RELATIONSHIP OF SIZE TO GONADAL MATURATIO N AND SPAWNING IN ARTIFICIALLY CONDITIONED MANILA CLAMS

Manila clams Ruditapes philippinarum of 16.1-36.0 mm in shell length, collected from Tokyo Bay, were divided into 5 size groups every 4.0 mm (groups A to E) and conditioned for 100 days at 18-degrees-C with sup plementary feeding of cultured microalgae Pavlova lutheri. Groups A an d B were estimated to be almost 0 yr old by previous investigation; gr oup C, 0-1 yr; group D, 1 yr; group E, greater-than-or-equal-to 1 yr. In the experiment, sampled clams were subjected to thermal spawning st imuli at intervals of 14-15 days. In each size group, gonads developed to a ripe stage as the condition factor was continuously increased, a nd spawning was successfully induced on and after 72 days. The relativ e growth rate reduced with increasing clam size. The histological ripe stage was attained earlier in large size groups and the number of egg s released per spawned female reached a maximum at day 100 in groups A to C, at day 86-100 in group D, and at day 86 in group E. Small clams at 0 yr old (groups A and B) could produce as many eggs per soft tiss ue weight as larger clams at greater-than-or-equal-to 1 yr (groups D a nd E). No significant differences in egg diameter, percent fertilizati on, and larval survival from straight-hinge to pediveliger stage were observed between clam size groups.