SHELL ARCHITECTURE AND CAUSES OF DEATH OF MICRONESIAN REEF SNAILS.

Abstract

Predation is generally believed to be an important cause of death and a powerful source of natural selection for shelled marine invertebrates (see Vermeij, 1978b, for a review). One form of predation that is particularly common in shallow tropical waters involves breakage of the external skeleton. Various architectural features of gastropods, including a thick shell, dentate outer shell lip, strong external sculpture (nodes, spines, and ribs), and tight shell coiling have been interpreted as adaptations that confer protection against crushing. Laboratory and field experiments have offered strong support for these interpretations (Kitching et al., 1966; Kitching and Lockwood, 1974; Stein et al., 1975; Vermeij, 1976; Zipser and Vermeij, 1978; A. R. Palmer, manuscript). Nevertheless, very little is known about the relative importance of crushing as a means of predation and as a cause of death under natural conditions. Some shells might be more resistant to crushing than others; but if breakage is infrequent as a potential cause of mortality, then this advantage has little ecological or evolutionary significance. Available estimates of the prevalence of crushing suggest that breakage is a relatively uncommon fate of snail shells in temperate waters. Fotheringham (1971) found in southern California that 8.5% of hermit crab-containing Maxwellia (= Shaskyus) festiva Hinds were broken in some way; for shells of Roperia (=Ocenebra) poulsoni Carpenter, this figure is 21.3%. Only 3.0% of subtidal hermit-containing Olivella biplicata Sowerby in southern California were broken shells (Edwards, 1969). Hendler (1977) reported that 28% of Cypraecassis testiculus L. that were collected near Octopus shelters on the Atlantic coast of Panama were broken. Short-term experiments in which snails were kept with predatory crabs in cages on the sea bottom in Ireland and New Zealand show that some snails are vulnerable to crushing, whereas other species or morphs are more resistant to breakage (Kitching et al., 1966; Kitching and Lockwood, 1974). The proportion of shells crushed by temperate crabs under natural conditions has not been estimated. My objectives in this paper are (1) to present data on the frequencies of breakage and drilling in natural populations of reef-associated snail shells as inferred from field collections made in Micronesia, and (2) to show that breakage is a common fate of shells, and that certain habitat refuges and architectural features confer protection against breakage.