Weak directional selection on inducible defensive shell traits in a marine gastropod by the inducing shell-breaking predator

Abstract

Predation can drive variation in prey phenotypes by selecting for both constitutive and inducible defensive traits, which can act in concert to protect prey from predators. To understand the adaptive evolution of prey traits it is important to determine which traits are true targets of natural selection. Selection gradient analysis is a useful tool for doing this. We combined field surveys of the extensive intraspecific variation in defensive shell traits of the intertidal whelk Nucella lamellosa with laboratory experiments to determine which traits were associated with risk of crab predation, and which traits were under selection by the whelks’ major crab predator, Cancer productus. Field surveys showed strong associations between risk of crab predation and shell shape, shell thickness, and apertural area, but not apertural teeth. Laboratory choice experiments in which crabs were offered a variety of whelk shell morphs revealed that thinner–shelled whelks with wider apertures were consumed significantly more often than thicker–shelled whelks with narrower apertures, confirming previous findings, and resulting in positive selection differentials for these traits. However, multiple logistic regression revealed that only shell thickness, and none of the other traits measured, had a significant effect on whelk survival, indicating only weak directional selection for this trait. The mechanism for crabs selecting for shell thickness in whelks is likely through increased handling time and decreased profitability. Given that shell thickness is a phenotypically plastic trait induced by C. productus, future work should examine the degree to which selection by crabs acts on both constitutive levels and inducibility of shell thickness in N. lamellosa.