Experiments on the influence of oysters on the distribution, abundance and sizes of the gastropod Bembicium auratum in a mangrove swamp in New South Wales, Australia

Abstract

Mean density of the littorinid snail Bembicium auratum varied with height above low water in a mangrove swamp (Avicennia marina) in New South Wales, Australia. Snails were more abundant on oysters (Crassostrea commercialis) than on other substrata (pneumatophores, mud and on the trunks of trees). On all substrata, the densities of snails were also greatest at the heights where clumps of oysters were found naturally. At these, relatively low levels on the shore, there was a greater proportion of smaller snails than found elsewhere. Smaller snails also occurred more frequently on oysters than on other substrata. Experimental removal of oysters from areas where they occurred naturally (with associated controls for disturbance of the substratum, the oysters and the snails) rapidly resulted in decreased densities and increased mean size of snails. More snails emigrated or disappeared from plots without than with oysters. More juveniles than adults emigrated or vanished. When snails were transplanted to higher levels in the swamp with oysters, they remained in experimental plots in larger densities than in plots where no oysters were provided. Again, greater proportions of juveniles than adults emigrated or disappeared. Virtually identical results were reproduced in three series of experiments in autumn, 1 month and 1 yr apart, but, in summer (December), no snails moved or disappeared. Oysters are an important influence on distribution, abundance and sizes of snails in mangrove swamps. Previously proposed causes of observed distribution of B. auratum include influences of availability of food. This was not supported by measurements of chlorophyll as an index of microalgae. We propose that oysters provide refuges from predation (by crustaceans or fish) during high tide.