The ecology of an assemblage dominant:: the encrusting bryozoan Fenestrulina rugula

The cheilostome bryozoan Fenestrulina rugula is a major component of the encrusting fauna of physically disturbed shallow water habitats in Antarctica. On rocks collected from Rothera Point, Adelaide Island, F. rugula was the dominant occupier of space (88% of all bryozoans, 76% of all fauna), though spirorbid polychaetes showed more colonisation events per unit area. Growth rate was relatively rapid in comparison with other polar bryozoans, but slower than temperate or tropical species. In fully reproductive colonies almost 80% of zooids carried ovicells. Colonies whose growth brought them into contact with other encrusting fauna (usually another colony of F. rugula) produced smaller zooids and initiated ovicell production earlier than unrestrained colonies. Analysis of overgrowth interactions showed that F. rugula was a relatively poor competitor compared with other encrusting bryozoans, and most within-species interactions were indeterminate. Population mortality was relatively high, averaging 89% per annum, although not as high as in some other species from ephemeral habitats. F. rugula is thus a typical early coloniser in being relatively fast-growing, quick to mature, short-lived and a relatively poor competitor. The population dynamics and ecology of this assemblage dominant appear to have been influenced primarily by the ephemeral nature of its habitat.