Consumption of gastropods by the reef crab, Ozius truncatus H. Milne-Edwards: the role of opportunistic foraging behaviour

Abstract

Ozius truncatus was presented with intertidal snails in haphazard order of size. All snails were attacked, but as the crab approached satiation, it began to reject larger snails. Both Nodilittorina australis and Nerita atramentosa were attacked by shearing small pieces from along the spiral axis of the whorl. To do this, force was applied to the inner rim of the shell by the proximal molar of the dactylus. O. truncatus is less specialized in its feeding characteristics than the tropical congener, O. verreauxii. Whereas O. truncatus has a moderately enlarged proximal molar, with mechanical advantage of only 0.92, O. verreauxii has an exaggerated proximal molar with mechanical advantage of 1.17. When feeding on Nerita, O. verreauxii applies the proximal molar to the columellar shelf, breaking within a few seconds all shells below the critical size for crushing. O. truncatus, constrained by the relatively small size of the proximal molar, does not attack the columellar shelf of Nerita, but resorts to breaking the whorl, a method that involves increasingly long handling times as shell size increases. Opportunistic acceptance of prey is a general characteristic of crabs feeding upon gastropods, these prey normally being associated with low encounter rates and relatively unpredictable profitabilities. Opportunistic feeding, reported for crabs feeding on gastropods, and selective feeding, reported for those feeding on mussels, can be explained by a hypothetical mechanism (Jubb et al., 1983) involving the interaction of hunger level with the relative strengths of reinforcing stimuli from the prey under attack and competing stimuli from other prey perceived simultaneously.