FISH PREDATION AND THE EVOLUTION OF GASTROPOD SHELL SCULPTURE: EXPERIMENTAL AND GEOGRAPHIC EVIDENCE.

Marine gastropod molluscs exhibit a wide range of shell forms, many of which are believed to provide some defense against predation. Vermeij (1974) has suggested that narrow or occluded apertures, low spires, thickened shells and strong sculpture act in various ways to deter the attacks of shell-breaking or shell-entering predators. In addition, Vermeij (1976, 1977a) has presented evidence suggesting that inter-oceanic differences in crab predation intensity may have been responsible for the evolution of less vulnerable shells among Indo-West Pacific gastropods when compared with Eastern Pacific and Tropical Atlantic ones. Similarly, one might expect regional differences in the intensity of a particular mode of predation to lead to geographic variation in the frequencies and degree of development of a mode-specific morphological defense. Crabs and fishes are almost certainly the most important shell-breaking predators on lower intertidal and subtidal gastropods. Their modes of feeding, however, are rather different. Crabs are able to manipulate shells more precisely and can apply their chelae to a shell in a variety of ways. They should be less deterred by strong external sculpture than by thickened shells, reduced spires, or reinforced or occluded apertures, except when shells are small relative to chelae. On the other hand, fishes that crush gastropods must use their jaws and hence can only apply stress to very restricted portions of a shell in a restricted manner. They should be more effectively deterred by certain types of strong external sculpture than by thickened lips or restrictive apertures. More specifically, stout spines or nodes appear to reduce susceptibility to being crushed by fish. Such sculpture may act in at least four ways (Fig. 1): 1) it increases the effective diameter of the shell, thus decreasing the mechanical advantage of the predator's crushing apparatus; 2) it distributes the stress over a larger area of the shell; 3) it localizes the stress at the thickest parts of the shell; and 4) it focuses the applied force to a very restricted area of the crushing surface, increasing the chance of damage to the predator. The structural similarity of strong axial ribbing suggests it should perform in a similar manner. Shell thickness and to some degree overall shell shape may also affect susceptibility to crushing. In this paper I directly test the importance of stout spines as a potential defense against crushing by fish. Observations on the type of damage inflicted by crushing fishes serve as the basis for some speculation about how other morphologies might also reduce vulnerability. Finally I consider the geographic distribution of stout spinose sculpture and how it relates to the distribution of shell-destroying fishes. Experimental studies that examine the importance of gastropod shells as a defense against predation have almost exclusively employed crabs as predators (Ebling et al., 1964; Kitching et al., 1966; Kitching and Lockwood, 1974; Vermeij, 1976; Zipser and Vermeij, 1978; but see Ivlev's 1961 study of freshwater carp, roach and perch on Limnaea ovata of different shell thicknesses, p. 102). The present study represents the first experimental examination of the types of shell morphologies that offer an effective defense against shell-crushing fishes and rays. Although I have only