The Paracyclops fimbriatus-complex (Copepoda, Cyclopoida): a revision.

The type species of Paracyclops, P. fimbriatus (Fischer, 1853), is redescribed based on material from the vicinity of St Petersburg in Russia, one of the type localities. The taxonomic status of P. fimbriatus, P. chiltoni (Thomson, 1882) and P. imminuta Kiefer, 1929 are stabilised and a neotype is designated for P. fimbriatus. It is revealed that failure to separate P. imminuta from the other two species of the complex was the main factor reponsible for the taxonomic confusion concerning all three. Characters traditionally used by taxonomists, such as proportional measurements of the caudal rami and the terminal spines of the third endopodal segment of leg 4 are found to vary considerably within populations as well as between populations. Since there is extensive overlap in variability expressed between these three variable species, it is therefore concluded that most early records are unreliable. Previously overlooked characters which can help to differentiate between them are introduced, including the ornamentation of the frontal surface of the coxobasis of antenna, of the posterior surface of leg 1, and of the caudal rami.