Physiological and biochemical traits correlate with differences in growth rate and temperature adaptation among groups of the eastern oyster Crassostrea virginica.

We tested two hypotheses in this study: first, that intraspecific growth variations in a marine bivalve are correlated with physiological (basal metabolic rate and scope for growth) and biochemical (membrane lipids) characteristics, and, second, that this bivalve shows intraspecific variations in physiological and biochemical adaptations to temperature. To test these hypotheses, five genetically distinct groups of juvenile oysters Crassostrea virginica that showed differences in their growth rates were maintained in the laboratory (1) for further measurements of growth and standard metabolic rates and (2) subjected to acclimation at 4 degrees C, 12 degrees C and 20 degrees C and further examined for scope for growth and determination of membrane lipid composition. Our results show that a lower basal metabolic rate and lower unsaturation index of membrane lipids coincides with higher growth rates and a higher scope for growth in oysters. We provide evidence that intraspecific differences in basal metabolic rate in oysters are related to membrane unsaturation as predicted by Hulbert's theory of membranes as metabolic pacemakers. Furthermore, our results suggest that the theory of membranes as metabolic pacemakers is related to intraspecific differences in growth. A perfect negative relationship was observed between the acclimation temperature and the unsaturation index of membrane lipids in oysters, as predicted by the homeoviscous adaptation theory. However, changes in the unsaturation index in response to temperature were mainly due to variations in the eicosapentaenoic (20:5n-3) fatty acid in fast-growing oysters, whereas slow-growing animals changed both docosahexaenoic acid (22:6n-3) and 20:5n-3. Thus, the pattern of biochemical compensation in response to temperature in this species shows intraspecific variation.

Lipid remodeling in wild and selectively bred hard clams at low temperatures in relation to genetic and physiological parameters.

A temperature decrease usually induces an ordering effect in membrane phospholipids, which can lead to membrane dysfunction. Poikilotherms inhabiting eurythermal environments typically counteract this temperature effect by remodeling membrane lipids as stipulated in the homeoviscous adaptation theory (HVA). Hard clams, Mercenaria mercenaria, can suffer high overwintering mortalities in the Gulf of St Lawrence, Canada. The selectively bred M. mercenaria var. notata can have higher overwintering mortalities than the wild species, thus suggesting that the two varieties have different degrees of adaptation to low temperatures. The objective of this study was to investigate the changes in lipid composition of soft tissues in wild and selected hard clams in relation to their metabolic and genetic characteristics. Clams were placed at the northern limit of their distribution from August 2003 to May 2004; they were exposed to a gradual temperature decrease and then maintained at <0 degrees C for 3.5 months. This study is the first to report a major remodeling of lipids in this species as predicted by HVA; this remodeling involved a sequential response of the phospholipid to sterol ratio as well as in levels of 22:6n-3 and non-methylene interrupted dienoic fatty acids. Hard clams showed an increase in 20:5n-3 as temperature decreased, but this was not maintained during overwintering, which suggests that 20:5n-3 may have been used for eicosanoid biosynthesis as a stress response to environmental conditions. Selectively bred hard clams were characterized by a higher metabolic demand and a deviation from Hardy-Weinberg equilibrium at several genetic loci due to a deficit in heterozygote frequency compared with wild clams, which is believed to impose additional stress and render these animals more vulnerable to overwintering mortality. Finally, an intriguing finding is that the lower metabolic requirements of wild animals coincide with a lower unsaturation index of their lipids, as predicted by Hulbert's theory of membranes as pacemakers of metabolism.