REPRODUCTIVE PATTERN OF THE CLAMS, RUDITAPES-DECUSSATUS AND R-PHILIPPINARUM ON INTERTIDAL FLATS IN BRITTANY

Since the introduction of the Manila clam Ruditapes philippinarum (Mol lusca: Bivalvia) to aquaculture clam beds, this species has spread to several sites. As a result, an extensive fishery has developed around this species. Its reproductive cycle and that of a native clam species , Ruditapes decussatus (Mollusca: Bivalvia) from two Brittany sites ar e examined here and the seasonal variations in the condition index, th e scale of maturation stages, the histology and the oocyte size-freque ncies of both species are compared. The differences observed between t he species and between sites were attributed to environmental conditio ns. Ruditapes philippinarum showed a more extended breeding period and a greater number of spawning events than R. decussatus. In the Morbih an Gulf, the reproductive activity of R. philippinarum appeared partic ularly great in terms of number of spawning events, and speed of matur ation. For both species, synchronism was more apparent during the firs t maturation, while spawning, and particularly partial spawning, tende d to decrease the level of synchronization.