Progressing diversification and biogeography of the mesopelagic Nematoscelis (Crustacea: Euphausiacea) in the Atlantic

Evolutionary mechanisms driving the diversification of pelagic animals remain poorly understood, partly due to the high levels of gene flow in the open ocean. We use molecular phylogenetics, morphological, and phylogeographic approaches to test the allopatric speciation model in respect to the Atlantic krill genus Nematoscelis. Based on our observations, we hypothesize that diversification in genus Nematoscelis may occur through three progressing stages: (i) populations separated geographically and divergence occurred in the mitochondrial COI gene only (Nematoscelis megalops, one clade of Nematoscelis tenella), (ii) morphology diverged (clades of Nematoscelis microps and Nematoscelis atlantica), and (iii) the nuclear H3 gene diverged (clades of N. tenella). Our results confirm allopatric expectations and the dispersal-limiting speciation model. We propose that the primary driver of diversification is geographic isolation coupled with hydrology-linked gene barriers at ∼14–22°N (new) and ∼30°S. The second driver preventing hybridization of diverged populations is linked to external morphology, i.e. enlarged photophores and chitin saddles on the pleon of males, which facilitate sexual selection by female choice. Same-male forms, even belonging to different species, rarely co-occur, which makes the selection effective and not biased. Our results implicate a significant role of non-copulatory characters in Nematoscelis speciation.