A new species of deep-sea mussel (Bivalvia: Mytilidae: Gigantidas) from the South China Sea: Morphology, phylogenetic position, and gill-associated microbes

A new species of deep-sea mussel, Gigantidas haimaensis n. sp., was described from the Haima Cold Seep on the northwestern slope of the South China Sea. This new species superficially resembles Gigantidas platifrons, but can be distinguished from it by more elongated shells, subterminal umbones, and a nearly straight dorsal shell margin. Phylogenetic reconstruction based on three mitochondrial genes and two nuclear genes revealed that this new species belongs to the Gigantidas clade. Meanwhile, phylogenetic analyses based on the mitochondrial genes cytochrome oxidase I (cox1) and NADH dehydrogenase subunit 4 (nad4) uncovered G. platifrons, Gigantidas childressi, and Gigantidas mauritanicus as the three most closely related species to this new species. Pairwise comparison between G. haimaensis n. sp. and these three species revealed sequence divergence from 4.38% to 6.30% for cox1, and from 4.67% to 5.72% for nad4. The morphological and genetic divergences with the congeneric species of Gigantidas warrant the taxonomic recognition of G. haimaensis n. sp. Metagenomic sequencing the microbial 16S rRNA gene in the gills of G. haimaensis n. sp. indicated that this mussel species mainly relies on methanotrophic Gammaproteobacteria for nutrition.