THE ADAPTIVE SIGNIFICANCE OF A LUNAR OR SEMILUNAR REPRODUCTIVE-CYCLE IN MARINE ANIMALS

Many marine invertebrates and fishes have a lunar or semi-lunar reprod uctive cycle. There are several possible explanations for the advantag es of reproductive rhythmicity. Such rhythmicity may (1) increase the rate of mating; (2) increase the rate of fertilization; (3) help to re tain free-living larvae in nursery sites; (4) help to spread free-livi ng larvae over some adult habitats; (5) decrease predator pressure on free-living larvae; or (6) increase reproductive success by the direct influence of lunar cyclic environmental effects such as moonlight. In this study, the adaptive significance of reproductive rhythmicity in increasing the reproductive rate, is examined by analyzing the model w hich has three major controlling parameters, i.e. population density, sex ratio, and the duration of breeding interval. The objective functi on, that is adaptiveness, of per capita oviposition number per unit ti me is calculated under the non-cyclic or the cyclic reproductions. It appears, from the analysis of the model, that the cyclic type is accom panied by a higher rate of oviposition than the non-cyclic one under c onditions of high sex ratio (female > male) and/or of low population s ize. In particular, under conditions of low population size, the cycli c reproduction decreases the lower limit of population growth and, the reby, reduces the probability of extinction of the population.