Ecology of the starfish Anasterias rupicola at Marion Island (Southern Ocean)

Abstract

The starfish Anasterias rupicola (Verrill) is the most conspicuous littoral macro-inverte brate predator at Marion Island; it has a mean density of 17.5 individuals m-2 in the intertidal and infratidal zones. A. rupicola tends to congregate in relatively sheltered sites and attains maximum densities of 171 m-2. The diet of A. rupicola is recorded from 404 cases of predation, 40 % of which occurred on the limpet Nacella delesserti. Two other important prey are the polychaete Platynereis australis and the isopod Dynamenella huttoni. A. rupicola may feed either as solitary individuals or clustered together in groups on single prey. Solitary starfish show size-limited predation and are limited to capturing prey smaller than themselves. A. rupicola which collectively attack large prey are able to overcome the restriction of size-limited predation. Counts of feeding versus non-feeding starfish in the field show that there is an average of 11.7 % of the A. rupicola population feeding at any one time. Starfish confined with N. delesserti in cages were monitored each month and provided data for a linear regression to predict the daily intake of food by a given mass of A. rupicola. It is estimated from calculations that the starfish removes 31.0 N. delesserti m-2yr_1. The only discernible growth was recorded in brooded juveniles which showed an overall increment in diameter of 1.26 mm in 6 to 8 mo. It is suggested that A. rupicola takes at least 39 yr to attain its maximum diameter of 110 mm. Intertidal A. rupicola are preyed on by the lesser sheathbill Chionis minor and the kelp gull Larus dominicanus. A. rupicola is a key predator of littoral organisms at Marion Island; its cooperative feeding behaviour, brooding habit and slow growth make it an interesting species worthy of further study.