Ozius verreauxii feeds opportunistically on gastropods. Specialized chelal morphology enables the crab to break open the shells of virtually all Nerita scabricosta below a critical size. For mechanical reasons, attack efficiency declines rapidly as snails approach the critical size. O. verreauxii is able to recognize this narrow size-zone by revolving the prey in its mouthparts for a few seconds, rejecting the prey before wasting much time or energy on a potentially unsuccessful attack. Such precision in the evaluation of prey is associated with the modified chelal morphology, consistent with the hypothesis that tropical species tend to be ecologically more specialized than temperate forms.