Lm. Kruger et Cl. Griffiths, SEA-ANEMONES AS SECONDARY CONSUMERS ON ROCKY SHORES IN THE SOUTH-WESTERN CAPE, SOUTH-AFRICA, Journal of Natural History, 32(5), 1998, pp. 629-644
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.