Sources of nutrition in intertidal sea anemones from the south-western Cape, South Africa

Abstract

Respiration rates of seven species of intertidal sea anemones from the south-western Cape, South Africa were measured with and without illumination to determine whether zooxanthellae contributed to their nutritional needs. Light had no significant effect on oxygen exchange in any of the species, suggesting that all are azooxanthellate. Diets of anemones from two sites, Wooley’s Pool in False Bay and Blouberg on the Atlantic coast some 20 km north of Cape Town, were examined by gut content analysis. Seven species occurred at Wooley’s Pool, but only one (Bunodactis reynaudi) at Blouberg. At Wooley’s Pool 39.4% of anemones contained food, while at Blouberg only 7.4% did so. A wide spectrum of prey taxa were consumed by the various anemone species, although considerable dietary overlap occurred. Pelecypods, gastropods and isopods were ingested by all the species examined. Pelecypods dominated the diet of B. reynaudi at Blouberg (91% occurrence), but were recorded in only 42% of the same species at Wooley’s Pool. The frequency of occurrence of pelecypods in the guts of the other species ranged from 9% (Anthothoe stimpsoni) to 28% (Actinia equina). Gastropods were the main prey items eaten by Anthopleura michaelseni (67% occurrence), Pseudactinia flagellifera (43%) and Pseudactinia varia (42%), while isopods were the most frequently occurring constituent in the diets of A. stimpsoni (62%) and Bunodosoma capensis (16%). A. equina was the only species for which insects were a regular part of the diet (26% occurrence). Platyhelminths were found only in the guts of B. capensis (11%); holothurians only in B. reynaudi at Wooley’s Pool (2%), and crinoids only in P. flagellifera (2%). Large quantities of algal material and indigestible debris were also ingested, indicating that sea anemones are non-selective feeders. Cluster analysis and multi-dimensional scaling techniques revealed four distinct feeding groups among the anemone assemblages. These are characterized as microphagous (A. stimpsoni), generalist (S. capensis and A. equina), macrophagous (S. reynaudi at Wooley’s Pool, A. michaelseni, P. flagellifera and P. varia) and specialist bivalve- feeder (B. reynaudi at Blouberg).