Two South Australian rocky intertidal platforms were sampled in April 1981 and December 1982 (Marino Rocks) and in December 1982 (Lady Bay), respectively. Three snail species, Nerita atramentosa Reeve, 1855, Bernbicium nanum (Lamarck, 1822) and Austrocochlea concamerata (Wood, 1828), exhibited shore-level sizegradients, with smaller individuals occupying the higher intertidal levels. The reef crab (Ozius truncatus M.-Edwards, 1834), whose distribution overlaps that of the gastropods, is an important predator of these gastropods. The shore-level distribution of shell sizes can be explained by differential selection against smaller individuals by reef crabs. The decreased density of small snails at lower intertidal levels may be the result of either their consumption by reef crabs and/or their movement to higher shore levels where crab predation is less intense. balance between physical factors (e.g. desiccation and wave action) and biotic interactions (Bertness, 1977).
Predation by birds (Hartwick, 1981), fish (McCormack, 1982), seastars (Paine, 1969;Markowitz, 1980), crabs (Kitching and Ebling, 1967) and predatory whelks (Connell, 1970) can influence the distributions of some intertidal gastropods. While few studies have examined the effect of predation on Australian intertidal gastropods, predatory whelks (Black, 1978), shallow-water fish and oystercatchers (Parry, 1982) are important predators of limpets. The reef crab Ozius truncatus M.-Edwards, 1834 is a common rocky intertidal species of South Australian shores (Hale, 1927). In this study we describe the effect of crab predation on the distribution of three prosobranch gastropod species Nerita atramentosa Reeve, 1855, Bembicium nanum (Lamarck, 1822) and Austrocochlea concamerata (Wood, 1828) on two intertidal platforms in South Australia.