Sea anemones as secondary consumers on rocky shores in the south-western Cape, South Africa

Abstract

The species composition, abundance and distribution patterns of intertidal sea anemone assemblages were determined at two sites on the Cape Peninsula, South Africa. A single species, Bunodactis reynaudi (Milne-Edwards) dominated at Bloubergstrand in Table Bay, with a density of 901 m-1 (i.e. per linear metre of coastline). Seven species, with a combined density of 660 m-1, were found at Wooley's Pool in False Bay. Of these Actinia equina Linn. and Anthothoe stimpsoni (Verr.) occurred primarily in the upper intertidal, Anthopleura michaelseni (Pax), Bunodosoma capensis (Less.) and Bunodactis reynaudi in the mid- to low intertidal and Pseudactinia varia Carlgren and P. flagellifera (Hertw.) only in sheltered pools near the sublittoral fringe. Density data are combined with previously reported gut content analyses and digestion rate estimates to calculate total annual consumption rates of natural populations of anemones. At Wooley's Pool the anemones colonizing each linear metre of shore consume an estimated 103 337 isopods, 14 973 amphipods, 21 050 cirripedes, 40 468 pelecypods and 30 102 gastropods annually. At Bloubergstrand the B. reynaudi population consume 38 437 polychaetes, 129 744 pelecypods and 116 229 gastropods m-1y-1. These figures considerably exceed estimated population consumption rates for other major invertebrate predators. However, the bivalves and cirrepedes taken by anemones are dislodged individuals, hence anemones are unlikely to play any significant role in structuring populations of these species. By contrast, consumption of motile prey, such as isopods and gastropods, may impact significantly on these populations. Overall annual consumption by naturally occurring anemone assemblages at Bloubergstrand were calculated to be 182 284 kJ m-2y-1, six times higher than at Wooley's Pool (30 851 kJ m-2y-1). These very high rates suggest that anemones are major secondary consumers in rocky intertidal communities in the south-western Cape.