Aspects of the Population Biology of the Marine Isopod Excorallana Tricornis Occidentalis Richardson, 1905 (Crustacea: Isopoda: Corallanidae) at Cano Island, Pacific Costa Rica

Diel and seasonal aspects of occurrence, density, size and sex ratios of the marine isopod Excorallana tricornis occidentalis from Cano Island, Costa Rica, were studied during 1984 and 1985. Surface and vertical plankton tows were taken over 24-hour periods. 2,916 individuals were collected; 42% were juveniles, 39% females and 17% males. Only one ovigerous female was collected (5.5 mm in length). Females probably remain hidden in the substrate while brooding embryos and mancas. Highest densities were found in November and January 1984, but differences in monthly densities were not statistically significant. E. tricornis occidentalis is a member of the demersal zooplankton community. Significant differences in isopod densities between sampled hours indicate that it has a nocturnal migratory behavior; in both surface and vertical tows it is found in the water column only between 1800 and 0600 h, with greatest emergence after midnight (between 2400 and 0300 h). This species is probably a nocturnal predator that feeds at night in the water column. Mean length for males was 5.17 mm, for females 4.76 mm, and for juveniles 2.48 mm; mean length for all 2,916 individuals was 3.91 mm. There were significant differences in isopod lengths between sampled years, months and hours. Adults predominated in most months. The largest percentages of juveniles occurred in July and November, indicating probable Spring and Fall recruitment bursts. Same-sex pairwise comparisons of mean body lengths showed E. tricornis occidentalis to be significantly smaller than its Atlantic con-specific Excorallana tricornis tricornis, and another Atlantic congener, E. oculata.