Constitutive and inducible defensive traits in co‐occurring marine snails distributed across a vertical rocky intertidal gradient

Summary 1.  Among‐species differences in the expression of constitutive and plastic traits may shape species distributions within a community. For many systems, the relationships among individual species’ phenotypes, particularly morphological plasticity, and performance and distribution are poorly understood. 2.  I tested whether three co‐occurring species of marine snails in the genus Nucella differed in their constitutive and inducible defences against crabs, in terms of growth and shell architectural defence. I also tested whether among‐species trait differences were associated with interspecific differences in antipredator performance (resistance to crushing) and distributions along a vertical tidal emersion gradient on rocky shores where the snails co‐occur. 3.  Species distributed lower on the shore and thus exposed to crabs for longer periods of time exhibited higher constitutive growth rates than the upper‐shore species. Lower‐shore species also exhibited greater inducible reductions in somatic growth and increases in shell thickening than the upper‐shore species. Plastic alterations of the scaling relationship between shell‐thickness and somatic growth (i.e. greater shell mass per body mass) enabled the two lower‐shore species to produce stronger, more crab‐resistant shells in the presence of crabs than their upper‐shore congener. 4.  Results suggest that species‐specific differences in both constitutive and inducible shell defence and growth reflect growth/mortality tradeoffs that are associated with species’ vertical distributions along a predation risk gradient. Thus, adaptive differences in inducible morphological defenses should be considered along with more traditionally studied traits such as behaviour and constitutive morphological defenses, when examining traits that influence patterns of species distribution along environmental gradients.