Population Dynamics of the Isopod Cirolana Imposita Barnard in a Kelp-Bed

Abstract

A population of the sub-tidal isopod Cirolana imposita in a kelp-bed at Oudekraal (Cape Peninsula), South Africa was sampled for 18 months at monthly intervals. Field studies were conducted in conjunction with laboratory breeding and rearing experiments to investigate reproductive cycles, population structure, growth, and mortality. Results indicate that the lifespan is two years, and that some recruitment occurs throughout the year, except during summer months when cold upwelled water was recorded. Females consistently outnumbered males, often by more than 2 : 1. Juveniles occurred in all samples, accounting for 15-52% of the population. New cohort entered the population in June and October-November. The brood period lasts four months and females carry 15-33 eggs per brood. Field results showed rapid growth (0.57 mm per month). Mortality was high amongst juveniles (40-70% per month) but decreased with age to 16%.