Morphological variability and systematics of Ponto-Caspian cladocerans of the genus Cercopagis Sars, 1897 (Branchiopoda: Onychopoda: Cercopagidae)

In recent decades, significant changes have occurred in the composition of zooplankton, especially in the Ponto-Caspian Basin. An important component of zooplankton in this region is cladocerans of the genus Cercopagis. However, the introduction of invasive species means that many species of this genus have probably become extinct. Because their disappearance molecular study of these organisms is no longer possible. Therefore, a comparative analysis of morphological variability has become more important. We studied the morphological variability of all 12 currently known species of the genus. Samples from the Caspian, Azov, Black, and Baltic Seas were analyzed. The number of species in the genus was probably overestimated. Two species (C. beklemishevi and C. longicaudata) are likely to be first-generation individuals hatched from resting eggs of C. micronyx and C. spinicaudata, respectively. C. neonilae is perhaps a junior synonym of C. socialis, and C. anonyx is a junior synonym of C. micronyx. The geographic variation of C. pengoi suggests that the Caspian populations of this species differ significantly from all others. We propose to consider them as a separate subspecies.