SEA-ANEMONES AS SECONDARY CONSUMERS ON ROCKY SHORES IN THE SOUTH-WESTERN CAPE, SOUTH-AFRICA

The species composition, abundance and distribution patterns of intert idal 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 9 01 m(-1) (i.e. per linear metre of coastline). Seven species, with a c ombined density of 660m(-1), were found at Wooley's Pool in False Bay. Of these Actinia equina Linn. and Anthothoe stimpsoni (Verr.) occurre d primarily in the upper intertidal, Anthopleura michaelseni (Pax), Bu nodosoma capensis (Less.) and Bunodactis reynaudi in the mid-to low in tertidal 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 r ate estimates to calculate total annual consumption rates of natural p opulations of anemones. At Wooley's Pool the anemones colonizing each linear metre of shore consume an estimated 103 337 isopods, 14 973 amp hipods, 21 050 cirripedes, 40 468 pelecypods and 30 102 gastropods ann ually. At Bloubergstrand the B. reynaudi population consume 38437 poly chaetes, 129744 pelecypods and 116229 gastropods m(-1)y(-1). These fig ures considerably exceed estimated population consumption rates for ot her major invertebrate predators. However, the bivalves and cirrepedes taken by anemones are dislodged individuals, hence anemones are unlik ely to play any significant role in structuring populations of these s pecies. By contrast, consumption of motile prey, such as isopods and g astropods, may impact significantly on these populations. Overall annu al consumption by naturally occurring anemone assemblages at Bloubergs trand were calculated to be 182284 kJ m(-2)y(-1), six times higher tha n at Wooley's Pool (30 851 kJ m(-2) y(-1)). These very high rates sugg est that anemones are major secondary consumers in rocky intertidal co mmunities in the south-western Cape.