SUMMARYWe 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°C, 12°C and 20°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 Hulbertʼs 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 slowgrowing 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.

Soft-shell clam, Mya arenaria, culture on the east coast of Canada is characterized by high loss following seeding. To evaluate the importance of passive transport due to currents, an experimental flume study was designed. The purpose was to measure the effects of hydrodynamic conditions, substrate, and clam size on dispersal in controlled laboratory condition and to interpret these results in relation to field measurements. Unidirectional currents with gradual increasing velocities (0 to 60 cm s − 1) were applied to three substrates (muddy sand, medium sand, and coarse sand) in which clams from one of three size classes (10, 15, and 20 mm) had burrowed. We also examined the resulting effects of turbulent kinetic energy on the erosion of medium-grain sand and clams from the three size classes. Turbulent energy was created with a homemade device acting on the unidirectional currents. Nearly 95% of buried clams (all substrates and size classes together) were unaffected by unidirectional currents of up to 60 cm s − 1 , but only 10% withstood turbulent kinetic energy of 10.1 J m − 3 , a level that is lower than that measured in the field during an autumnal storm. The transport of clams was found to be directly related to substrate erosion-levels.

A B S T R A C TThere are several factors affecting scallops during the metamorphosis process that could explain the relatively low post-larvae yield observed in hatcheries. Competent bivalve larvae respond to different settlement cues to undergo metamorphosis and without adequate cues, larvae delay their metamorphosis. The objective of this study is to improve the settlement ratio of the two scallop species, Placopecten magellanicus and Pecten maximus by physical cues associated with hydrodynamic conditions, stocking density in settlement units and larval size at time of transfer to settling units. For each treatment, physiological condition was determined by fatty acid analysis to determine the energetic reserves and structural lipids. We observed similar results for the two important commercial pectinid species and validate the hypothesis on the positive effect of increased flow rate and larval size after transfer to settlement systems on settlement success. Increasing flow rate also affects positively the physiological condition of settled post-larvae by a higher accumulation of total fatty acids in neutral lipid fractions. Furthermore, no effect of larval stocking density until 90 larvae cm −2 in the downwelling sieves was observed. To our knowledge this study is the first to characterize the effect of seawater flow rate on settlement success of different pectinid species cultured under similar conditions. The experiments were performed in a close to commercial scale and thus are relevant to industry situations.

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