Four New Species of the Deep-Sea Genus Hymenaster (Velatida: Pterasteridae) from the North Pacific, with a Discussion on the Phylogeny of the Family

Sea stars of the family Pterasteridae (Asteroidea: Velatida) are among the characteristic deep-sea echinoderm taxa. The family includes eight extant genera, of which the genus Hymenaster is the most diverse (52 species) and widely distributed. However, its taxonomic diversity remains insufficiently studied, owing to the fragility of the specimens and poorly understood diagnostic characters. In this study, we describe four new Hymenaster species from the abyssal and hadal North Pacific using morphological and molecular data. The updated number of Hymenaster species known from the North Pacific is fourteen. Molecular analyses place the four new species in three different phylogenetic lineages. A concatenated dataset comprising three mitochondrial and one nuclear marker is used to provide a preliminary phylogeny of the family Pterasteridae, the first to include all but one extant genus. Phylogenetic reconstructions recover the genus Myxaster (Myxasteridae) within Pterasteridae, thus confirming the paraphyletic status of the family. The two most diverse pterasterid genera, Hymenaster and Pteraster, are also recovered as paraphyletic. The molecular data indicate that some morphological characters traditionally used to distinguish pterasterid genera may be homoplastic and highlight the need to identify additional characters that would better align taxonomy with molecular phylogeny.