The population biology of Ligia oceanica (L.)(Crustacea: Oniscoidea)

Abstract

The dynamics and life history of an isolated population of Ligia oceanica (L. ) inhabiting a sea-wall exposed to moderate wave action have been studied over a period of three years. Sex ratio, size at maturity, growth, breeding phenology, size and age structure, population density and size, female breeding size, fecundity regression and broodpouch mortality have been determined. From these data population natality, and schedules of survivorship, fecundity and reproductive value are calculated. Although the supralittoral zone of rocky shores is generally regarded as a severe and stressful habitat, the dynamics of the population exhibit only minor differences between years, and no direct influence of wave exposure can be determined. This population is compared with another nearby but isolated population occupying an harbour wall completely protected from wave action. These populations are presumed to suffer different levels of density-independent mortality due to the wave exposure characteristics of the sites. The sheltered site population shows no significant differences between the two years of study. Interpopulation differences in age and size structure, growth rate, population breeding phenology and breeding sizes are slight. However the sheltered population shows increased adult survival and longevity, decreased weight specific fecundity and reproductive allocation, and increased brood-pouch mortality. Because winter survival is high, when mortality due to wave action is expected to be severe, this factor is rejected as an important determinant of the life history. The reduced survival at the exposed site can be interpreted as a consequence of the increased reproductive allocation by individuals at this site. The majority of females are semelparous, but within both populations two distinct waves of breeding activity occurred each year. Larger, faster growing individuals breed at the end of their first ýear, LATE in the season, and have high weight specific fecundities and low absolute and weight specific egg sizes. Smaller, slower growing individuals delay reproduction and breed in their second year, EARLY in the season. Although gaining greatly in size through growth the effect on fecundity is offset by a decrease in weight specific fecundity and parallel increases in absolute and weight specific egg size. The following trade-offs are recognised for individual female Ligia : between growth and reproduction (within breeding groups), growth and reproduction (between breeding groups), reproduction and survival (between breeding groups), weight specific egg size and weight specific fecundity (between breeding groups), and between absolute egg size and fecundity (within breeding groups). Two possible life history strategies are recognised. (i) An ability of a female parent to alter egg size seasonally may optimise total offspring fitness in the different environments they experience.(ii) An ability of small, mature females to delay reproduction until the following year may also increase their fitness. By assuming that offspring survival is related to egg size it is possible to model this tactic. The population breeding phenology is seen as a direct consequence of the life history and has the indirect effect of stabilizing the population. The ability of individual females to vary their life history traits depending on individual circumstances (size and season) may represent a flexible approach to the environment which is not envisioned in the more formal models of life history evolution