Distribution and dispersion of the predatory intertidal gastropod Morula marginalba

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Abstract
In New South Wales, the muricid gastropod M. marginalba occurs intertidally on most rocky shores in lower estuaries and on the open coast. M. marginalba is absent from most wave-battered headlands and shores made up of massed large boulders. Experiments showed that physical dislodgement by waves was a sufficient explanation for absence of the whelks from areas of strong wave action. The xanthid crab Ozius truncatus is abundant on boulder shores and, in the laboratory, preyed on M. marginalba by breaking its shell. The crabs ate many more small than large snails. There is a size-related incidence of chipped shells in populations of M. marginalba adjacent to the boulder habitat, suggesting that the effects of predation by crabs on the snails ranges from slight and size-dependent mortality through to total exclusion. At times the snails form aggregations. Unlike some other species of muricid gastropods, M. marginalba did not aggregate to breed, feed or shelter from heavy waves. Clumping behavior was positively related to the density of non-feeding snails. Whelks fed most when periods of submersion were longest, either due to spring tides or to big waves washing the shore. Since aggregations invariably occurred in pools or crevices, snails were probably sheltering from the effects of long periods of emersion and aggregations were seen when there were enough sheltering snails to form clusters. The snails changed their habitat with increasing body size, from moist, low-shore areas with very small prey, through to a higher intertidal habitat with large prey. Preferred barnacle prey were frequently abundant higher on the shore than the snails were naturally found. Snails caged at these higher levels were able to survive and feed, though at a reduced rate compared with those lower on the shore. These findings support a model of dispersion in which snails tend to remain at the lowest shore level at which suitable prey are sufficiently available; availability being a function of feeding rate and therefore shore height as well as abundance of prey.