The Relationship between the Rhizothrichidae Por (Copepoda: Harpacticoida) and the Cletodoidea Bowman & Abele, Including the Establishment of a New Genus and the Description of a New Species

Abstract

During expedition EMB-238 of RV EM BORGESE to the Fehmarn Belt (Baltic Sea, Germany) in 2020, specimens of a new species of the Rhizothrichidae Por (Crustacea, Copepoda, Harpacticoida) were found. Currently, Rhizothrichidae enclose two genera: Rhizothrix Sars and Tryphoema Monard. The assignment of the new species is based on the following commonly derived characters: the formation of a strong spinulose (sub)apical element on the last segment of the female A1, and the formation of a long brush seta on the last exopodal and endopodal segments of the P1. The new species could not be assigned to either Rhizothrix or Tryphoema because it lacks their autapomorphies but presents exclusive derived characters, including the absence of the abexopodal seta on the A2 allobasis, the possession of only one seta on the endopods of P2–P4, and the fusion of the female P5 exopod and baseoendopod. Therefore, it is assigned to a new genus. Besides the species description, its phylogenetic position in the Rhizothrichidae as well as its possible relationship with Rhizothrix and Tryphoema are discussed. Furthermore, the affiliation of the Rhizothrichidae with the Cletodoidea is justified on the basis of 17 apomorphies. A key to the rhizothrichid genera is provided.