PHENOTYPIC PLASTICITY IN THE FOOT SIZE OF AN INTERTIDAL SNAIL

Abstract

The risk of dislodgment due to hydrodynamic forces on rocky intertidal shores is greater on wave-exposed than on protected shores, and this is believed to represent an important selective force in intertidal communities. For intertidal snails, the probability of dislodgment by a given flow is determined, in part, by shell size and shape, and by the attachment strength of the foot. This study addressed two questions. First, do habitat-specific differences in traits that reduce the risk of dislodgment of an intertidal snail (Littorina obtusata) parallel differences in wave energies? To address this question, I measured variation in (1) shell size (defined as the maximum projected surface area, MPSA); (2) foot size; (3) maximum shear dislodgment force; and (4) tenacity (dislodgment force per unit foot size) of two wave-exposed and three protected snail populations. Second, are habitat-specific differences in foot size, and hence attachment strength, the product of selection or of phenotypic plasticity in response to increased hydrodynamic stress? I conducted field transplant experiments and a laboratory flume experiment that manipulated water velocity to test for plasticity in foot size. Wave-exposed snails exhibited traits that reduce the risk of dislodgment. Their shells were smaller (MPSA) and more squat (shell height relative to MPSA and shell length) than were shells of protected conspecifics. Wave-exposed snails also had larger foot sizes and were thus able to resist greater shear dislodgment forces than protected conspecifics of similar MPSA. Neither wave exposure nor collection site influenced tenacity, indicating that the greater dislodgment forces of wave-exposed snails were due to their larger foot sizes. Assuming that MPSA is proportional to speed-specific drag, which is reasonable for bluff bodies, I found that dislodgment force was proportional to drag. Foot size scaled isometrically with MPSA, and dislodgment force scaled isometrically with foot size, indicating that the risk of dislodgment does not change as snails grow. The field and laboratory flume experiments provide the first clear demonstration that increased hydrodynamic stress induces plastic increases in foot size. Protected snails raised on a wave-exposed shore produced a larger foot than controls raised on their native shore. Selection on the foot size of protected snails raised on both shores was not detected in either habitat. Protected snails also produced larger foot sizes when raised in high-velocity flumes relative to snails raised under low-velocity flow. In contrast, wave-exposed snails exhibited no change in foot size when raised under both high- and low-velocity flow. Hence, variation in the foot size of wave-exposed snails is less flexible than that of protected conspecifics. If natural selection favors plasticity in heterogeneous environments, an asymmetry may be favored as well, especially if there are risks associated with improper adjustments to unpredictable environmental cues.