Elements of reproductive strategy in oysters,Crassostrea gigas, from the “Rade de Brest”, France

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Abstract
In France oyster farming is very important from an economical point of view (150,000 t/y), but curiously, no data exist concerning the reproductive cycle of the cultivated species Crassostrea gigas. New selection programs in oyster hatcheries are based on the use of adults having proper characteristics (growth, resistance to diseases and known sexuality) for which we need to know how sex determination can be influenced by environmental factors. C. gigas individuals from a natural population of the “Rade de Brest”, Brittany, France, were examined every 2 weeks during a 2-year period from February 1996. Reproductive parameters including their condition index and their sex ratio were determined, and seasonal and annual differences assessed. Under microscopic examination the presence of sperm or oocytes or the absence of gametes allowed us to define individuals as male, female, hermaphrodite or sexually undetermined oysters. The maximum percentage of female oysters, 70%, was found in July, whereas males were observed almost throughout the year. Females exhibited a very distinct seasonal pattern of gametogenesis. The frequency of hermaphrodites varied from less than 1% during July—September to about 37% during May—April. The sex ratio was three females to one male just before the spawning season. In conclusion, sex determination of C. gigas oysters is strongly influenced by environmental factors. Therefore, further study of the influence of such factors is of great interest for practical applications in the hatchery such as the sex determination of breeders.