F. Lango-Reynoso, N. Devauchelle, M. Le Pennec, P.J. Hatt
Abstract:
In France oyster farming is very important from an economical point of view (150 000 l/year) and 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 that we need to know how the sex determination can be influenced by environmental factors.
Crassostrea gigas individuals from a natural population of the Rade de Brest, France were examined every 2 weeks during a 2-year period from Februry 1996. Reproductive parameters including their condition index and their sex ratio were determined, seasonal and annual differences assessed. Under microscopic examination the presence of sperm, ovocyte or the absence of gametes allowed us to define individuals as male, female, hermaphrodite or sexual undetermined oysters. The population sex ratio was recorded for various shell lengths.
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 gametogenesis pattern. The frequency of hermaphrodites varied from less than 1% in July-September to about 37% in May-April. The sex ratio was 3 females: 1 male just before the spawning season.
Seasonal gonad changes in oysters were examined using histological methods. In the reproductive cycle of C. gigas 5 stages were distinguished according to Yakovlev (1978): spawning, postspawning, reduction, growth-and-maturation and prespawning. The major spawning in the Rade de Brest was commonly observed in July. Postspawning was followed by the reduction stage that extended from early September to last October. During the growth and maturation stages, gametogenetic activity was initially observed in January, evolved from February to March and stopped in April. At the end of May the gonad was morphologically mature. Hermaphrodites were characterized by mosaic gonads.
In conclusion, the sex determination of Crassostrea gigas oysters is easily influenced by environmental factors. Therefore, going deeper in the knowledge of such factors influence is of the utmost interest for practical applications in hatchery such as the sex determination of breeders.
(IUEM, Lab. BIOFLUX, Place Nicolas Copernic, 29280 Plouzane, France)