Description of a new species of Histampica (Ophiuroidea: Ophiothamnidae) from cold seeps in the South China Sea and analysis of its mitochondrial genome

Ophiuroids are widely distributed marine benthic organisms, but very few species of ophiuroids have been reported from deep-sea chemosynthetic habitats including hydrothermal vents and cold seeps. In this study, a new species of Ophiothamnidae, Histampica haimaensis sp. nov., collected from cold seeps in the South China Sea, was described based on morphological and genetic analyses. The new species, representing the first known occurrence of Histampica from chemosynthetic habitats, is distinct from its congeners in having long triangular or half elliptical radial shields that are not in contact with each other, four stout, tapering and blunt arm spines, and three or four lateral oral papillae on each side of the jaw, including distally two large and broad papillae and proximally one or two small and narrow papillae. Phylogenetic analysis based on mitochondrial COX1 gene supports the morphological identification. A complete mitochondrial genome (mtgenome) of the new species was characterized by genome skimming. It is 15,759 base pairs in length and contains 37 genes including 13 protein coding genes, two ribosomal RNA genes and 22 transfer RNA (tRNA) genes. Among the tRNA genes, trnL and trnS are duplicated. When compared with the mtgenomes from the same family of ophiuroids, the mtgenome of H. haimaensis sp. nov. has undergone four potential historical gene order reversals, and one tandem duplication random loss event. ND1, ND2 and ND5 each contains a positively selected site, while COX2 contains three positively selected sites This study provides morphological description to H. haimaensis sp. nov. and reports the first cold-seep ophiuroid mitochondrial genome with a novel gene order arrangement and signals of positive selection, which may have contributed to adaption of Ophiuroidea to the deep-sea chemosynthetic environments.