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Juvenile oysters (Crassostrea gigas) (produced in November 2009) reared under uniform hatchery conditions for 4 months were selected for extreme growth rate differences by repeatedly taking larger and smaller individuals to achieve weight differences >30 x between fast (F) and slow (S) growers. The physiological basis of differential growth was analyzed in experiments in June 2010, where components of energy gain (clearance and ingestion rates and absorption efficiency), energy loss (metabolic rates) and resulting scope for growth (J [h.sup.-1]) were compared for groups of F and S oysters fed three different ration levels (≍ 0.5, 1.5 and 3.0 mg of total particulate matter [L.sup.-1]). In both F and S oysters, a higher food ration promoted asymptotic increases in energy gain rates through regulatory adjustments to clearance rates, which maintained similar absorption efficiencies across the food concentrations. No significant differences were found between growth groups in mass-specific physiological rates (i.e., per unit of body mass). However, the scaling of these rates to a common size in both groups using allometric coefficients derived for C. gigas revealed higher energy gain rates coupled with lower metabolic costs of growth in fast growers. Thus, appropriate size-standardization is essential in accounting for observed differences in growth rate. Present results are in accordance with previous reports on other bivalve species on the physiological processes underlying endogenous growth differences, suggesting that the same interpretation can be applied to the extremes of these differences., Introduction Growth rates vary among individuals in natural bivalve populations (Mallet and Haley 1983; Dickie et al. 1984; Brown 1988) even when exposed to similar environmental conditions. This indicates the [...]

Infection by Bonamia ostreae has caused extensive mortalities of oysters Ostrea edulis through European and United States coasts for at least 25 years. The development of a bonamiosis-resistant strain seems a promising strategy to fight against the disease. As a first step, evaluation of variability in productive traits and disease susceptibility of European populations was performed to identify favourable oyster populations with which to start selective breeding in Galicia (NW Spain). Oysters taken from Greece, Ireland, Ortigueira (Galicia) and Coroso (Galicia) were used as brood stock, and 19 seed families were produced (4-5 families from each origin). The oyster families were used to assess variability through on-growing in an area of the Rja de Arousa heavily affected by bonamiosis. Results showed significant differences in growth, mortality and susceptibility to bonamiosis and other diseases, both between origins and between families under origins. Bonamiosis was associated with mortality in the late stage of oyster on-growing. Indications of natural selection of bonamiosis less-susceptible oysters due to the long exposure of the Ortigueira population to bonamiosis were found. Other symbionts and pathological conditions were detected, of which herpes-like viral infections and disseminated neoplasia could also cause mortality. An index of the overall incidence of pathological conditions (OIPC) was estimated for each family. A significant correlation between the OIPC and the cumulative mortality of the families was noted. On average, oysters from autochthonous origins showed better performance. The results obtained with the best-performing families suggest that the profitability of oyster farming in Galicia would improve, even under bonamiosis pressure, by using appropriate oyster spat.