Does the transport of larvae throughout the south Atlantic support the genetic and morphometric diversity of the Sally Lightfoot Crabs Grapsus grapsus (Linnaeus, 1758) and Grapsus adscensionis (Osbeck, 1765) (Decapoda Grapsidae) among the oceanic islands?

Grapsus grapsus and Grapsus adscensionis are supralittoral crabs that are known to inhabit oceanic islands and depend on surface currents to recruit in the rocky shores. The ornamentation of the cephalothorax is very distinct among species, but morphological differences are controversial, and integrative studies with different approaches are needed. This study investigated the genetic variation among the populations of G. grapsus from Saint Peter and Saint Paul Archipelago (SP), Fernando de Noronha (FN), Rocas Atoll (RA) and Trindade Island (TR) in the western Atlantic, and G. adscensionis from the islands of Ascension (AI) and Saint Helena (SH) in the mid-Atlantic. Morphology was assessed by geometric morphometric analyses of the carapace and chelae, and numerical analyses of tubercles in the frontal plate of SP, FN, RA, TR and AI populations. In addition, dispersal of the larvae in the Atlantic Ocean was simulated performing a Lagrangian analysis using HYCOM reanalysis dataset as the ocean surface velocity field. The data obtained for the mitochondrial D-loop gene confirmed the distinctness of the two putative species and demonstrated the connectivity between the populations of G. grapsus from the three equatorial islands. The TR population presented unique haplotypes, as well as AI and SH. The geometric morphometric analyses showed differentiation between the carapace shapes for G. grapsus and G. adscensionis, however, the chelae shape does not allow to distinguish between the species or the population. The morphometric and molecular results were consistent with the pattern of particles dispersion in the Atlantic ocean. The larvae of SP, FN and RA mix after two months of drift, while the larvae of TR, AI, SH circulate only around the respective islands. The results reinforce the validation of the two species and the isolation of populations of G. grapsus in TR and of G. adscensionis in AI and SH. The populations of these islands might be maintained by self-recruitment, through larval behavior associated with the local current system, and therefore should be the target of conservation measures.