A new Caribbean species of the giant isopod genus Bathynomus (Crustacea, Isopoda, Cirolanidae) with notes on its evolutionary history

Abstract The new species Bathynomus tuebingoborikensis sp. nov., discovered in an historic museum collection, is described. We provide preliminary phylogenetic analyses of the genus, including extant and fossil species, using morphological characters. We reconstruct the evolutionary origin of the genus in the Caribbean, and suggest ten different migration waves all over the world oceans, including multiple recolonizations of the Americas. Diversifications correspond with two areas of speciation: the Australasian and Caribbean archipelagos. Our results show a rather variable body size in the more rootward taxa, whereas an erratic increase of body size by 30% is noticed in a more advanced section of the genus tree, a radiation likely corresponding to a colder climate after the Mio-Pliocene border. Body size is correlated to the change of several morphological characters, but—unexpectedly—no correlation could be found with the reported living depths of the species. This suggests that physiological adaptation to large size appeared only once, at the origin of the genus, and a great environmental plasticity is present within the group.