Autumn conditioning of the oyster Crassostrea gigas: a new approach

Gametogenesis, emitted gametes and larval yield (D larvae) of the oyster, Crassostrea gigas (Thunberg), were simultaneously examined in laboratory under two different conditions of temperature and photoperiod. One batch (“natural” cycle, NC) followed the equivalent cycle of average conditions of these parameters measured in La Tremblade (Charente-Maritime, France) during the last 15 years. The second batch (“accelerated” cycle, AC) was maintained under a variation two times faster of these parameters. Three conditioning experiences were performed during October, November and December 1998 with oysters from both NC and AC conditions. “Accelerated individuals” produced oocytes in growing stage from October and mature oocytes were noticed from December. In contrast, oysters from NC produced growing oocytes until January. Animals in AC produced 42% (October), 56% (November) and 96% (December) of mature oocytes after conditioning. Only the “natural” lot conditioned in December showed growing (23%) and mature oocytes (8%). Higher Walne–Mann index (WMI) values were recorded for “accelerated oysters” and significant differences (P<0.05) between treatments were noticed during October and November, suggesting nutrient accumulation before conditioning in the “accelerated” condition. Gamete emission and D larval yield results were equivalent to those reported in literature during early spring for the same species. The effect of temperature and photoperiod is discussed in order to understand their relationship with gametogenesis under these particular experimental conditions. It was demonstrated that internal clocks regulating gametogenesis can be change if stimulating factors (environmental cues) also change.