A new species of (Copepoda, Calanoida, Arietellidae) from the hyperbenthic layer of Japan, with a molecular phylogenetic analysis of some representative genera of the Arietellidae.

A new species of the continental shelf hyperbenthic genus is described, the first from the Indo-Pacific. This is the second species of known, and the first description of a male in the genus. The new species is easily distinguished from other species of () based on: (1) short caudal rami, approximately 1.5 times longer than wide; (2) 2 setae on the mandibular endopod; (3) 6 setae on the maxillular coxal epipodite; and (4) in the female, a short left antennule reaching the posterior border of the genital double-somite. The new diagnosis of differs from in the separation of ancestral segments IX-XII and XIV-XV of the antennule, and the presence of 5-6 setae on the maxillular praecoxal arthrite. is also separated from based on the absence of a seta on the third ancestral segment of the antennary exopod, the symmetry of legs 1-3, the presence of a medial basal seta on the female leg 5, and 2 lateral exopodal spines on the female leg 5. A molecular phylogenetic analysis of some representative genera of the family Arietellidae, including the present new species, recovers two arietellid clades (- and -clades) as in previous studies. Dichotomous keys for the genera of Arietellidae and the species of are included.