Locked in the icehouse: Evolution of an endemic Epimeria (Amphipoda, Crustacea) species flock on the Antarctic shelf.

The Antarctic shelf's marine biodiversity has been greatly influenced by the climatic and glacial history of the region. Extreme temperature changes led to the extinction of some lineages, while others adapted and flourished. The amphipod genus Epimeria is an example of the latter, being particularly diverse in the Antarctic region. By reconstructing a time-calibrated phylogeny based on mitochondrial (COI) and nuclear (28S and H3) markers and including Epimeria species from all oceans, this study provides a temporal and geographical framework for the evolution of Antarctic Epimeria. The monophyly of this genus is not supported by Bayesian Inference, as Antarctic and non-Antarctic Epimeria form two distinct well-supported clades, with Antarctic Epimeria being a sister clade to two stilipedid species. The monophyly of Antarctic Epimeria suggests that this clade evolved in isolation since its origin. While the precise timing of this origin remains unclear, it is inferred that the Antarctic lineage arose from a late Gondwanan ancestor and hence did not colonize the Antarctic region after the continent broke apart from the other fragments of Gondwanaland. The initial diversification of the clade occurred 38.04Ma (95% HPD [48.46Ma; 28.36Ma]) in a cooling environment. Adaptation to cold waters, along with the extinction of cold-intolerant taxa and resulting ecological opportunities, likely led to the successful diversification of Epimeria on the Antarctic shelf. However, there was neither evidence of a rapid lineage diversification early in the clade's history, nor of any shifts in diversification rates induced by glacial cycles. This suggests that a high turnover rate on the repeatedly scoured Antarctic shelf could have masked potential signals of diversification bursts.

Genetic and morphological divergences in the cosmopolitan deep-sea amphipod Eurythenes gryllus reveal a diverse abyss and a bipolar species.

Eurythenes gryllus is one of the most widespread amphipod species, occurring in every ocean with a depth range covering the bathyal, abyssal and hadal zones. Previous studies, however, indicated the existence of several genetically and morphologically divergent lineages, questioning the assumption of its cosmopolitan and eurybathic distribution. For the first time, its genetic diversity was explored at the global scale (Arctic, Atlantic, Pacific and Southern oceans) by analyzing nuclear (28S rDNA) and mitochondrial (COI, 16S rDNA) sequence data using various species delimitation methods in a phylogeographic context. Nine putative species-level clades were identified within E. gryllus. A clear distinction was observed between samples collected at bathyal versus abyssal depths, with a genetic break occurring around 3,000 m. Two bathyal and two abyssal lineages showed a widespread distribution, while five other abyssal lineages each seemed to be restricted to a single ocean basin. The observed higher diversity in the abyss compared to the bathyal zone stands in contrast to the depth-differentiation hypothesis. Our results indicate that, despite the more uniform environment of the abyss and its presumed lack of obvious isolating barriers, abyssal populations might be more likely to show population differentiation and undergo speciation events than previously assumed. Potential factors influencing species' origins and distributions, such as hydrostatic pressure, are discussed. In addition, morphological findings coincided with the molecular clades. Of all specimens available for examination, those of the bipolar bathyal clade seemed the most similar to the 'true' E. gryllus. We present the first molecular evidence for a bipolar distribution in a macro-benthic deep-sea organism.