The movements of two species of predatory whelks (Muricidae: Gastropoda) were examined on several rocky seashores in New South Wales, Australia, from 1980 to 1984. The smaller species, Morula marginalba Blainville, moved less far than the larger species, Thais orbita (Gmelin). Intermittent but rapid movement allowed T. orbita to shelter in exposed places low on the shore, but to feed on their preferred prey, barnacles, much higher up to shore. This resulted in a dynamic and unpredictable pattern of vertical foraging movements for this species. Compared to areas with intact prey or with barnacles that had been killed but not removed, M. marginalba moved quickly away from areas from which barnacles had been experimentally removed. Many M. marginalba left experimental plots on two shores without prey or crevices in which to shelter, whereas some stayed over 30 d in areas with either prey or crevices (but not both), and most remained in plots where both prey and crevices were available. Thus, the presence of suitable prey and crevices seems to influence the movements of both species, which respond in different ways, corresponding to their different mobilities. These results are discussed in relation to the behavioural cues involved and predation by the whelks.
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