Intraspecific trait cospecialization of constitutive and inducible morphological defences in a marine snail from habitats with different predation risk

Summary 1.  Studies examining the integration of constitutive and inducible aspects of multivariate defensive phenotypes are rare. 2.  I asked whether marine snails ( Nucella lamellosa ) from habitats with and without abundant predatory crabs differed in constitutive and inducible aspects of defensive shell morphology. 3.  I examined multivariate shell shape development of snails from each habitat in the presence and absence of waterborne cues from feeding crabs ( Cancer productus ). I also examined the influence of constitutive and inducible shell morphology on resistance to crushing. 4.  Regardless of the presence of crabs, snails from high‐risk (HR) habitats developed rotund, short‐spired shells, while snails from low‐risk habitats developed elongate shells, tall‐spired shells, indicating among‐habitat divergence in constitutive shell shape. Moreover, allometry analyses indicated that constitutive developmental patterns underlying this variation also differed between habitats. However, snails from HR habitats showed greater plasticity for apertural lip thickness and apertural area in the presence of crab cues, indicating among‐habitat variation in defence inducibility. 5.  Both shell shape and apertural lip thickness contributed to shell strength suggesting that constitutive shell shape development and inducible lip thickening have evolved jointly to form an effective defence in habitats where predation risk is high.