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.

捕食可以通过选择本构性和诱导性防御性状来推动猎物表型的变化，这可以协同作用来保护猎物免受捕食者的侵害。要了解猎物性状的适应性进化，重要的是确定哪些性状是自然选择的真正目标。选择梯度分析是执行此操作的有用工具。我们结合了实地调查，对潮间带海藻Nucella lamellosa的防御壳性状的种内广泛变异与实验室实验进行了比较，以确定哪些性状与蟹的捕食风险有关，以及哪些性状是由蟹的主要蟹肉捕食者癌症生产者选择的。。现场调查显示，捕食螃蟹的风险与壳的形状，壳的厚度和开孔面积（而不是开孔牙齿）之间有很强的联系。实验室选择实验向螃蟹提供了多种变体形变质的螃蟹，这些结果表明，孔径较大的较薄壳的螺than比孔径较小的较厚壳的螺consumed的消耗要多得多，这证实了以前的发现，并导致这些性状的阳性选择差异。然而，多重逻辑回归分析表明，只有壳的厚度以及所测得的其他任何性状都没有对螺状体存活产生显着影响，表明该性状的定向选择较弱。螃蟹选择海螺壳厚度的机制可能是通过增加处理时间和降低获利能力来实现的。C.productus，今后的工作应该研究蟹类的选择对lamellosa的构成水平和壳厚度诱导的作用。