Egg-size and -number variations related to maternal size and age, and the relationship between egg size and larval characteristics in an annual marine gastropod, Haloa japonica (Opisthobranchia; Cephalaspidea)

Abstract

Contrary to the traditional egg size-number trade-off model, egg size of the small opisthobranch Haloa japonica varies significantly within a population. To examine the cause of this variation, I estimated (1) the effects of maternal size and age (i.e. the number of days after the onset of breeding the egg was spawned) on egg size and number, and (2) the relationship between egg size and several larval characteristics, including larval size, length of survival under conditions of starvation, and time to hatching. Egg size and number decreased in the course of the breeding season under laboratory conditions and egg size increased with maternal size. Length of survival under conditions of starvation was positively correlated with egg volume. This reproductive pattern is similar to that of other taxonomic groups, such as annual plants, insects and a marine polychaete. The decrease of reproductive investment is best explained as a maternal investment strategy related to maternal mortality, as proposed by Begon and Parker (1986; Oikos 47:293-302). My data support the prediction of their theory.