SIZE-DEPENDENT REPRODUCTIVE TRAITS OF YOLDIA-NOTABILIS (BIVALVIA, PROTOBRANCHIA)

Between-site variation in age and size at maturity, egg size, fecundity and reproductive effort was measured in a population of the bivalve Yoldia notabilis in Otsuchi Bay, northeastern japan, and tests were conducted to address whether these reproductive traits are dependent on size or age. The diameter of mature oocytes increased with age and size, as did fecundity and reproductive effort. Most individuals were mature at 3 yr of age (Age 3) at the shallower station (10 m depth) and at Age 4 at the deeper station (14 m), although the size at maturity was similar (between 15 and 20 mm shell length) at the. 2 stations. This is the size at which the bivalve escapes heavy predation from the crab Paradorippe granulata. Age-specific reproductive effort was higher at the shallower station than at the deeper one; however, reproductive effort did not differ significantly between stations when compared on the basis of size, These lines of evidence suggest that the onset of reproduction and the schedule of reproductive effort depend more on size than age. The observed size-dependency of reproduction is considered to be associated with size-dependent mortality rate due to predation, as well as with size-limited reproductive capacity due. to morphological constraints in shell volume.