Insights into the diversification of deep-sea endoparasites: Phylogenetic relationships within Dendrogaster (Crustacea: Ascothoracida) and a new species description from a western Pacific seamount

Parasitic crustacean Dendrogaster is completely endoparasitic and is not known from any marine invertebrates other than asteroids. Understanding the diversity of this group is considerably lagging, as it is an internal parasite that is not easily detected without dissecting the host. The group is thought to have coevolved with asteroids due to its endoparasitism; however, this has not been verified due to a lack of phylogenetic studies. A molecular phylogenetic analysis was required to understand the evolution of this very specialized genus. During a biodiversity survey at An'ei Seamount established as an offshore marine protected area in Japan, we discovered an unknown species of Dendrogaster inside a recently described species of asteroids. In this study, we attempted to elucidate the phylogenetic relationships of multiple species of Dendrogaster, including a new species, Dendrogaster nike sp. nov., collected from the An'ei Seamount of the Nishi-Shichito Ridge. We determined the COI and 18S gene sequences of some Dendrogaster species and reconstructed a phylogenetic tree. Upon plotting the information of the host and habitat depth onto the phylogenetic tree, evidence of coevolution between Dendrogaster and its host, which is commonly observed in endosymbiotic organisms, was not observed. Instead, our findings suggest that species diversification in Dendrogaster may have occurred based on habitat depth rather than coevolution with the host.