the laboratory, the total and soft body weights and height (largest shell distance in an anterior to posterior orientation) of each scallop were registered to the nearest 0.1 mm using vernier calipers. Scallops were dissected to obtain gonads (without crystalline style and gut content), digestive glands, mantles, and adductor muscles, and their weights were registered and tissues were stored at -80[degrees]C for further biochemical analysis. Before freezing, a portion of gonad including both sexes was fixed in a formalin solution (10%) for further histologic analysis.

Indicators of Quantity and Quality of the Available Food

During the study period, seawater samples of the scallop-sampling area were collected using a Van Dorn bottle. The seawater samples were collected at 6.5 m of depth, close to the sandy bottom on which the scallops live. Water was screened through a 200-[micro]m Nitex mesh to eliminate zooplankton and large particles before analysis (Park et al. 2001). For each analysis sample, in triplicate, a known volume of seawater was immediately filtered under gentle vacuum and in dark conditions through washed, precombusted, preweighed Whalman GF/F filters (47-mm diameter). The filters were washed with distilled water to remove residual salt.

Chlorophyll a was extracted on acetone, and its concentration was determined using the fluorometric method, as modified by Parsons et al. (1984).

The sum of carbohydrate, lipid, and protein concentration in the water, expressed as a percentage of the total seston, represents a useful index of the quality of the food available to suspension-feeders (Navarro & Thompson 1995). Food quality index (FI) was calculated according to Widdows et al. (1979), using the equation:

FI = (FQ/TPM) x 100

where FQ is the food quantity ([micro]g/L), and TPM is the total particulate matter or total seston ([micro]g/L).

To calculate FQ. the major biochemical components of seston were analyzed: protein by the method of Lowry et al. (1951), carbohydrates by the phenol sulfuric acid method (Dubois et al. 1956). and lipids by the sulfophosphovanillin method (Barnes & Blackstock 1973) using a commercial reactive and standard solutions (Merck, Darmstadt, Germany). FQ was then determined as the sum of the concentration of these components.

To calculate TPM, filters were dried at 80[degrees]C for 24 h and were reweighed after cooling in a desiccator. TPM was determined as the difference between the filter preweight (before filtering) and the dried filter weight.

Analysis of the Reproductive Activity

Transversal gonad sections including female and male portions were dehydrated in an ethanol series of progressive concentrations and embedded in paraplast X-tra. Sections (7 [micro]m) were obtained and stained with hematoxilin-eosin (Humason 1979). Gametogenesis of N. subnodosus was categorized based on the qualitative characteristics of the gonad, separately for each sex, and by considering the five stages of gonadal maturation (undifferentiated, developing, ripe, partially spawned, and spent) proposed by Reinecke-Reyes (1996) for this species. The relative frequencies of the stages of gonadal development throughout the year were obtained with female and male stages combined, as there was not a difference in the temporality pattern between sexes.

To obtain quantitative values that represent the reproductive activity, the monthly gonadosomatic index (GSI) was calculated by dividing the gonad wet weight by the total soft body wet weight and by expressing the results as a percentage (Sastry 1970). Oocyte diameters were measured from digitized images of histologic sections using the SCAN PRO software (version 5.0, Systat Software, Inc., Richmond, CA). Oocytes sectioned through the nucleus were individually traced with the pointer, and the major and minor axis lengths were measured automatically by the software. Then, an average of both dimensions was calculated and considered as the estimated diameter, which was calculated monthly. At least 100 oocytes per scallop were measured. Individuals with few measurable oocytes and extensive pbagocytosis ("spent" specimens) were not considered, using the criteria of Grant & Tyler (1983).

Additionally, indices of digestive gland (DGI), mantle (MI), and adductor muscle (AMI) were calculated as follows:

Organ index = (Organ weight/Total soft body weight) x 100

Tissue Biochemical Analyses

To obtain a crude extract of digestive glands and gonads (female and male portions separately), 0.3 g of tissue was homogenized in 3 mL of cold saline solution (NaCl 35 ppt). This extract was used for all biochemical analyses. Muscle and mantle samples were processed in a different way when determinations were made (explained later for each case) because of their toughness.

Soluble protein determination was done by the Bradford (1976) technique, using commercial chromogen reagent (Sigma-Aldrich, St. Louis, MO) and bovine albumin serum (Sigma-Aldrich) as standard solution. Samples were first digested during 2 h with 0.1 N NaOH at a 1:10 dilution. For mantle and muscle, 0.02 g of tissue were directly digested in 2 mL of 0.5 N NaOH during 24 h. Then, 1 mL of Bradford reactive solution was added to 10 [micro]L of digested sample, and absorbance was read at 595 nm.

Total carbohydrates were determined by the anthrone method (Van Andel 1965) after protein precipitation with trichloroacetic acid (TCA). For gonads and digestive gland, the crude homogenate was diluted 1:2 with 20% TCA, whereas samples of muscle and mantle (0.5 g muscle or 0.9 g mantle) were directly homogenized in 5 mL of 10% TCA. In both cases, samples were centrifuged at 4000 rpm at 5[degrees]C for 10 min. Then, 1 mL of anthrone solution (0.1% dissolved in 76% sulfuric acid) was added to 0.1 mL of supernatant and incubated at 86[degrees]C for 2 to 10 min, depending on the concentration of carbohydrates in each tissue and sampling period. The reaction was stopped by cooling at 4[degrees]C in an ice bath, and absorbance was read at 620 mm. Total carbohydrates were quantified as glucosyl units, using a calibration curve of glucose treated in the same way as particular samples, especially for the incubation times.

Total lipids were determined by the sulfophosphovanillin method (Barnes & Blackstock 1973) using a commercial reactive and standard solutions (Merck). A volume of 0.5 mL of concentrated sulfuric acid was added to 50 [micro]L of crude homogenate of gonads and digestive glands, incubated at 90[degrees]C for 10 min and immediately cooled at 4[degrees]C in an ice bath. Then, 20 [micro]L were mixed with 200 [micro]L sulfuphosphovanillin solution in a microplate, incubated for 40 min, and absorbance read at 540 nm with a microplate reader (Biorad 550, Tokyo, Japan).

Acylglycerides were determined by a colorimetric-enzymatic kit (GOP-PAP, Merck). The crude homogenate of gonads and digestive glands was centrifuged at 4000 rpm at 5[degrees]C for 10 min. Then, 20 [micro]L were mixed with 200 [micro]L commercial chromogen solution in a microplate, incubated for 20 min, and absorbance read at 490 nm with a microplate reader (Labsystems, Uniscan II). Lipids and acylglycerides were not analyzed in the muscle and mantle due to the lack of sensitivity of these methods on those particular tissues, which have a very low lipid content.

All the biochemical techniques were previously standardized for each tissue. The results of each biochemical constituent were expressed in mg [g.sup.-1] of dry weight of each organ tissue.

Statistical Analyses

One-way ANOVA, followed by mean comparisons post hoc Tukey test were made to assess significant differences in biochemical composition and indices among months. Pearson product moment correlations were used between IGS and DGI, AMI, and MI, and the different biochemical variables.

Because GSI, DGI, AMI, and MI values are reported as percentages, they were arcsine transformed (Zar 1996) to reduce the dependence of the sample variance on the mean and to normalize the data distribution. Data are reported as means. STATISTICA for Windows (version 6.0) was used for analyses. The level of significance was pre-set at P < 0.05.

RESULTS

Gonad Maturation Process

Monthly qualitative assessment of the reproductive condition is illustrated in Figure 1. In December 2000 and from January to March 2001, most lion's paw scallops (between 75% and 100%) were inactive (undifferentiated and spent stages). The development stage was present in low percentage in January (12.5%) and with high incidence starting in April and increasing gradually to July (60-90%). Ripe scallops were found from June to November. The highest proportion of ripe scallops was observed in August (100%). Small proportions of ripe and partially spawned scallops were found in February and March. A massive spawning period was observed from September to November (25-75% of the population).

[FIGURE 1 OMITTED]

Oocyte Diameters

The mean oocyte diameter (Fig. 2) showed significant differences among months (one-way ANOVA, P < 0.01) with a clear seasonality (Tukey post hoc test). The minimum mean oocyte diameter (23.5 [micro]m) was observed in January, and the maximum (51 [micro]m) was observed in August. In December and March, the scallops were spent or undifferentiated, and thus oocyte diameter was not estimated.

[FIGURE 2 OMITTED]

Organ Indices

Significant differences (one-way ANOVA, P < 0.01) were observed for GSI throughout the year (Fig. 3), GSI values were low from December to March (5.5-7.4%); they began to increase gradually starting in April, reaching their maximum in August (26.6%), which was significantly different from other months. From September to November, the GSI values fell to levels similar to December 2000.

[FIGURE 3 OMITTED]

DGI values changed significantly (one-way ANOVA, P < 0.01) throughout the year (Fig. 3). The highest value was observed in January (7.96%) and the lowest in October (5.79%) with intermediate values for the other months. DGI showed a significant negative correlation with GS1 ([r.sup.2] = -0.43, P < 0.001).

Values of AMI were more than 32% of total biomass from December 2000 to June 2001 and then significantly decreased (one-way ANOVA, P < 0.01) to values around 30% of biomass from July to November (Fig. 3). The AMI showed a significant negative correlation with GSI ([r.sup.2] = -0.59, P < 0.001).

Small, although significant (one-way ANOVA, P < 0.0l) changes were observed for MI, which presented the highest value in December 2000 (24.2%) and the lowest in August 2001 (17.6%) (Fig. 3). The MI showed a significant negative correlation with GSI ([r.sup.2] = -0.65, P < 0.001).

Indicators of Quantity and Quality of the Food Available

Photosynthetic pigment concentration (chlorophyll a [micro]g [L.sup.-1]) and food index values in the Laguna Ojo de Liebre showed a similar variation throughout the year (Fig. 4). High chlorophyll a values (over 1 [micro]g [L.sup.-1]) were observed in March and September-October in accordance with higher food index values (more than 11%). However, in September-October the higher values of chlorophyll a (1.19 and 1.2 [micro]g [L.sup.-1]) were observed, but food index values were not the highest (under 12%).

[FIGURE 4 OMITTED]

Tissue Biochemical Composition

Protein

Protein was the major biochemical component of gonads. In the ovary portion, significantly lower values were observed from December to March. From April, the protein concentration increased gradually until significantly higher values were reached from June to October (Fig. 5). In the testis portion, a different pattern occurred (Fig. 5): protein increased gradually from January to April; then a decrease in protein concentration occurred from May to July, followed by a slight, though not significant, increase in August and September. Protein concentration in the ovary portion was significantly correlated to GSI ([r.sup.2] = 0.55, P < 0.05) but not in testis.

[FIGURE 5 OMITTED]

The digestive gland had a significantly lower concentration of proteins in April-May (Fig. 5). Higher values were observed in February and from June to August.

Protein levels in the adductor muscle were low from March to August (Fig. 5), whereas significantly higher values were observed from September to November.

In the mantle, the concentration of proteins decreased significantly from February to May (Fig. 5). Then, a significant increase was observed in July-August, followed by a significant decrease by September and then a further increase by November.

Carbohydrates

In the ovary portion, the significantly lowest values were present from December to April. By May, carbohydrates concentration increased gradually until September, when the significantly highest values were observed. Then, the concentration of carbohydrates declined in October-November (Fig. 6). In the testis portion, a similar pattern was observed, although the significantly highest concentration of carbohydrates was present in August, and the decline occurred in September (Fig. 6). The GSI showed a significant correlation with carbohydrates of ovary ([r.sup.2] = 0.6, P < 0.05) and testis ([r.sup.2] = 0.48. P < 0.05) portions.

[FIGURE 6 OMITTED]

The levels of carbohydrates in digestive glands showed a ten p to increase progressively throughout the year, with significantly higher values in August-September (Fig. 6). Carbohydrates in digestive glands showed a significant correlation to GSI ([r.sup.2] = 0.41, P < 0.05).

The concentration of carbohydrates in adductor muscle in creased from December to June, when the significantly highest value was observed (Fig. 6). In July, a significant decrease was observed, and values continued to decrease until November.

The significantly highest concentrations of carbohydrates in mantle were observed from December to February (Fig. 6). Then, they decreased to intermediate levels from March to May and to the lowest values from June to November. Carbohydrates in mantle showed a significant negative correlation to GSI ([r.sup.2] = -0.43, P < 0.05).

Lipids

Lipids in the ovary portion showed a seasonal pattern. The lowest values were observed from December to April (Fig. 7). A significant increase was then observed from May, the highest values were attained from June to August, followed by a significant decrease from September to November. In the testis, lipids concentration did not change significantly throughout the year (Fig. 7). Concentration of lipids in the ovary portion showed a significant correlation to GSI ([r.sup.2] = 0.47, P < 0.05) but not in testis.

[FIGURE 7 OMITTED]

The concentration of lipids in digestive glands oscillated around high values from December to June with two peaks in February and May (Fig. 7). By July, lipids concentration fell significantly until November. Lipids showed a significant negative correlation to GSI ([r.sup.2] = -0.38. P < 0.05).

Acylglycerides

In the ovary portion, the lowest concentrations of acylglycerides were observed from December to March (Fig. 8). They increased from April, reaching significantly higher values from June to September, followed by a significant decrease in October and November. In the testis portion, acylglycerides levels showed a gradual but significant (P < 0.01) decrease from January to November (Fig. 8). Acylglycerides in ovaries were positively correlated ([r.sup.2] = 0.51, P < 0.05), whereas in testis they were negatively correlated ([r.sup.2] = -0.33, P < 0.05) to GSI.

[FIGURE 8 OMITTED]

The significantly highest concentrations of acylglycerides in digestive glands were observed from March to June (Fig. 8), whereas in the other months the values were significantly lower. Acylglycerides in digestive glands were negatively correlated to GSI ([r.sup.2] = -0.41. P < 0.05).

DISCUSSION

Gonad Maturation Process

Based on histologic data, the current study shows that the reproductive cycle of Nodipecten subnodosus may be divided into four periods: (1) period of quiescence from December to March (with a small reproductive activity in February-March), (2) period of initial reproductive activity from April to May, (3) period of maximum ripeness from June to September, and (4) period of spawning from September to November. This pattern is in accordance with previous studies of this species at the same locality (Reinecke-Reyes 1996) and at a southern locality (Racotta et al. 2003). However, in addition to the main spawning period, an earlier minor spawning is common in other species such as Mya arenaria (Brousscau 1987), Pecten maximus, Pacopecten magellanicus, and in the genus Chlamys (see Thompson 1977). Such a spawning was considered as facultative because it occurs only when environmental conditions are favorable (Brousseau 1987). In this sense, it is possible that IV. subnodosus from Laguna Ojo de Liebre could begin a facultative spawning, as a slight reproductive activity was observed in February-March as indicated by the presence of some partially spawned scallops and by an increase in their oocyte diameter. However, in this period the IGS was low (<7%) as a consequence of the high proportion of spent (40%) and the presence of undifferentiated scallops (30%).

The positive correlations between all biochemical compounds analyzed in the ovary portion and the carbohydrates in the testis portion and GSI reflected their close relation to the reproductive activity. In relation to protein content in ovaries of pectinids, two patterns have been observed. Some studies showed an important accumulation of proteins in gonads during the ripening process (Barber & Blake 1981, Epp et al. 1988), whereas other studies reported that levels of protein in gonad tissue did not increase during gametogenesis (Thompson 1977, Couturier & Newkirk 1991, Pazos et al. 1996, Racotta et al. 1998, Ruiz-Verdugo et al. 2001. Racotta et al. 2003). However, these patterns probably are not a species-specific strategy, because N. subnodosus shows both patterns depending on the locality (Racotta et al. 2003 vs. current study). In the testis portion, a different variation in the protein content was found, as it decreased in accordance with the ripening process in this study, in other functional hermaphrodite pectinids, contrasting results have been obtained. In P. maximus, the protein contents of the testis and ovary portions were nearly identical throughout the study (Lubet et al. 1987), whereas in Argopecten purpuratus, the protein level was higher in the female than in the male portion of the gonad in mature organisms (Martinez 1991). A possible explanation of our results is that proteins were transferred from the testis portion to the ovary portion during ripening and accumulated as yolk. Further work is necessary in functional hermaphroditic bivalves to test this hypothesis.

Despite that previous studies in pectinids have not reported an increase in the levels of carbohydrates in gonads associated with reproductive activity (Barber & Blake 1981, Couturier & Newkirk 1991, Martinez 1991, Pazos et al. 1996), our results indicated a significant increase during the entire maturation process and a decrease in the spawning period. It has been suggested that the accumulated carbohydrates in developing gonads can be used as an immediate available energy source for lipid synthesis or spawning (Racolta et al. 1998, Ruiz-Verdugo et al. 2001, Racotta et al. 2003) and also as precursors for lipid synthesis during the formation of gametes, which is known to occur in mollusks (Gabbott 1975. Lubet 1976, Barber & Blake 1985). Our results suggest that in N. subnodosus, the carbohydrates of testis could be used as an energy source for sperm motility.

The accumulation of total lipids and acylglycerides that we found in the ovary portion is an important and well documented process occurring during ripening in scallops (Barber & Blake 1991, Couturier & Newkirk 1991, Martinez 1991, Pazos et al. 1997, Racotta et al. 2003). These reserves are stored in ripening eggs and may subsequently be used by the larvae as an energy source during the first life stages of growth and metamorphosis (Holland 1978, Barber & Blake 1991, Couturier & Newkirk 1991). In contrast, in our results, acylglycerides levels in testis decreased significantly in parallel with the ripening process, and their concentration was negatively correlated to GSI. As for proteins, acylglycerides could be transferred from the testis portion to the ovary portion during the ripening process for the accumulation in gametes as yolk. In P. magellanicus (Thompson 1977, Robinson et al. 1981, Couturier & Newkirk 1991), Chlamys septemradiata (Ansell 1974), and A. purpuratus (Martinez 1991), the ovarian lipid content was higher than in the testis portion in ripe organisms.

Energy and Nutrient Supply for Gametogenesis

It is known that the mollusk digestive gland is responsible for nutrient storage and transfer of assimilated food to body tissues, and that the initiation of the oocyte growth phase is dependent on the accumulation and transfer of nutrient reserves from the digestive gland to the gonad (Sastry & Blake 1971, Gabbolt & Bayne 1973, Vassallo 1973, Barber & Blake 1983, Pazos et al. 1997, Lodeiros et al. 2001). The variations of lipid, acylglyceride, and protein concentrations during gonad ripening observed in this study are in accordance with this transfer of energy and nutrients to the gonad for ripening in N. subnodosus. The rapid decrease in protein content between March and May (phase of early oocyte development) indicates that this substrate could be used for structural purposes during oocyte differentiation. On the other hand, the decline of lipids and acylglycerides between July and October indicated that they are used for yolk formation. Carbohydrates tend to increase slightly in the digestive gland, despite the development of gonads, thus suggesting that this organ did not transfer carbo hydrates to the gonad. In contrast to the apparent mobilization of proteins and lipids, Racotta et al. (2003) did not obtain any changes of these substrates in the digestive gland of N. subnodosus in Bahia Magdalena during gonad development.

The storage and mobilization of metabolic substrates from adductor muscle to the gonad during gametogenesis has been reported in bivalves, particularly for several pectinids such as A. irradians (Barber & Blake 1981, Epp et al. 1988), Chlamys opercularis (Taylor & Venn 19791, P. maximus (Comely 1974), A. nentricosus (Racotta et al. 1998), A. purpuratus (Martinez 1991), and N. subnodosus (Racotta et al. 2003). The depletion of proteins observed in adductor muscle from March to August suggests that lion's paw scallops use this reserve for supporting the whole process of gametogenesis. These results are in agreement with those reported for this (Racotta et al. 2003) and other pectinids, including P, maximus (Comely 1974), C, opercularis (Taylor & Venn 1979), A. irradians (Epp et al. 1988), P. magellanicus (Couturier & Newkirk 1991, Faveris & Lubet 1991), and A. ventricosus (Racotta et al. 1998). However, these results contrast with those of Barber and Blake (1981, 1985) and Brokordt et al. (2000), who suggest that protein reserves are used only to meet the cost of maintenance during final ripening and spawning.

In contrast, muscle carbohydrates increased from February to June, when oocytes started growing, but concentration decreased significantly during the final ripening phase (July to October) and during spawning (September to November). From the above, it is evident that N. subnodosus in Laguna Ojo de Liebre used carbohydrates stored in muscle to meet the final ripening and spawning needs.

The relative importance of the role of adductor muscle protein and carbohydrate reserves in supporting energy demands during reproduction in scallops is controversial. In some pectinids, the adductor muscle protein appears as the major energy reserve, which supplies the demand during gametogenesis (Barber & Blake 1985, Epp et al. 1988, Couturier & Newkirk 1991, Faveris & Lubet 1991, Pazos et al. 1997, Racotta et al. 1998, Racotta et al. 2003). In other species, the energy demands during gametogenesis are mainly supported by adductor muscle glycogen (Comely 1974, Taylor & Venn 1979, Barber & Blake 1981, Robinson et al. 1981).

The mantle tissue as a storage organ and its relation to gametogenesis has been observed in mussels (Gabbott 1975, Bayne et al. 1982, Mathieu & Lubet 1993) and scallops (Lodeiros et al. 2001). In the current study, a decrease in mantle proteins during early gonad development (March-May), as well as a further decrease in mantle carbohydrates during final ripening and spawning (June-November), may be considered as evidence that the lion's paw scallops store nutrients in the mantle to be transferred to the gonad during ripening.

The seasonal changes of biochemical composition in several somatic tissues of the lion's paw scallop N. subnodosus in this locality indicate a clear energy storage and use cycle strongly related to reproductive activity, as is common in other pectinids (Gabbott 1975, Barber & Blake 1991, Mathieu & Lubet 1993). This is further supported by significant negative correlations between several biochemical components in somatic tissues and GSI, as well as by the significant negative correlations of all somatic organ indices with GSI. The relation between organ indices and gonad development have been reported for scallops as an indicator of energy storage and use patterns (Barber & Blake 1981, Barber & Blake 1983, Villalejo-Fuerte & Ceballos--Vazquez 1996, Barrios 1997).

The current study shows that gametogenesis in N. subnodosus, at the locality of Laguna Ojo de Liebre, relies on previously stored reserves and thus behaves as a conservative species (Bayne 1976). In contrast, the results obtained by Racotta et al. (2003) for the same species, but at a southern locality (Bahia Magdalena), indicate that reproductive activity depends minimally on previously stored reserves, showing an opportunistic behavior (Bayne 1976). It is known that both strategies can be adopted by the same species In support gametogenesis, depending on food availability on a specific locality, which in turn depends on environmental variables that influence primary productivity (Barber & Blake 1991, Thompson & McDonald 1991, Luna-Gonzalez et al. 2000).

In this sense, Bahia Magdalena is considered a eutrophic lagoon with high primary productivity (Gomez-Gutierrez et al. 2001), whereas our results show that Laguna Ojo de Liebre has a low food availability (maximum 1.2 [micro]g [L.sup.-1] chlorophyll a) year round, which agrees with previous works (Millan et al. 1987, Delgadillo-Hinojosa et al. 2002). Additionally, the food index (FI) values observed in our work (maximum 17%) can be considered as indicative of a low nutritional quality of the food available to suspension-feeders. Sites considered with a nutritionally dilute environment had FI values between 8% and 11% (Soniat et al. 1984, Luna-Gonzalez et al. 2000), whereas sites with a nutritionally rich environment had FI values as high as 55% (Navarro & Thompson 1995).

This could explain the contrasting results between our study and the previous work of Racolta et al. (2003). Then, the reproductive strategy (opportunistic vs. conservative) of N. subnodosus could be considered as an adaptation to food availability in a particular environment. A similar switch between both strategies was reported for A. ventricosus (Luna-Gonzalez et al. 2000) and P. maximus (Pazos et al. 1997), which use the available food in the environment, more than reserves, for gonadal ripening when the food is abundant, but they use the muscle reserves when the food abundance is poor.

Finally, changes in temperature, salinity, or photoperiod have been considered as physical environmental variables inducing spawning of bivalves (Mann 1979, Lubet et al. 1987). Park et al. (2001) mentioned that food availability also may have an effect on the initiation of spawning, and Starr et al. (1990) demonstrated that phytoplankton levels during blooms should be sufficient to induce spawning in Strongylocentrotus droebachiensis and Mytilus edulis. For other scallops, it has been suggested that spawning may be triggered by changes in temperature (Luna-Gonzalez et al. 2000) or phytoplankton blooms (Arsenault & Himmelman 1998). In N. subnodosus from Laguna Ojo de Liebre, the start of spawning coincides with the higher concentrations of chlorophyll a; then it is possible that the spawning of N. subnodosus was triggered by the higher concentrations of chlorophyll a, which, additionally, may ensure the food supply for larvae (Newell et al. 1982).

ACKNOWLEDGMENTS

We are grateful to Compania. Exportadora de Sal, Sociedad Cooperativa Pescadores Unidos de Guerrero Negro, and Reserva de la Biosfera El Vizcaino. This study was supported by the following projects: CGPI-200229, CGP1-2002072, PAC15, and SEMARNAT-CONACyT 2002-C01-0277. We are grateful to Institurn Politecnico Nacional for the grants (EDI and COFFA) to the authors. M. A. M. is a doctoral student-fellow of CONACyT. Our thanks also to Real English Agency for editing the manuscript.

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M. ARELLANO-MARTINEZ, (1), * I. S. RACOTTA, (2) B. P. CEBALLOS-VAZQUEZ, (1) AND J. F. ELORDUY-GARAY (1)

(1) Centro Interdisciplinario de Ciencias Marinas, Instituto Politecnico Nacional, La Paz, Baja California Sur 23000, Mexico; (2) Centro de Investigaciones Biologicas del Noroeste, La Paz, Baja California Sur 23000. Mexico

* Corresponding author. Fax: [phone]: E-mail: marellam@ ipn.mx