Partial revision of the genus Cercopagis Sars, 1897 (Crustacea: Cladocera: Onychopoda), with the redescription of two species and comments on morphology, taxonomy, reproduction, species richness, zoogeography, and origin

A partial revision of the genus Cercopagis Sars, 1897 was carried out with a redescription of the type species of the genus C. socialis (Grimm, 1877) and the species C. pengoi (Ostroumov, 1892).The nearby related genus Apagis Sars, 1897 was abolished, since its individuals represent only a temporary stage of the life cycle -females of the first generation hatched from resting eggs.Other species, C. micronyx Sars, 1897, C. longiventris Mordukhai-Boltovskoi, 1962, C. spinicaudata Mordukhai-Boltovskoi, 1962, as well as representatives of the "Apagis" forms are described briefly, since only little materials were available for them.There was no material available for the other three species of the genus Cercopagis.The partial nature of the revision is due to the fact that the author did not have sufficient material at his disposal, in particular, because the previously extensive collections of zooplankton from the reservoirs of the Ponto-Caspian-Aral basin have not been preserved.The representatives of the genus Cercopagis are discussed in the aspects of comparative morphology, taxonomy, peculiarities of sexual reproduction, geographical distribution, species richness, and origin.It is assumed that in recent decades, the species richness of the genus had undergone significant degradation due to large-scale changes in the aquatic ecosystems of the basin, in particular, in connection with the introduction of numerous alien species into the Caspian Sea.