Isolated but phenotypically similar: comparing oceanic island populations of the land crab Johngarthia lagostoma

Abstract Restricted gene flow often leads to structured populations that may experience distinct selective pressures. Understanding the extent to which genetic and phenotypic variation are related can therefore reveal the evolutionary mechanisms shaping disconnected lineages. Oceanic island endemic taxa offer valuable opportunities for exploring this relationship, as they inhabit geographically isolated, usually geologically young, and environmentally stable systems where population genetic structure is strong. The yellow land crab Johngarthia lagostoma, restricted to South Atlantic islands, comprises three genetically distinct populations based on previous genetic study, yet little is known about its phenotypic variation. Here, we compared growth patterns and shapes of the carapace, abdomen, and chelipeds between populations from two equatorial islands (Rocas Atoll and Fernando de Noronha) and one subtropical island (Trindade). Chelar polymorphism was consistent across locations, with balanced frequencies between homochelous and heterochelous morphs. For all traits, we found overlaps in morphometric spaces among populations and structure-related selective pressures, without any match between genetic and phenotypic distances. Our findings are consistent with ancestral polymorphism retention and/or stabilizing selection acting on J. lagostoma populations as the processes underlying the observed patterns. These mechanisms are consistent with geological history and ecological homogeneity among islands, resulting in phenotypic similarity despite genetic divergence.