Six new Krithe from the Kuril-Kamchatka Trench, with the first insight into phylogeography of deep-sea ostracods

Hadal depths are one of the least explored ecosystems on the planet, with little data on biodiversity and even less on phylogegraphy of its inhabitants, especially of meiofaunal groups. Ostracods are an important component of meiofaunal communities, but with less than a dozen species described from the world’s ocean trenches. Therefore, six species we describe from the Kuril-Kamchatka Trench greatly contribute to our knowledge of the hadal ostracod fauna. The new species belong to the genus Krithe Brady, Crosskey a Robertson, 1874, one of the most common ostracod genera from the continental slopes to abyss. Most of the 150 named species are fossil or subrecent, and described from shells only. Our discovery brings the list of species with known soft parts to 11, and provides further insights into morphology of these small ostracods. Our analysis shows that sexual dimorphism in the sixth leg should be excluded from the generic diagnosis. A relatively high abundance and wide distribution of the new species allowed us to study phylogeny (including estimation of the divergence time), genetic diversity, and phylogeography, providing the first such data for ostracods and one of the rare ones for all meiofaunal hadal (but also general deep-sea) groups. We use three genetic markers: mitochondrial cytochrome oxidase I (COI), internal transcribed spaces (ITS), and 18S rRNA; COI and ITS contained enough variability for the molecular phylogeny, and the former one for the population genetics and phylogeography analyses. Molecular clock analysis suggests that the most recent common ancestor (MRCA) of the six species lived 13 million years ago, supporting previously published estimates that the hadal fauna is relatively young. A high haplotype diversity coupled with a low nucleotide diversity are indicative of a rapid population expansion from a small effective population size. This is also supported with negative, statistically significant, values of Tajima’s D and Fu’s tests. Bayesian Skyline Plot places the time of expansion in the last interglacial period (120,000 years ago) and corresponds to the influx of cold, nutrient rich waters of the Oyashio current. The isolation by distance model is rejected, and almost non-existent population structuring is supported by a combination of low Fst values, and often a higher number of mutational changes between haplotypes sampled at the same locality than between those from different localities.