Redescription of Enhydrosoma sarsi (Scott, 1905) (Copepoda, Harpacticoida, Cletodidae T. Scott) from the western Baltic Sea (Germany) and remarks on the systematics of Enhydrosoma Boeck, 1873

Findings of Enhydrosoma sarsi (Copepoda, Harpacticoida, Cletodidae T. Scott) in the Fehmarn Belt (western Baltic Sea, Germany) during the “MGF Ostsee” research mission enabled a comprehensive and detailed redescription of the species. This prompted a morphology-based phylogenetic study aiming to establish (i) Enhydrosoma as a monophylum and (ii) E. sarsi as a phylogenetically justifiable, distinct species. The phylogenetic characterisation of Enhydrosoma, based on 79 morphological characters, was subject to some reservations, because the four recognised apomorphies also occurred, at least sporadically, in the here compared genera Geehydrosoma, Kollerua, Limnocletodes, Schizacron, Spinapecruris and Strongylacron.

In contrast, E. sarsi could be clearly characterised as a distinct species on the basis of six autapomorphies in mainly the antennula. Mitochondrial COI and nuclear 18S (V1–V2) sequence data obtained for selected Enhydrosoma species were used to reinforce morphological identifications and to support the phylogenetic framework. In the course of our phylogenetic analysis, it was noted that two of the 37 species currently assigned to Enhydrosoma must be excluded from the genus.

Enhydrosoma pectinatum was transferred to the genus Geehydrosoma as G. pectinatum comb. nov., and E. wellsi to Cletodes as C. wellsi comb. nov. Further inconsistencies with regard to the characterisation of Enhydrosoma and the systematic relationships within the Cletodidae are illustrated with examples. They underline the need for a revision of the Cletodidae, with particular importance dedicated to the redescription of already known species.