ECOPHYSIOLOGY OF MATURATION AND SPAWNING IN OYSTER (CRASSOSTREA-GIGAS) - METABOLIC (RESPIRATION) AND FEEDING (CLEARANCE AND ABSORPTION RATES) RESPONSES AT DIFFERENT MATURATION STAGES

The study of maturation and spawning of the oyster is part of a resear ch program to investigate the summer mortalities of the oysters, Crass ostrea gigas in Marennes-Oleron Bay. Four maturity stages were simulta neously obtained by diet and thermal conditioning (immature, low matur ation, mature and post-spawning stages). Measurements of clearance, fi ltration, absorption and respiration rates allowed a calculation of th e scope for growth and hence an estimation of the oyster's energetic b udget at various maturity stages. Male and female oysters had similar physiological responses. The filtration rate ranged from 2.4 to 2.6 l. h(-1) at the early stages of maturation and decreased to 1.8 l.h(-1) d uring the maturity stage. Growth rate resulting from gonad development did not induce filtration rate changes. Mature 2.5 and 1.5-year-old o ysters showed a negative energy budget reaching -15 and -90 J.h(-1) re spectively. By contrast, non-ripe oysters had scope for growth in the range 110 to 170 J.h(-1). A negative energy budget during the high mat uration stage resulted from a reduced absorption efficiency. A new all ometric relationship for the respiration model of C. gigas was defined during vitellogenesis with a 0.574 coefficient value. Based on our re sults, the oyster's physiological weakness during vitellogenesis shoul d be considered as a part of explanation for spring and summer mortali ties of cultured oysters in Marennes-Oleron Bay.