PHENOTYPIC PLASTICITY IN AN INTERTIDAL SNAIL: THE ROLE OF A COMMON CRAB PREDATOR.

Abstract

Prey species may reduce their risk of predation in many ways (for reviews, see Edmunds 1974; Vermeij 1978, 1987). Although avoidance behaviors and chemical defenses can act as deterrents to predation in marine environments, morphological changes have received the greatest emphasis. For example, spines in a marine bryozoan (Harvell 1984), barnacle shape (Lively 1986a), and numerous gastropod shell traits (Vermeij 1978) are known to deter predation. In intertidal environments, predation by invertebrate predators often parallels gradients in wave exposure and is often most important on shores protected from direct wave energy (Menge 1978). Likewise, variation in the shell thickness of intertidal gastropods also tends to parallel gradients in wave exposure and predation intensity. Thick-shelled morphs are typically found on protected shores where crab predators are more abundant, whereas thin-shelled morphs are found on wave-exposed shores (Kitching et al. 1966; Kitching and Lockwood 1974; Reimchen 1982; Palmer 1985a, 1990). Extreme turbulence probably inhibits crab foraging on waveexposed shores (Kitching et al. 1966; Crothers 1968; Menge 1978). Shell thickness of intertidal gastropods varies among intertidal shores, in part because of differential predation by crabs (Kitching et al. 1966; Kitching and Lockwood 1974; Vermeij 1978, 1987; Seeley 1986). Although natural selection by crabs has been suggested to promote the evolution of thicker-shelled snails (Seeley 1986), predator-induced phenotypic plasticity may also influence shell thickness (Appleton and Palmer 1988; Palmer 1990). Predator-induced morphological defenses are a unique example of phenotypic plasticity in which the predator or a cue associated with the predator (often chemical in nature) induces the formation of structures in the prey that deter predation. This phenomenon is common in aquatic rotifer and zooplankton assemblages (for a review, see Dodson 1989). Most examples of predator-induced defenses involve plants (Baldwin and Schultz 1983; Haukioja and Neuvonen 1985; Rhoades 1985) or clonal (e.g., the cladoceran zooplankton examples cited in Dodson 1989) and colonial organisms (Harvell 1984, 1986). Cases of predator-induced defenses in solitary organisms appear to be rare. Curiously, cases involving solitary organisms, with the exception of Bronmark and Miner (1992), have been found in intertidal environments (Lively