Rhachotropis (Eusiroidea, Amphipoda) from the North East Atlantic

Abstract

The genus Rhachotropis has the widest geographic and bathymetric distribution of all amphipod genera worldwide. Molecular and morphological investigations of specimens sampled around Iceland and off the Norwegian coast allow the first insights into the relationships of North East Atlantic Rhachotropis. The 31 cytochrome oxidase subunit I (COI) sequences generated for this study were assigned 13 Barcode Index Numbers (BINs) in the Barcode of Life database (BOLD), of which 12 are new to the database. Molecular analyses of COI and 16S sequences could not confirm a theory that depth has a greater influence on the phylogeny of Rhachotropis than geographic distance. Although the North East Atlantic is a well-studied area, our molecular investigations revealed the genus Rhachotropis may contain cryptic species, which indicates a higher biodiversity than currently known. For example, the specimens which key to Rhachotropis helleri is a complex of three COI clades, two of which cannot be identified with morphological traits. One specimen of each of the clades in the cladogram was documented by high definition photographs. A special focus was on the visual morphology of the eyes, as this character shows interspecific differences within the genus Rhachotropis in response to fixation in ethanol. Detailed morphological investigation showed that some clades thought to be indistinguishable can be separated by minute but consistent morphological characters. Datamining Genbank to examine all registered COI-sequences of R. aculeata, the only previously known Rhachotropis BIN in the North Atlantic and sub-Arctic, showed R. aculeata to be subdivided by an Arctic and a North Atlantic population.

A new predator connecting the abyssal with the hadal in the Kuril-Kamchatka Trench, NW Pacific

“…Four chosen individuals representing three different BINs were taken for 16S gene analysis. It was amplified using the primer pair 16SFt_amp (GCRGTATIYTRACYGTGCTAAGG) and 16SRt_amp2 (CTGGCTTAAACCGRTYTGAACTC) ( Lörz et al., 2018 ). PCR cycling conditions were as follows: initial denaturation in 95 °C for 2 min, followed by 35 cycles of 30 s at 95 °C, 30 s at 50°C, 45 s at 72 °C and final elongation in 72 °C for 5 min.…”

“…The published COI sequence of the genetically closest species of Rhachotropis ( R. cf. proxima, GenBank accession number MG521128 , Lörz et al., 2018 ) was used as outgroup ( Table S1 ). To visualize molecular divergence of COI haplotypes, a Minimum Spanning Network was generated using PopART 1.7 ( Bandelt, Forster & Röhl, 1999 ).…”

Amphipod family distributions around Iceland

“…Eusirids are abundant members of the deep-sea fauna off Iceland (Weisshappel 2000), known to be predators (Enequist 1949) with good swimming capabilities (Bousfield and Hendrycks 1995). Rhachotropis Smith, 1883 was the dominant genus (see Lorz et al 2018), with three other genera represented; namely Eusirus, Cleonardo , and Eusirella . About half of the specimens collected, 355 individuals, were from a single station at 169 m, which was also the shallowest sampled (station 866).…”

A genetic fingerprint of Amphipoda from Icelandic waters – the baseline for further biodiversity and biogeography studies

“…300 m) but from very different localities: the Iceland-Faroe Ridge and the Iceland Basin. As the genus Rhachotropis is the subject of another publication in this issue (Lörz et al 2018) the details of taxonomic rank of R. helleri are not presented here.…”