Intertidal or subtidal/circalittoral species: which appeared first? A phylogenetic approach to the evolution of non-planktotrophic species in Atlantic Archipelagos

Volcanic oceanic archipelagos are fascinating natural laboratories of evolutionary patterns and processes in remote, unique conditions. In the insular marine realm, deepwaters and sea-surface circulation hamper dispersal and, for marine invertebrates, this ability is linked to larval development: planktotrophic organisms disperse easily, whereas non-planktotrophic species usually have restricted ranges. Similarly, bathymetric zonation also influences dispersal: intertidal species are more prone to engage in the process than subtidal/circalittoral species. Therefore, the presence of endemic congeneric non-planktotrophic marine gastropods in two Atlantic archipelagos, hundreds of kilometers apart, inspired a biogeographical hypothesis. It predicts that when two congeneric non-planktotrophic gastropod species, with different bathymetric specific ranges, simultaneously occur and are restricted to two remote archipelagos, the subtidal/circalittoral species is expected to be evolutionarily older than the intertidal species. The present study aims to test this theoretical prediction from a multidisciplinary perspective, with a molecular, Bayesian, fossil-calibrated, phylogenetic analysis of selected Rissoidae species to test the theoretical predictions. We hereby corroborate the earlier speciation of the subtidal/circalittoral Alvania sleursi, compared to the congeneric intertidal Alvania mediolittoralis. Supported by ecological and palaeontological observations in the Azores and Madeira archipelagos, our study provides the first phylogenetic approach to this biogeographical hypothesis, unveiling the evolution of Rissoids in two insular Atlantic systems. In a broader perspective, combining molecular and palaeontological data contributes to better understand past processes that shaped current diversity in Atlantic Archipelagos. This approach can be further replicated in other related non-planktotrophic invertebrates in remote Archipelagos, to corroborate the biogeographical hypothesis in other marine taxa.