Over the last decades significant progress was achieved in the investigations of water fleas (Crustacea: Cladocera). Their morphology, taxonomy and biogeography can be considered as well-studied, but still there are genera almost ignored by taxonomists. The genus Ceriodaphnia Dana, 1853 (Cladocera: Daphniidae) belongs to such problematic groups. From previous publications, it is obvious that different taxa from the genus have a very different morphology of their ephippia. Here, we study ephippium morphology in six common taxa from central-northern European Russia (Ceriodaphnia megops Sars, 1862; C. laticaudata P.E. Willer, 1867; C. rotunda (Straus, 1820) sensu Sars, 1862; C. quadrangula (O.F. Muller, 1785); C. puichella Sars, 1862; C. reticulata (Jurine, 1820)) using light and scanning electron microscopy. A key to their identification based on ephippium morphology is proposed. This could be the starting point for revisions based on morphological characters with special emphasis on ephippia. Also, we propose that Ceriodaphnia taxa at least from the Holocene subfossil samples could be identified to the species group level according to structure of their ephippia. This approach to identification of Ceriodaphnia remains in sediments could improve palaeoecological reconstructions. However a global revision of the genus is still needed.

Remains of Rhynchotalona latens (Sarmaja-Korjonen, Hakojärvi et Korhola 2000) were found in the bottom sediments of several glaciogenic lakes in northwestern Russia. Subfossil remains of the species were noted both in the bottom sediments of the Late Pleistocene and Mid-Holocene. We discovered a rare species, R. latens, in the bottom sediments of Lake Medvedevskoye (Karelian Isthmus). This species prefers shallow oligo-mesotrophic lakes with organic sediments and has attracted the interest of scientists around the world as it is considered a glacial relict and has recently been found in surface sediments and as a living population in Finland and Russia.