Crab Shell-crushing Predation and Gastropod Architectural Defense

The shell-breaking behavior of the crabs 0:iu.y verrec~uxri Saussure 1853 and Eriphia .squtmwta. Stimpson 1859 from the Bay of Panama is described. The master claws of both these crabs are well designed for breaking shells. Small shells, relative to the size of a crab predator, are crushed by progressively breaking off larger segments of a shell’s apex, while larger shells are peeled by inserting a large dactyl molar into the aperture of a shell and progressively chipping away the lip of the shell. Heavy gastropod shells are shown to be less vulnerable to crab predators than lighter shells, and narrow shell apertures and axial shell sculpture are demonstrated to be architectural features that deter crab predation. The incidence of architectural features which deter crab predation appears to be higher for smaller gastropod species than for larger gastropods which are too large for most crab predators. Large fish predators prey upon both gastropods and shell-crushing crabs. To avoid fish predators. both these prey groups seek refuge under rocks when covered by the tide. Fish predation thus appears to enforce a close sympatry between smaller gastropods and their crab predators. Shell-crushing predation by decapod crustaceans has been shown to be an important process in the evolution of shell (Kitching & Lockwood, 1974; Vermeij, 1976, 1977, 1978; Zipser & Vermeij, 1978) and crab claw design (Shoup, 1968; Vermeij, 1977). This coevolutionary relationship has been shown to vary latitudinally with heavier predation pressure by crabs in tropical latitudes (Vermeij, 1977. 1978), leading to heavier shells and shell armor in tropical gastropods (see Vermeij, 1978, for review). In addition, it has been suggested that interpopulation variation in the shell thickness of direct-developing littoral gastropods is correlated with the occurrence of predatory crabs, such that individuals in populations subject to heavy predation have thicker shells than conspecifics from populations where predation is not as severe (Bertness, 1977; Kitching & Lockwood, 1974). Teleost fish such as the spiny puffer fish (Diodon spp.) and smooth puffer fish (Arothron spp.) also prey upon shelled organisms by crushing them in their strong mouthparts or pharyngeal mills (Vermeij, 1978; Palmer, 1979). Palmer (1979) has shown that spines on some gastropod shells protect them from teleost predators