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Biochemical composition and energy allocation in the tropical scallop Lyropecten (Nodipecten) nodosus during the months leading up to and following the development of gonads
Aquaculture 199 Ž2001. 63–72 www.elsevier.nlrlocateraqua-online
Biochemical composition and energy allocation in the tropical scallop Lyropecten žNodipecten/ nodosus during the months leading up to and following the development of gonads Cesar ´ J. Lodeiros a,) , Jose´ J. Rengel a, Helga E. Guderley b, Osmar Nusetti c , John H. Himmelman b a
Lab. Acuicultura, Dept. de Biologia Pesquera, Instituto Oceanografico de Venezuela, UniÕersidad de ´ Oriente, Apdo Postal 245, Cumana´ Estado Sucre 6101, Venezuela b Departement de Biologie, UniÕersite´ La Õal, Quebec City, Quebec, Canada G1K 7P4 ´ c Departamento de Biologıa, ´ UniÕersidad de Oriente, Cumana´ 6101, Venezuela Received 27 April 2000; accepted 13 December 2000
Abstract We quantified biochemical constituents of the major body components of the scallop Lyropecten nodosus Žinitially measuring 9.4 mm in shell height. cultured at 8, 21 and 34 m in depth in the Golfo de Cariaco, Venezuela, to evaluate the allocation and mobilization of energy in the organisms during periods of somatic and gonadal growth, and during periods of environmental stress. A marked decrease in muscle carbohydrates with depth was associated with a general decrease in the growth rate of the scallops. Protein in the digestive gland and carbohydrates in the muscle and remaining tissues dropped during maximal gonadal growth, suggesting that these tissues contributed energy for gonadal production. Furthermore, during the reproductive period, scallops at 21 m made greater use of lipids in the digestive gland than individuals at 8 m, probably due to the decreased availability of phytoplankton at 21 m. During the last 2 months of our study, when scallops that were energetically depleted by reproduction were faced with low food availability, high temperatures and colonization by fouling organisms, protein levels in remaining tissues decreased and lipid levels stabilized. The relative importance of growth and deposition of
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Corresponding author. Tel.: q58-93-512276; fax: q58-93-315902. E-mail address: [email protected] ŽC.J. Lodeiros..
0044-8486r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 4 - 8 4 8 6 Ž 0 1 . 0 0 5 0 5 - 1
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C.J. Lodeiros et al.r Aquaculture 199 (2001) 63–72
reserves in somatic tissues was influenced by reproductive state and prevailing biological and thermal conditions. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Scallop; Lyropecten nodosus; Biochemical composition; Growth; Venezuela
1. Introduction The biochemical composition of marine bivalves is affected by exogenous factors, such as food availability and temperature, and endogenous factors, such as energetic demands for reproduction ŽGiese and Pearse, 1974; Gabbott, 1983; Martinez, 1991; Thompson and MacDonald, 1991; Sarkis, 1993; Claereboudt and Himmelman, 1996; Boadas et al., 1997.. However, how tissue biochemical composition changes as individuals grow has not been well studied. Tissue biochemical composition could remain constant as size increases until reproductive maturity. Then, when somatic growth slows in favor of reproductive investment, the biochemical composition of somatic tissues may change according to the requirements of reproduction. Alternately, increases in the size of bivalves could lead to shifts in tissue biochemical composition, if growth is accompanied by shifts in habitat use, food availability and the relative size of different tissues. For example, the increased effort needed for swimming by larger scallops may explain the shift to a relatively larger and more centrally positioned adductor muscle ŽManuel and Dadswell, 1993.. Once reproductive maturity is attained, the use of assimilated material for somatic or reproductive growth will vary according to environmental conditions and food availability ŽMartinez, 1991; Almeida et al., 1997; Lodeiros and Himmelman, 2000.. Thus, the allocation of assimilated energy may vary during growth, and is certain to shift with reproductive maturity. We previously reported how depth-related environmental factors markedly affect the growth and survival of Lyropecten Ž Nodipecten. nodosus, initially measuring 9.4 mm in shell height, maintained in suspended culture in the Golfo de Cariaco, Venezuela ŽLodeiros et al., 1998.. Growth rates decreased with the reduction in phytoplankton availability with depth. The growth of somatic tissues slowed during gonadal production and periods of unfavorable environmental conditions. In the present paper, we examine changes in the biochemical composition of the major body components of L. nodosus cultured at 8, 21 and 34 m in depth in the above study to evaluate Ž1. the impact of growth on the biochemical composition of the body components, Ž2. the relative importance of various body components for storage of carbohydrate, protein and lipid reserves, Ž3. the allocation of assimilated energy between somatic and gonadal production, and Ž4. the effects of environmental stress on energy allocation. This study is one of few studies that focuses on biochemical changes and energy allocation in a bivalve during the months leading up to and following the development of gonads. L. nodosus is an hermaphroditic bivalve found in the tropical Carribean and as far south as Rio de Janeiro ŽAbbott, 1974; Smith, 1991; Lodeiros Seijo et al., 1999.. Although this species usually occurs in low densities in natural habitats, and thus does not support a fishery, it can attain a large size Žf 130 mm in shell height. and the large adductor muscle is attractive for human consumption. It is thus a potential species for aquaculture ŽVelez ´ and Lodeiros, 1990..
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2. Methods Our study was conducted from 18 December 1993 to 20 July 1994 at the Turpialito Station Ž10826X 56Y N, 64802X 00Y W. of the Instituto Oceanografico de Venezuela of the ´ Universidad de Oriente, in the Golfo de Cariaco, northeastern Venezuela. Juveniles with a mean shell height of 9.4 mm ŽSD s 1.3. were placed in 6-mm mesh pearl nets Ž35 = 35 cm bottoms. and suspended at 8, 21 and 34 m in depth from a long line located at f 300 m from shore. Three pearl nets, each containing 90 juveniles, were placed at each depth. Variations in shell height among treatments at the onset of the experiment Ž18 December 1993. were not significant ŽANOVA, P s 0.31.. At about monthly intervals, all scallops from each treatment Ždepth. were collected and combined in a large tank and then 10–15 individuals were randomly selected for determinations of tissue masses and biochemical composition. The remaining scallops were returned to the same experimental depths in new pearl nets. The density per net was reduced as the scallops grew and adjusted so that the maximum cover of the pearl net floor did not exceed 30%. The density at any given date was the same at all depths. Dry mass of muscle, digestive gland, gonad and remaining soft tissues was obtained by drying at 608C for 72 h. To explore changes in tissue composition during the culture at different depths, we quantified protein, carbohydrate and lipid contents for 10 randomly selected scallops at each depth on each sampling date Žexcept n s 5 at 34 m in July. using colorimetric methods. For each scallop, we initially pulverized the dry tissues and then homogenized a 10-mg sample in 1 ml of de-ionized water. Subsamples of the homogenate were taken for the biochemical determinations. Protein concentrations were determined by the Lowry method ŽLowry et al., 1951., using bovine serum albumin as the standard. Carbohydrates were quantified using the phenol-sulphuric acid method ŽDubois et al., 1956. and lipids were measured with phosphovanillin reagent, using cholesterol as a standard ŽBligh and Dryer, 1959; Postma and Stroes, 1968.. To test for differences among depth treatments on each sampling date, we compared the values for each biochemical component of each body part at the three depths, using one way ANOVAs followed by post hoc comparisons using Scheffe´ test Ž a s 0.05.. The same procedure was used to compare different sampling dates at each depth. Prior to performing these analyses, the data were tested for normality using the Shapiro–Wilk W test ŽZar, 1984.. Homogeneity of variances was verified by graphically examining the distribution of variance residues. In cases where the dispersion of residues was relatively high, the data were arcsine transformed so that variances became homogeneous. 3. Results 3.1. Mass of body components The increase in tissue mass of the scallops was greatest at 8 m in depth, intermediate at 21 m and least at 34 m ŽFig. 1.. At 8 and 21 m in depth, a rapid increase in the mass of somatic tissues Žmuscle, digestive gland and remaining tissues. occurred during the first 5 months. Then during the last 2 months, tissue growth tended to slow at 21 m in depth, and decreases in mass occurred at 8 m.
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Gonadal production was first noted on the same date, 21 February 1994, at 8 and 21 m depth, even though somatic tissue mass was considerably greater at 8 m than at 21 m. Thereafter, at 8 and 21 m, gonadal mass increased until 24 May 1994 progressively ŽANOVAs, Scheffe´ tests, P - 0.05., and marked increases occurred between 22 April and 24 May 1994. Subsequent drops in gonadal mass indicated spawning. On all dates, gonadal mass was greater at 8 than at 21 m in depth. At 34-m depth, gonads were only observed on the last sampling date, 20 July 1994. Thus, our data permitted evaluation of the biochemical composition of scallop tissues in the months leading up to and following the development of gonads and comparisons of the tissue composition of scallops with and without gonads Žscallops at 8 and 21 m compared to scallops at 34 m.. 3.2. Biochemical components As the relative protein content of the muscle was similar at the three depths and also over time ŽANOVAs, P ) 0.05. ŽFig. 1., it was little affected by the requirements of growth and reproduction. Mean values were always over 345 mg gy1 dry mass. In contrast, carbohydrate content varied markedly with depth, decreasing from 8 to 34 m ŽANOVAs, Scheffe´ tests, P - 0.05., particularly during the phase of limited reproductive investment Žprior to 22 April.. At each depth, and particularly at 8 and 21 m, carbohydrates decreased in the latter part of the study ŽScheffe´ tests, P - 0.05.. The decreases between 22 April and 24 May 1994 coincided with the period of high gonadal production, which suggested mobilization of muscle carbohydrate for gametogenesis. The further decreases in carbohydrate levels during June and July coincided with a period of unfavorable environmental conditions Žlow food availability and high temperatures, Lodeiros et al., 1998.. The high carbohydrate levels attained in the muscle at certain periods and marked changes with depth and over time suggested that muscle carbohydrate was used as an energetic reserve or transferred to support gamete production. Lipid levels in the muscle increased progressively during the study at the three depths. Protein levels in the digestive gland varied markedly at both 8 and 21 m. They were ) 360 mg gy1 dry mass until late March 1994, decreased significantly ŽScheffe´ tests, P - 0.05. during April and May Žby 203 and 170 mg gy1 dry mass at 8 and 21 m, respectively., and then increased ŽScheffe´ tests, P - 0.05. by late July. The decreases in protein during April and May coincided with gonadal growth. In contrast, at 34 m in depth where gonadal development was delayed until the end of our study, protein levels did not vary ŽANOVA, P ) 0.05; the mean level was 352 mg gy1 dry mass.. Lipid levels rose steadily until late May, showing the opposite changes as protein levels ŽFig. 1.. Carbohydrate levels were similar at the three depths and did not vary markedly over time, only changing significantly ŽScheffe´ test, P - 0.05. during the latter part of the Fig. 1. L. nodosus. Total dry mass of the muscle, digestive gland, remaining somatic tissues, gonad, protein, carbohydrate and lipid composition of each body component, for scallops, initially measuring 9.4 mm in shell height, maintained in suspended culture at 8, 21 and 34 m in depth at Turpialito, in the Golfo de Cariaco, Venezuela, from 18 December 1993 to 20 July 1994 Žsampling began in January 1994.. Vertical lines represent standard errors Ž ns10 on all instances except at 34 m in July where ns 5..
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study at 34 m in depth. Lipids were considerably more concentrated in the digestive gland than in other organs and differences among depths were marked Žgenerally decreasing with depth.. These observations suggest that a major role of the digestive gland is lipid storage. At all three depths, the lipid concentration in the digestive gland increased until 24 May 1994 and then remained stable except at 21 m where a decrease occurred ŽScheffe´ test, P - 0.05.. The lipid storage role of the digestive gland started during early growth and was generally maintained even during periods of marked gonadal growth. The biochemical composition of the remaining somatic tissues did not differ markedly with depth. The major changes over time were decreases in carbohydrates at 8 and 21 m between 22 April and 24 May 1994 ŽScheffe´ tests, P - 0.05., the period of most rapid gonadal growth, and in proteins at 8 and 21 m during the last 2 months of the study ŽScheffe´ tests, P - 0.05. ŽFig. 1.. During the first months of our study, lipid levels tended to increase at 8 and 21 m and varied irregularly at 34 m, and thereafter were almost stable at all depths. Relative carbohydrate levels of the gonads showed only minor changes from late February 1994, when gonad tissues were first present, until the end of the study. In contrast, lipid and protein levels showed inverse changes during the period of gonadal growth, i.e., February through May 1994.
4. Discussion The biochemical composition of the body components of the tropical scallop L. nodosus cultured in the Golfo de Cariaco revealed tissue specific changes during growth. The drop in muscle carbohydrate levels with depth was concomitant with a general decrease in the growth of the scallops. Similarly, lipid levels in the digestive gland of the scallops showed an inverse relation with depth. We previously showed that the decreasing growth rate with depth was likely explained by decreasing food availability with depth Žmean chlorophyll a levels were 6.8 times greater at 8 than at 34 m; Lodeiros et al., 1998.. These observations support the suggestion by Robinson et al. Ž1981. that muscle carbohydrates reflect the condition of scallops. Thus, in periods of high food availability, scallops do not simply accrue more tissues of a constant biochemical composition; rather they deposit glycogen in muscle and lipids in the digestive gland ŽRobinson et al., 1981., perhaps in view of subsequent protein synthesis in other tissues ŽFig. 2.. Once gonads become apparent in L. nodosus, levels of muscle carbohydrates and digestive gland proteins drop during periods of maximal gonadal production, suggesting that gonadal production was supported by these reserves. Interestingly, lipid reserves in the digestive gland were spared. The greater stability of tissue levels of biochemical constituents in the scallops cultured at 34 m was likely due to their lack of reproductive activity. The muscle and digestive gland have been indicated as storage organs for various bivalves ŽGabbott, 1983; Barber and Blake, 1991.. Other studies of scallops report that gonad growth is mainly supported by muscle reserves ŽTaylor and Venn,
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Fig. 2. Variations in temperature ŽA., total dry seston ŽB., organic content of the seston ŽC. and chlorophyll a concentration ŽD. from December 1993 to October 1994 at 8, 21 and 34 m in depth at Turpialito, Golfo de Cariaco, Venezuela Žafter Lodeiros et al., 1998..
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1979; Barber and Blake, 1991; Epp et al., 1988; Faveris and Lubet, 1991; Couturier and Newkirk, 1991.. We further documented decreases in carbohydrate in the remaining somatic tissues of L. nodosus between late April and late May, when gonadal mass more than doubled, suggesting that reserves were also withdrawn from these components. Although the remaining somatic tissues mainly represent organs involved in maintenance Že.g., mantle, gills and excretory organs., the mantle may contribute energy for gonadal production as suggested for several other bivalves Žde Zwaan and Mathieu, 1992., including pectinids ŽEpp et al., 1988.. In L. nodosus, digestive gland lipids were controlled independently of muscle carbohydrates. Scallops cultured at 21 m virtually depleted muscle carbohydrates during gonadal growth, but simultaneously increased digestive gland lipids Žthereby decreasing relative levels of digestive gland proteins. and muscle size. At 8 m, muscle carbohydrates were only slightly decreased during gonadal growth, while the mass of the digestive gland and muscle rose markedly. The greater food availability at 8 than 21 m ŽLodeiros et al., 1998. may have contributed to the faster growth of scallops at 8 m and limited depletion of somatic reserves during gonadal production. The greater depletion of muscle carbohydrates and digestive gland lipids during gonadal growth at 21 m than at 8 m supports the hypothesis that gonadal growth is favored over somatic growth when food resources are limited during reproductive periods in bivalves ŽBrowne and RusselHunter, 1978; Toumi et al., 1983.. Such preferential allocation of reserves to gonadal growth also was suggested for EuÕola ziczac in the Golfo de Cariaco ŽLodeiros and Himmelman, 2000.. During the latter part of our study, the water column was stratified and characterized by high temperatures and low phytoplankton biomass ŽLodeiros et al., 1998.. Such conditions decrease physiological capacities in the scallop E. ziczac in natural beds in the Golfo de Cariaco ŽBoadas et al., 1997.. L. nodosus may be particularly susceptible to such stressful conditions when they are weakened due to reproduction. Both at 8 and 21 m, L. nodosus released their gametes during the last 2 months of our study, apparently in association with a rise in temperature ŽLodeiros et al., 1998.. The growth of fouling organisms on the scallops, particularly at 8 m ŽLodeiros et al., 1998., may have exacerbated the effects of high temperatures and low food availability. During the last 2 months, protein levels in the remaining tissues decreased, muscle carbohydrates of the scallops at 8 and 21 m were depleted and digestive gland lipids in scallops at 21 m decreased. Whereas lipid levels in scallops increased during the period of rapid gonadal growth, they reached a plateau or even decreased in most body components Žexcept muscle. during the last 2 months. This generalized depletion of somatic reserves is suggestive of energetic limitation, possibly related to stress associated with stratification of the water column and low phytoplankton levels. This suggestion is strengthened by the total mortality that occurred 1 month later at 8 m ŽLodeiros et al., 1998.. Our study of energy storage and mobilization in relation to reproduction and environmental conditions only covered the period of growth up to a shell length of 35 mm Žat 34 m. to 56 mm Žat 8 m., which is much smaller than the size which L. nodosus attains in natural populations Ž110–130 mm.. Further studies are required to evaluate if energy allocation undergoes additional modifications as scallops further increase in size. In scallops, reproductive output often continues to increase throughout the lifetime of the
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animal, whereas somatic production remains constant or declines ŽThompson and MacDonald, 1991.. Our study indicates that the muscle and digestive gland are the principal deposits of energetic reserves in L. nodosus. Reproductive output can be supported by reserve mobilization from various tissues, particularly when food availability is reduced. Decreases in the protein levels of remaining somatic tissues seem related to exposure to stressful conditions. Carbohydrate levels of the muscle and lipid levels in the digestive gland of L. nodosus would appear to be useful indices of the condition of the scallops. However, our conclusions are based upon individuals grown in culture conditions, where stress could be due, in part, to the culture techniques. Our conclusions for scallops with developing or recently developed gonads may not be representative of the allocation of energetic reserves in large individuals. Effectively, the use of assimilated energy for both growth and reproduction distinguishes smaller scallops from larger ones in which assimilated energy is used primarily for reproduction ŽThompson and MacDonald, 1991.. Studies on partitioning of energy in L. nodosus at larger sizes are needed to corroborate our conclusions.
Acknowledgements This study was possible due to the facilities provided by the Instituto Oceanografico ´ de Venezuela, Universidad de Oriente. We are particularly indebted to M. Nunez ˜ for his help with the fieldwork. The research was supported by grants from the Consejo de Investigacion ´ de la Universidad de Oriente to C.J.L and the National Sciences and Engineering Council of Canada to H.E.G. and J.H.H.
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de Zwaan, A., Mathieu, M., 1992. Cellular biochemistry and endocrinology. In: Gosling, E. ŽEd.., The Mussel Mytilus: Ecology, Physiology, Genetics and Culture. Developments in Aquaculture and Fisheries Science, vol. 25, Elsevier, New York, pp. 223–307. Dubois, M., Gilles, J., Hamilton, J., Reber, P., Smith, F., 1956. Colorimetric method for determination of sugars and related substances. Anal. Chem. 28, 2350–2356. Epp, J., Bricelj, M., Malouf, R., 1988. Seasonal partitioning and utilization of energy reserves in two age classes of the bay scallop Argopecten irradians irradians ŽLamarck.. J. Exp. Mar. Biol. Ecol. 121, 112–136. Faveris, R., Lubet, P., 1991. Energetic requirements of the reproductive cycle in the scallop Pecten maximus ŽLinnaeus, 1758. in Baie de Seine ŽChannel.. In: Shumway, S.E., Sandifer, P.A. ŽEds.., An International Compendium of Scallop Biology and Culture. World Aquaculture Workshops, vol. 1, World Aquaculture Society, Baton Rouge, LA, pp. 67–73. Gabbott, P.A., 1983. Developmental and seasonal metabolic activities in marine molluscs. In: Hochachka, P.W. ŽEd.., The Mollusca, vol. 2, Academic Press, New York. Giese, A.C., Pearse, J.S., 1974. Introduction: general principles. In: Giese, A.C., Pearse, J.S. ŽEds.., Reproduction of Marine Invertebrates, vol. I, Academic Press, New York, pp. 1–49. Lodeiros, C.J.M., Himmelman, J.H., 2000. Identification of factors affecting growth and survival of the tropical scallop EuÕola Ž Pecten. ziczac in suspended culture in the Golfo de Cariaco, Venezuela. Aquaculture 182, 91–114. Lodeiros, C.J., Rengel, J., Freites, L., Himmelman, J.H., 1998. Growth and survival of the tropical scallop Lyropecten Ž Nodipecten. nodosus maintained in suspended culture at three depths. Aquaculture 165, 41–50. Lodeiros Seijo, C.J.M., Marin Espinosa, B., Prieto Arcas, A., 1999. Catalogo de moluscos marinos de las ´ costas nororientales de Venezuela, clase Bivalvia. Edicion ´ APUDONS, 109 pp., 9 Plates. Lowry, O., Rosebrough, N., Farr, A., Randall, R., 1951. Protein measurement with the Folin phenol reagent. J. Biol. Chem. 193, 265–275. Manuel, J.L., Dadswell, M.J., 1993. Swimming of juvenile sea scallops, Placopecten magellanicus ŽGmelin.: a minimum size for effective swimming? J. Exp. Mar. Biol. Ecol. 174, 137–175. Martinez, G., 1991. Seasonal variation in biochemical composition of three size classes of Chilean scallop Argopecten purpuratus Lamarck, 1819. Veliger 34, 335–343. Postma, T., Stroes, A.P., 1968. Lipid screening in clinical chemistry. Chem. Acta 22, 569–578. Robinson, W.E., Wehling, W.E., Morse, M.P., McLeod, C.G., 1981. Seasonal changes in soft-body component indices and energy reserves in the Atlantic deep-sea scallop, Placopecten magellanicus. Fish. Bull. 79, 449–458. Sarkis, S., 1993. Seasonal changes in the gross biochemical composition of the turkey wing, Arca zebra ŽSwainson, 1883., in Bermuda. J. Shellfish Res. 12, 329–336. Smith, J.T., 1991. Cenozoic giant pectinids from California and Tertiary Caribbean Provinces: Lyropecten, Macrochlamys, Vertipecten, and Nodipecten species. U.S. Geological Survey Professional Paper, vol. 1391, U.S. Government Printing Office, Washington, 166 pp., 39 Plates. Taylor, A., Venn, T., 1979. Seasonal variations in weight and biochemical composition of tissues of the queen scallop Chlamys opercularis from the Clyde Sea Area. J. Mar. Biol. Assoc. U.K. 59, 605–621. Thompson, R.J., MacDonald, B.A., 1991. Physiological integrations and energy partitioning. In: Shumway, S.E. ŽEd.., Scallops: Biology, Ecology and Aquaculture. Developments in Aquaculture and Fisheries Science, vol. 21, Elsevier, pp. 347–376. Toumi, J., Hakala, T., Haukioja, E., 1983. Alternative concepts of reproductive effort, costs of reproduction and selection in life history evolution. Am. Zool. 23, 25–34. Velez, A., Lodeiros, C., 1990. El cultivo de moluscos en Venezuela. In: Hernandez, A. ŽEd.., Cultivo de ´ moluscos en America Latina. Red Regional de Entidades y Centros de Acuicultura de America Latina, ´ CIID-Canada, pp. 345–369. Zar, J.H., 1984. Biostatistical Analysis. Prentice-Hall, Englewoods Cliff, NJ.
Biochemical composition and energy allocation in the tropical scallop Lyropecten žNodipecten/ nodosus during the months leading up to and following the development of gonads Cesar ´ J. Lodeiros a,) , Jose´ J. Rengel a, Helga E. Guderley b, Osmar Nusetti c , John H. Himmelman b a
Lab. Acuicultura, Dept. de Biologia Pesquera, Instituto Oceanografico de Venezuela, UniÕersidad de ´ Oriente, Apdo Postal 245, Cumana´ Estado Sucre 6101, Venezuela b Departement de Biologie, UniÕersite´ La Õal, Quebec City, Quebec, Canada G1K 7P4 ´ c Departamento de Biologıa, ´ UniÕersidad de Oriente, Cumana´ 6101, Venezuela Received 27 April 2000; accepted 13 December 2000
Abstract We quantified biochemical constituents of the major body components of the scallop Lyropecten nodosus Žinitially measuring 9.4 mm in shell height. cultured at 8, 21 and 34 m in depth in the Golfo de Cariaco, Venezuela, to evaluate the allocation and mobilization of energy in the organisms during periods of somatic and gonadal growth, and during periods of environmental stress. A marked decrease in muscle carbohydrates with depth was associated with a general decrease in the growth rate of the scallops. Protein in the digestive gland and carbohydrates in the muscle and remaining tissues dropped during maximal gonadal growth, suggesting that these tissues contributed energy for gonadal production. Furthermore, during the reproductive period, scallops at 21 m made greater use of lipids in the digestive gland than individuals at 8 m, probably due to the decreased availability of phytoplankton at 21 m. During the last 2 months of our study, when scallops that were energetically depleted by reproduction were faced with low food availability, high temperatures and colonization by fouling organisms, protein levels in remaining tissues decreased and lipid levels stabilized. The relative importance of growth and deposition of
)
Corresponding author. Tel.: q58-93-512276; fax: q58-93-315902. E-mail address: [email protected] ŽC.J. Lodeiros..
0044-8486r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 4 - 8 4 8 6 Ž 0 1 . 0 0 5 0 5 - 1
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C.J. Lodeiros et al.r Aquaculture 199 (2001) 63–72
reserves in somatic tissues was influenced by reproductive state and prevailing biological and thermal conditions. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Scallop; Lyropecten nodosus; Biochemical composition; Growth; Venezuela
1. Introduction The biochemical composition of marine bivalves is affected by exogenous factors, such as food availability and temperature, and endogenous factors, such as energetic demands for reproduction ŽGiese and Pearse, 1974; Gabbott, 1983; Martinez, 1991; Thompson and MacDonald, 1991; Sarkis, 1993; Claereboudt and Himmelman, 1996; Boadas et al., 1997.. However, how tissue biochemical composition changes as individuals grow has not been well studied. Tissue biochemical composition could remain constant as size increases until reproductive maturity. Then, when somatic growth slows in favor of reproductive investment, the biochemical composition of somatic tissues may change according to the requirements of reproduction. Alternately, increases in the size of bivalves could lead to shifts in tissue biochemical composition, if growth is accompanied by shifts in habitat use, food availability and the relative size of different tissues. For example, the increased effort needed for swimming by larger scallops may explain the shift to a relatively larger and more centrally positioned adductor muscle ŽManuel and Dadswell, 1993.. Once reproductive maturity is attained, the use of assimilated material for somatic or reproductive growth will vary according to environmental conditions and food availability ŽMartinez, 1991; Almeida et al., 1997; Lodeiros and Himmelman, 2000.. Thus, the allocation of assimilated energy may vary during growth, and is certain to shift with reproductive maturity. We previously reported how depth-related environmental factors markedly affect the growth and survival of Lyropecten Ž Nodipecten. nodosus, initially measuring 9.4 mm in shell height, maintained in suspended culture in the Golfo de Cariaco, Venezuela ŽLodeiros et al., 1998.. Growth rates decreased with the reduction in phytoplankton availability with depth. The growth of somatic tissues slowed during gonadal production and periods of unfavorable environmental conditions. In the present paper, we examine changes in the biochemical composition of the major body components of L. nodosus cultured at 8, 21 and 34 m in depth in the above study to evaluate Ž1. the impact of growth on the biochemical composition of the body components, Ž2. the relative importance of various body components for storage of carbohydrate, protein and lipid reserves, Ž3. the allocation of assimilated energy between somatic and gonadal production, and Ž4. the effects of environmental stress on energy allocation. This study is one of few studies that focuses on biochemical changes and energy allocation in a bivalve during the months leading up to and following the development of gonads. L. nodosus is an hermaphroditic bivalve found in the tropical Carribean and as far south as Rio de Janeiro ŽAbbott, 1974; Smith, 1991; Lodeiros Seijo et al., 1999.. Although this species usually occurs in low densities in natural habitats, and thus does not support a fishery, it can attain a large size Žf 130 mm in shell height. and the large adductor muscle is attractive for human consumption. It is thus a potential species for aquaculture ŽVelez ´ and Lodeiros, 1990..
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2. Methods Our study was conducted from 18 December 1993 to 20 July 1994 at the Turpialito Station Ž10826X 56Y N, 64802X 00Y W. of the Instituto Oceanografico de Venezuela of the ´ Universidad de Oriente, in the Golfo de Cariaco, northeastern Venezuela. Juveniles with a mean shell height of 9.4 mm ŽSD s 1.3. were placed in 6-mm mesh pearl nets Ž35 = 35 cm bottoms. and suspended at 8, 21 and 34 m in depth from a long line located at f 300 m from shore. Three pearl nets, each containing 90 juveniles, were placed at each depth. Variations in shell height among treatments at the onset of the experiment Ž18 December 1993. were not significant ŽANOVA, P s 0.31.. At about monthly intervals, all scallops from each treatment Ždepth. were collected and combined in a large tank and then 10–15 individuals were randomly selected for determinations of tissue masses and biochemical composition. The remaining scallops were returned to the same experimental depths in new pearl nets. The density per net was reduced as the scallops grew and adjusted so that the maximum cover of the pearl net floor did not exceed 30%. The density at any given date was the same at all depths. Dry mass of muscle, digestive gland, gonad and remaining soft tissues was obtained by drying at 608C for 72 h. To explore changes in tissue composition during the culture at different depths, we quantified protein, carbohydrate and lipid contents for 10 randomly selected scallops at each depth on each sampling date Žexcept n s 5 at 34 m in July. using colorimetric methods. For each scallop, we initially pulverized the dry tissues and then homogenized a 10-mg sample in 1 ml of de-ionized water. Subsamples of the homogenate were taken for the biochemical determinations. Protein concentrations were determined by the Lowry method ŽLowry et al., 1951., using bovine serum albumin as the standard. Carbohydrates were quantified using the phenol-sulphuric acid method ŽDubois et al., 1956. and lipids were measured with phosphovanillin reagent, using cholesterol as a standard ŽBligh and Dryer, 1959; Postma and Stroes, 1968.. To test for differences among depth treatments on each sampling date, we compared the values for each biochemical component of each body part at the three depths, using one way ANOVAs followed by post hoc comparisons using Scheffe´ test Ž a s 0.05.. The same procedure was used to compare different sampling dates at each depth. Prior to performing these analyses, the data were tested for normality using the Shapiro–Wilk W test ŽZar, 1984.. Homogeneity of variances was verified by graphically examining the distribution of variance residues. In cases where the dispersion of residues was relatively high, the data were arcsine transformed so that variances became homogeneous. 3. Results 3.1. Mass of body components The increase in tissue mass of the scallops was greatest at 8 m in depth, intermediate at 21 m and least at 34 m ŽFig. 1.. At 8 and 21 m in depth, a rapid increase in the mass of somatic tissues Žmuscle, digestive gland and remaining tissues. occurred during the first 5 months. Then during the last 2 months, tissue growth tended to slow at 21 m in depth, and decreases in mass occurred at 8 m.
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Gonadal production was first noted on the same date, 21 February 1994, at 8 and 21 m depth, even though somatic tissue mass was considerably greater at 8 m than at 21 m. Thereafter, at 8 and 21 m, gonadal mass increased until 24 May 1994 progressively ŽANOVAs, Scheffe´ tests, P - 0.05., and marked increases occurred between 22 April and 24 May 1994. Subsequent drops in gonadal mass indicated spawning. On all dates, gonadal mass was greater at 8 than at 21 m in depth. At 34-m depth, gonads were only observed on the last sampling date, 20 July 1994. Thus, our data permitted evaluation of the biochemical composition of scallop tissues in the months leading up to and following the development of gonads and comparisons of the tissue composition of scallops with and without gonads Žscallops at 8 and 21 m compared to scallops at 34 m.. 3.2. Biochemical components As the relative protein content of the muscle was similar at the three depths and also over time ŽANOVAs, P ) 0.05. ŽFig. 1., it was little affected by the requirements of growth and reproduction. Mean values were always over 345 mg gy1 dry mass. In contrast, carbohydrate content varied markedly with depth, decreasing from 8 to 34 m ŽANOVAs, Scheffe´ tests, P - 0.05., particularly during the phase of limited reproductive investment Žprior to 22 April.. At each depth, and particularly at 8 and 21 m, carbohydrates decreased in the latter part of the study ŽScheffe´ tests, P - 0.05.. The decreases between 22 April and 24 May 1994 coincided with the period of high gonadal production, which suggested mobilization of muscle carbohydrate for gametogenesis. The further decreases in carbohydrate levels during June and July coincided with a period of unfavorable environmental conditions Žlow food availability and high temperatures, Lodeiros et al., 1998.. The high carbohydrate levels attained in the muscle at certain periods and marked changes with depth and over time suggested that muscle carbohydrate was used as an energetic reserve or transferred to support gamete production. Lipid levels in the muscle increased progressively during the study at the three depths. Protein levels in the digestive gland varied markedly at both 8 and 21 m. They were ) 360 mg gy1 dry mass until late March 1994, decreased significantly ŽScheffe´ tests, P - 0.05. during April and May Žby 203 and 170 mg gy1 dry mass at 8 and 21 m, respectively., and then increased ŽScheffe´ tests, P - 0.05. by late July. The decreases in protein during April and May coincided with gonadal growth. In contrast, at 34 m in depth where gonadal development was delayed until the end of our study, protein levels did not vary ŽANOVA, P ) 0.05; the mean level was 352 mg gy1 dry mass.. Lipid levels rose steadily until late May, showing the opposite changes as protein levels ŽFig. 1.. Carbohydrate levels were similar at the three depths and did not vary markedly over time, only changing significantly ŽScheffe´ test, P - 0.05. during the latter part of the Fig. 1. L. nodosus. Total dry mass of the muscle, digestive gland, remaining somatic tissues, gonad, protein, carbohydrate and lipid composition of each body component, for scallops, initially measuring 9.4 mm in shell height, maintained in suspended culture at 8, 21 and 34 m in depth at Turpialito, in the Golfo de Cariaco, Venezuela, from 18 December 1993 to 20 July 1994 Žsampling began in January 1994.. Vertical lines represent standard errors Ž ns10 on all instances except at 34 m in July where ns 5..
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study at 34 m in depth. Lipids were considerably more concentrated in the digestive gland than in other organs and differences among depths were marked Žgenerally decreasing with depth.. These observations suggest that a major role of the digestive gland is lipid storage. At all three depths, the lipid concentration in the digestive gland increased until 24 May 1994 and then remained stable except at 21 m where a decrease occurred ŽScheffe´ test, P - 0.05.. The lipid storage role of the digestive gland started during early growth and was generally maintained even during periods of marked gonadal growth. The biochemical composition of the remaining somatic tissues did not differ markedly with depth. The major changes over time were decreases in carbohydrates at 8 and 21 m between 22 April and 24 May 1994 ŽScheffe´ tests, P - 0.05., the period of most rapid gonadal growth, and in proteins at 8 and 21 m during the last 2 months of the study ŽScheffe´ tests, P - 0.05. ŽFig. 1.. During the first months of our study, lipid levels tended to increase at 8 and 21 m and varied irregularly at 34 m, and thereafter were almost stable at all depths. Relative carbohydrate levels of the gonads showed only minor changes from late February 1994, when gonad tissues were first present, until the end of the study. In contrast, lipid and protein levels showed inverse changes during the period of gonadal growth, i.e., February through May 1994.
4. Discussion The biochemical composition of the body components of the tropical scallop L. nodosus cultured in the Golfo de Cariaco revealed tissue specific changes during growth. The drop in muscle carbohydrate levels with depth was concomitant with a general decrease in the growth of the scallops. Similarly, lipid levels in the digestive gland of the scallops showed an inverse relation with depth. We previously showed that the decreasing growth rate with depth was likely explained by decreasing food availability with depth Žmean chlorophyll a levels were 6.8 times greater at 8 than at 34 m; Lodeiros et al., 1998.. These observations support the suggestion by Robinson et al. Ž1981. that muscle carbohydrates reflect the condition of scallops. Thus, in periods of high food availability, scallops do not simply accrue more tissues of a constant biochemical composition; rather they deposit glycogen in muscle and lipids in the digestive gland ŽRobinson et al., 1981., perhaps in view of subsequent protein synthesis in other tissues ŽFig. 2.. Once gonads become apparent in L. nodosus, levels of muscle carbohydrates and digestive gland proteins drop during periods of maximal gonadal production, suggesting that gonadal production was supported by these reserves. Interestingly, lipid reserves in the digestive gland were spared. The greater stability of tissue levels of biochemical constituents in the scallops cultured at 34 m was likely due to their lack of reproductive activity. The muscle and digestive gland have been indicated as storage organs for various bivalves ŽGabbott, 1983; Barber and Blake, 1991.. Other studies of scallops report that gonad growth is mainly supported by muscle reserves ŽTaylor and Venn,
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Fig. 2. Variations in temperature ŽA., total dry seston ŽB., organic content of the seston ŽC. and chlorophyll a concentration ŽD. from December 1993 to October 1994 at 8, 21 and 34 m in depth at Turpialito, Golfo de Cariaco, Venezuela Žafter Lodeiros et al., 1998..
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1979; Barber and Blake, 1991; Epp et al., 1988; Faveris and Lubet, 1991; Couturier and Newkirk, 1991.. We further documented decreases in carbohydrate in the remaining somatic tissues of L. nodosus between late April and late May, when gonadal mass more than doubled, suggesting that reserves were also withdrawn from these components. Although the remaining somatic tissues mainly represent organs involved in maintenance Že.g., mantle, gills and excretory organs., the mantle may contribute energy for gonadal production as suggested for several other bivalves Žde Zwaan and Mathieu, 1992., including pectinids ŽEpp et al., 1988.. In L. nodosus, digestive gland lipids were controlled independently of muscle carbohydrates. Scallops cultured at 21 m virtually depleted muscle carbohydrates during gonadal growth, but simultaneously increased digestive gland lipids Žthereby decreasing relative levels of digestive gland proteins. and muscle size. At 8 m, muscle carbohydrates were only slightly decreased during gonadal growth, while the mass of the digestive gland and muscle rose markedly. The greater food availability at 8 than 21 m ŽLodeiros et al., 1998. may have contributed to the faster growth of scallops at 8 m and limited depletion of somatic reserves during gonadal production. The greater depletion of muscle carbohydrates and digestive gland lipids during gonadal growth at 21 m than at 8 m supports the hypothesis that gonadal growth is favored over somatic growth when food resources are limited during reproductive periods in bivalves ŽBrowne and RusselHunter, 1978; Toumi et al., 1983.. Such preferential allocation of reserves to gonadal growth also was suggested for EuÕola ziczac in the Golfo de Cariaco ŽLodeiros and Himmelman, 2000.. During the latter part of our study, the water column was stratified and characterized by high temperatures and low phytoplankton biomass ŽLodeiros et al., 1998.. Such conditions decrease physiological capacities in the scallop E. ziczac in natural beds in the Golfo de Cariaco ŽBoadas et al., 1997.. L. nodosus may be particularly susceptible to such stressful conditions when they are weakened due to reproduction. Both at 8 and 21 m, L. nodosus released their gametes during the last 2 months of our study, apparently in association with a rise in temperature ŽLodeiros et al., 1998.. The growth of fouling organisms on the scallops, particularly at 8 m ŽLodeiros et al., 1998., may have exacerbated the effects of high temperatures and low food availability. During the last 2 months, protein levels in the remaining tissues decreased, muscle carbohydrates of the scallops at 8 and 21 m were depleted and digestive gland lipids in scallops at 21 m decreased. Whereas lipid levels in scallops increased during the period of rapid gonadal growth, they reached a plateau or even decreased in most body components Žexcept muscle. during the last 2 months. This generalized depletion of somatic reserves is suggestive of energetic limitation, possibly related to stress associated with stratification of the water column and low phytoplankton levels. This suggestion is strengthened by the total mortality that occurred 1 month later at 8 m ŽLodeiros et al., 1998.. Our study of energy storage and mobilization in relation to reproduction and environmental conditions only covered the period of growth up to a shell length of 35 mm Žat 34 m. to 56 mm Žat 8 m., which is much smaller than the size which L. nodosus attains in natural populations Ž110–130 mm.. Further studies are required to evaluate if energy allocation undergoes additional modifications as scallops further increase in size. In scallops, reproductive output often continues to increase throughout the lifetime of the
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animal, whereas somatic production remains constant or declines ŽThompson and MacDonald, 1991.. Our study indicates that the muscle and digestive gland are the principal deposits of energetic reserves in L. nodosus. Reproductive output can be supported by reserve mobilization from various tissues, particularly when food availability is reduced. Decreases in the protein levels of remaining somatic tissues seem related to exposure to stressful conditions. Carbohydrate levels of the muscle and lipid levels in the digestive gland of L. nodosus would appear to be useful indices of the condition of the scallops. However, our conclusions are based upon individuals grown in culture conditions, where stress could be due, in part, to the culture techniques. Our conclusions for scallops with developing or recently developed gonads may not be representative of the allocation of energetic reserves in large individuals. Effectively, the use of assimilated energy for both growth and reproduction distinguishes smaller scallops from larger ones in which assimilated energy is used primarily for reproduction ŽThompson and MacDonald, 1991.. Studies on partitioning of energy in L. nodosus at larger sizes are needed to corroborate our conclusions.
Acknowledgements This study was possible due to the facilities provided by the Instituto Oceanografico ´ de Venezuela, Universidad de Oriente. We are particularly indebted to M. Nunez ˜ for his help with the fieldwork. The research was supported by grants from the Consejo de Investigacion ´ de la Universidad de Oriente to C.J.L and the National Sciences and Engineering Council of Canada to H.E.G. and J.H.H.
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