Influence of predation by a crab on the distribution of the size-groups of three intertidal gastropods in South Australia

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, Bembicium nanum (Lamarck, 1822) and Austrocochlea concamerata (Wood, 1828), exhibited shore-level size-gradients, 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.