Helen McCombie

Genetic polymorphism and trade-offs in the early life-history strategy of the Pacific oyster, Crassostrea gigas (Thunberg, 1795): a quantitative genetic study

We investigated genetic variability and genetic correlations in early life‐history traits of Crassostrea gigas. Larval survival, larval development rate, size at settlement and metamorphosis success were found to be substantially heritable, whereas larval growth rate and juvenile traits were not. We identified a strong positive genetic correlation between larval development rate and size at settlement, and argue that selection could optimize both age and size at settlement. However, trade‐offs, resulting in costs of metamorphosing early and large, were suggested by negative genetic correlations or covariances between larval development rate/size at settlement and both metamorphosis success and juvenile survival. Moreover, size advantage at settlement disappeared with time during the juvenile stage. Finally, we observed no genetic correlations between larval and juvenile stages, implying genetic independence of life‐history traits between life‐stages. We suggest two possible scenarios for the maintenance of genetic polymorphism in the early life‐history strategy of C. gigas.

Gill Development and Its Functional and Evolutionary Implications in the Blue Mussel Mytilus edulis (Bivalvia: Mytilidae)

Study of gill development in bivalve larvae and postlarvae provides information on the evolution of this organ and feeding mechanisms of early stages. Scanning electron microscopy was used to document the development of the filibranch homorhabdic gill in hatchery-reared larval, postlarval, and juvenile Mytilus edulis. Four key stages were identified during gill development: (1) transfer of the particle collection function from velum to gill at metamorphosis, with subsequent elongation of the gill filaments to form a gill basket, with complete frontal ciliation; (2) reflection of the inner demibranchs, and transition to a V-shaped gill; (3) delayed development of the outer demibranchs, occuring simultaneously along the gill axis, with transition to the adult final W-shape; and (4) formation of the ventral particle grooves and concomitant acquisition of dense abfrontal ciliation. These key stages signal shifts in the mechanisms of particle processing during the early development of M. edulis. Gill development in the homorhabdic filibranch M. edulis was similar to that of the early homorhabdic stages of the heterorhabdic filibranchs studied to date (Pectinidae), but different from that of the pseudolamellibranchs (Ostreidae), suggesting divergent evolution of this character. Similarly, the systems responsible for gill cohesion and structural integrity are common to both the homorhabdic and heterorhabdic filibranchs, suggesting evolutionary proximity, but they are patently different from those of the eulamellibranchs and pseudolamellibranchs, suggesting evolutionary divergence.

Individual growth variation and its relationship with survival in juvenile Pacific oysters, Crassostrea gigas (Thunberg)

In order to study individual growth variability and its relationship with survival in juvenile Crassostrea gigas, parental oysters were sampled at four sites located along the French Atlantic coast and bred under controlled hatchery conditions. Four groups of larvae were obtained by crossing five males and five females from each of the four sites, and a fifth group by crossing these 20 males and 20 females together in a pool. Larvae were reared under conditions allowing the maintenance of a maximum variability of size and gave five experimental groups. Oysters were individually monitored for growth and survival from 3 to 10 months after fertilization. The individual growth performances were relatively stable over time and no noticeable compensation for growth occurred. Early growth rate was a very good predictor of size later in life: 66% of variation in the live weight at 10 months could be explained by variation in the initial growth rate calculated between 3 and 4 months. A significant group effect was observed on survival and on growth rate. Mortality mostly occurred between 3 and 5 months and appeared to affect the slow-growing animals more. However, two groups can be distinguished among those which died during the experimental period, one which showed a decrease in weight and the other whose growth was similar to surviving oysters. These results are discussed in the light of usual oyster farming practices and selective breeding.