Structural differences between conic and predator-induced bent morphs of <i>Tetraclita rubescens</i> Nilsson-Cantell, 1931 (Cirripedia: Balanomorpha: Tetraclitidae) revealed by CT scanning

Abstract Morphological variants within species can arise as defences against predation. Several species of barnacles are known to exhibit predator-induced variants in the form of bent morphs, in which the rim of the opercular opening is perpendicular to the base. Bent morphs had only been described until recently for species of Chthamalus Ranzani, 1817, but the Northeast Pacific barnacle Tetraclita rubescens (Nilsson-Cantell, 1931) is now also known to exhibit a bent morph along the Pacific coast of the Baja California Peninsula, Mexico in the equatorward part of the species’ range. We show how the bent morph in T. rubescens is formed and how it compares with those of Chthamalus spp. We used X-ray computed microtomography (μCT) imaging technology to compare and quantify morphological differences between the bent and the typical, conic morphs of T. rubescens. All the bent specimens exhibited a bend in the same direction, towards the rostrum, and had a significantly shorter rostrum compared to the other shell wall plates. Conic specimens showed no significant disparity in length among their wall plates. The bent morph also has a relatively smaller opercular opening than the conic morph. We suggest that the bent morph has arisen as the result of recent range expansions of warm-water gastropod predators, an indirect result of climate warming (tropicalisation). A comparison with published information about the bent morphs of Chthamalus species revealed interspecific similarities and differences in shell shape, the direction of bend, and the predators that purportedly induce the bent phenotype. Further studies may clarify whether there are any life-history trade-offs and population impacts associated with bent morphs.