The reproductive conditions of the mussel, Mytilus edulis galloprovincialis, collected from four localities in Japan were studied by histological analyses of their gonads. Reproductive cycle of the mussels was divided into the six developmental stages: resting (a), pre-developmental (b-1), post-developmental (b-2); pre-spawning (c-1), post-spawning (c-2) and degenerative (d) stages. Their spawning seasons and the reproductive patterns were determined by these developmental stages of gonad.
The main spawning season of the mussel was considered longer in the southern areas than in the northern areas: from October to March in Yokosuka Harbor and Otsuchi Bay, from November to February in Urasoko Bay and from October to April in Omura Bay. The shorter spawning season of the Urasoko Bay seems due to the high saline water conditions of this area. In the spawning pattern there were two spawning types: total spawning and partial spawning types. In the former the gonad develops simultaneously and spawning occurs from the whole gonadal tissues. After this total spent gonads develop simultaneously again for the next spawning. In the latter case spawning occurs partially in the gonad tissues, histologically the gonads containing mixed developmental stages. The partial spawnings were more common in the individuals inhabiting the subtidal zone than those in the intertidal zone. In the subtidal zone partial spawning took place much more commonly in the 1-year than 0-year group.
Based on the reproductive patterns, it was suggested that the Japanese mussel is certainly identical to the Mediterranean species, Mytilus galloprovincialis LAMARCK.
