Discovery of a New Species and Host Record of Holophryxus Richardson, 1905 (isopoda: Dajidae) from the Central Arctic: a Model of Enhanced Descriptive Standards for Epicaridean Isopods

Adult isopods of the family Dajidae are exclusively ectoparasitic, typically infecting pelagic malacostracan crustaceans. It is assumed that their life cycle involves free-living and parasitic phases, with planktonic copepods acting as intermediate hosts. Most generic diagnoses proposed in the family have traditionally been incomplete, containing imperfect or misleading information, and characters whose states were wrongly assessed. In an attempt to analyse this state of affairs comprehensively, a taxonomic review and updated diagnosis of the species-rich genus Holophryxus are presented. Both traditional and novel morphological characters are critically assessed, forming the basis for updated differential diagnoses of all currently valid species. The presence of previously ignored sensory structures on the body and pereopods is highlighted and its significance discussed. Holophryxus citriformis sp. nov. is recorded on the common shrimp, Hymenodora glacialis, at 4300 m depth in the central Arctic Ocean. Its description, based on light microscopy and confocal laser scanning microscopy analysis of the female holotype, containing a dwarf male inside its marsupium, is proposed as a model of enhanced descriptive standards required in future morphological research on epicaridean isopods. Definitive host records and geographical distributions of all Holophryxus species are summarized as well as records of larval epicaridean stages associated with copepod hosts.