The family Clytemnestridae is one of the very few holoplanktonic harpacticoid lineages, typically occurring in the epipelagic zone of all oceans. Its monogeneric status and the cosmopolitan distribution of the only two species, Clytemnesrra scutellata Dana, 1847 and C. rostrata (Brady, 1883), have been universally accepted since 1891. Re-examination of the major expedition collections (Challenger 1873-76, Cambridge Suez Canal Expedition 1924, Great Barrier Reef Expedition 1928-29, Discovery) in the Natural History Museum proved both perceptions to be fal se. The generic concepts introduced by Claus ( 189 lb) but rejected by subsequent authors are revived, resulting in the recognition of two valid genera Clyternnestra Dana, 1847 (syn. Goniopelte Claus, 1891a) and Goniopsyllus Brady, 1883 (syn. Sapphir Car, 1890). Genera are separated on the basis of antennulary segmentation, caudal ramus sexual dimorphism and differences in the armature of the antenna, maxillule, maxilla, Pl and P2. Fundamental discrepancies are found in the female genital field and the male gonopores. Species discrimination prior to this revision was exclusively based on generic characters. Detailed examination of NHM material has quadrupled the number of species in t!ie family. Redescriptions are provided for both C. scutellata and G. rostratus, and descriptions are given for five new species previously confounded with these type species: C. farrani sp. nov., C. longipes sp. nov., C. asetosa sp. nov., G. clausi sp. nov. and G. brasiliensis sp. nov. Goniopelte gracilis Claus, 189 la is redescribed and reinstated as a valid species in Clytemnestra. It is believed to represent the Atlantic-Mediterranean sister-species of C. scutellata which presumably assumes only a restricted eastern Indo-Pacific distribution. Neotypes are designated for C. scutellata and C. gracilis. Mediterranean and other European records of G. rostratus in reality refer to G. clausi sp. nov. C. hendoiffi Poppe, 1890 is a junior subjective synonym of C. scutellata. The doubtful status of Sapphir rostratus Car, 1890, Clytemnestra tenuis Lubbock, 1860 and C. hendoiffi. var. quinquesetosa Poppe, 1890 is discussed. The intricate taxonomic history of the family is reviewed, including the nomenclatural confusion surrounding the priority of the family name. The phylogenetic relationships of the Clytemnestridae as well the onto genetic processes underlying the caudal ram us sexual dimorphism in Clytemnestra are discussed. The taxonomic impediment in marine plankton research caused by the failure to recognize pseudo-sibling or cryptic species is highlighted.
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