The adaptive significance of a lunar or semi-lunar reproductive cycle in marine animals

Abstract

Many marine invertebrates and fishes have a lunar or semi-lunar reproductive cycle. There are several possible explanations for the advantages of reproductive rhythmicity. Such rhythmicity may (1) increase the rate of mating; (2) increase the rate of fertilization; (3) help to retain free-living larvae in nursery sites; (4) help to spread free-living 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 which has three major controlling parameters, i.e. population density, sex ratio, and the duration of breeding interval. The objective function, that is adaptiveness, of per capita oviposition number per unit time is calculated under the non-cyclic or the cyclic reproductions. It appears, from the analysis of the model, that the cyclic type is accompanied by a higher rate of oviposition that the non-cyclic one under conditions of high sex ratio (female > male) and/or of low population size. In particular, under conditions of low population size, the cyclic reproduction decreases the lower limit of population growth and, thereby, reduces the probability of extinction of the population.