The life histories of the amphipods Lembos websteri Bate and Corophium bonnellii Milne Edwards in kelp holdfasts

Abstract

The life cycles of Lembos websleri Bate and Corophium bonnellii Milne Edwards are based on two generations per year. Eight cohorts have been identified in Lembos and nine for Corophium, each of which has a different longevity. In Lembos the summer generation proper lives ≈ 5–7 months whilst cohorts of the overwintering population may live up to 11.5 months. Males do not live as long as females, their longevity being ≈ 11% less in summer and 5% less in winter. C. bonnellii is parthenogenetic. Its summer generation lives ≈ 5 months and its winter generation 8–10 months. Corophium thus has a shorter life span than Lembos. It also grows faster. Growth is severely inhibited overwinter (but less so in Corophium) and picks up in spring. Year to year variations in timing of reproductive events may be related to temperature differences, sea state, and food supply. Population data for 1974 and 1975 were, however, generally very similar. Ovigerous females are present all the year in Lembos websteri but were never found in January in Corophium bonnellii. Egg-laying behaviour is described. Brood size is linearly related to maternal length in both species. A size-independent index of fecundity shows clear seasonal variation, with peak fecundity in summer. Minimum size of egg-bearing females is also a function of temperature in both species. A semi-lunar rhythmicity of brood production at summer temperatures is reported. Brood mortalities of ≈ 25–30% were found in both species which seemed unrelated to maternal size but which may (in Lembos) be seasonally variable (lower in winter). Egg size is similar in both species and egg volume more than doubles during development from Stage I to Stage IV. Corophium bonnellii eggs are bigger and individual female reproductive effort is higher than in other Corophium species. Corophium sometimes carried the ciliate Conidophrys on the basal segments of its antennae. Occasional endoparasites, namely cercariae and encysted metacercariae, were noted. Mortality due to fish predation was established to occur, the clingfish Apletodon microcephalus (Brook) and the sea snail Liparis liparis (L.) being important amphipod consumers.