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J. Moll. Stud. (1989), 55, 329-342
© The Malacological Society of London 1989

research-article

GROWTH AND MORTALITY OF THE MUSSELS CHOROMYTILUS MERIDIONALIS (KRAUSS) AND AULACOMYA ATER(MOLINA) AS INDICATORS OF BIOTIC CONDITIONS

A. BARKAI and G. M. BRANCH

Zoology Dept. University of Cape Town Rondebosch 7700, S. Africa

Two mussels, Choromytilus meridionalis (Krauss) and Aulacomya ater, (Molina) were used as indicators of biotic factors that apparently result in radically different sublittoral communities at two closely situated islands on the South African west coast. Specimens of both species were attached to submerged plates, which were either unprotected or protected from predation by cages. C. meridionalis grew faster and tolerated silting and smothering better than A. ater. At Malgas Island adults of C. meridionalis are completely absent and those of A. ater scarce due to intense predation by rock-lobsters. When protected against predation, growth of both species was significantly faster and survivership higher at Malgas than at Marcus Island, where C. meridionalis is abundant but A. ater absent. When the mussels were unprotected at Malgas, they were eaten in a few hours. At Marcus Island where rock-lobsters are virtually absent, caged mussels grow faster than unprotected individuals and survived longer. Drilling whelks (mostly Nucella squamosa (Lamarck)) were major predators at Marcus. Relative intensity of rock-lobster predation and competition between the mussels appear to control the structure and diversity of the sublittoral communities at these sites, rather than physical factors. Growth rate, shell thickness and tolerance of smothering by other species determine which species of mussel predominates.

(Received 5 April 1988; accepted 11 July 1988)


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