Morphological redescription and genetic characterization of the enigmatic Nannopus palustris Brady, 1880 (Copepoda, Harpacticoida, Nannopodidae)

Nannopus palustris Brady, 1880 is considered to be distributed worldwide and is the type species of Nannopus Brady, 1880 that, in turn, is the type genus of the family Nannopodidae Brady, 1880. However, its original description and subsequent amendments are discrepant and is insufficiently detailed, giving rise to considerable taxonomic confusion thus hampering species delimitation, further revisions, and other taxonomical acts. This paper presents a detailed morphological redescription of both sexes of N. palustris with the designation of a neotype from material recently collected from the type locality. We compared key characteristics of the newly collected specimens of N. palustris with those of other species of Nannopus . Morphological and molecular analyses, including 18S rDNA, support the subdivision of the genus into two species groups as proposed by Sciberras et al. (2021). Specimens from several distant populations, previously identified as N. palustris were reexamined. Specimens from the Westerschelde estuary (SW Netherlands) and Small Adzhalyk Estuary (NW Black Sea) were found to be identical to those from the type locality and were attributed to N. palustris . Specimens from the White Sea, however, turned out to be conspecific with N. procerus Fiers & Kotwicki, 2013. Specimens from the Vellar estuary (Bay of Bengal) and Lake Hinuma (Japan) likely represent distinct, yet undescribed species of Nannopus . An updated generic diagnosis is provided, offering a clearer framework for future taxonomic studies.