SHORT-TERM AND LONG-TERM ALTERATIONS IN THE ENERGY BUDGET OF YOUNG OYSTER OSTREA-EDULIS L IN RESPONSE TO TEMPERATURE-CHANGE

Young post-metamorphic oysters, Ostrea edulis L., previously reared at 20 degrees C, were exposed to 14 degrees C (cold treatment), 20 degre es C (control) and 26 degrees C (warm treatment) for 3 wk. Algal (Isoc hrysis galbana Parke) ingestion rate (IR), absorption efficiency (AE), oxygen consumption (V-O2) and ammonia excretion (V-NH4) were measured at the beginning, halfway and at the end of the experimental period. These rates were integrated in the energy budget and the net energy ga in available for biomass increase, or scope for growth (SFG), was calc ulated. All the physiological parameters, including AE, were significa ntly reduced in oysters acutely exposed to the cold treatment. However , following prolonged cold exposure an increase in IR and a slight dec rease in V-O2 allowed compensation of the SFG, which in the long-term achieved similar levels to control. After acute exposure to the warm t reatment, oysters showed slightly enhanced IR and V-NH4 but no major c hanges in the other physiological parameters? which yielded a slightly increased SFG. In the long-term, IR markedly increased in warm acclim ated oysters which, coupled with sustained metabolic costs, allowed an almost three times increase in the SFG. Different adaptive strategies of acclimation described for bivalves are discussed, under the common aim of SFG maximization following long-term exposure to a temperature change; and peculiarities of fast-growing juveniles are evidenced. It is concluded that juvenile oysters are able to respond in the long-te rm to either a decrease or increase of temperature, regulating the phy siological rates integrated in the energy balance in a manner that ren ders an overall improvement in the net energy available for growth.