Density, size structure and reproductive characteristics of fissiparous brittle stars in algae and sponges: evidence for interpopulational variation in levels of sexual and asexual reproduction

Abstract

This study documents density, size structure and levels of sexual and asexual reproduction in populabons of fissiparous brittle stars occupying algal turf and sponge habitats in Jamaica, Belize and Bermuda. Ophiocomella ophiactoides allowed comparison between algal turf habitats in Jamaica and Belize; Ophiactis savignyi allowed comparisons between algal turf and sponge habitats at each location. In algal turf growing in protected Jamaican coves. 0. ophjactoides was present at moderate to high densities (max. 26 dl-' of algae). Asexual reproduction predominated among the numerous small individuals, whereas sexual reproduction was confined to a few large individuals. In contrast, in algal turf growing in more exposed backreef and reef crest areas at Belize, this species was present at very low densities (max. 1 dl-l), showed a higher incidence of fission at all body sizes, and was exclusively asexual. Both mean body size and density of 0. savignyl were greater in sponges than in algal turf at each location. Sponge-dwelling populations contained asexual individuals as well as a comparatively small number of large sexually reproducing individuals, whereas epiphytic individuals were almost all asexual. These differences are considered in relation to habitat stability and other ecological factors. Algal clumps and sponges frequently contained all-male or all-female clusters of fissiparous brittle stars, suggesting clonal aggregations. Male 0. sarrignyi predominated in sponges: no sexual females were hscovered in samples taken from Harrington Sound, Bermuda, and females constituted only 11 and 17 O/O, respectively, of sexual individuals examlned from sponges in Jamaica and Belize.