Tatarocyclops dayanae — a new hyporheic genus and species of Cyclopidae (Copepoda, Cyclopoida) from the European part of Russia

A new genus and species of Cyclopidae (Copepoda, Cyclopoida) from the hyporheic zone of small rivers in Tatarstan (Russia) is described. The diagnosis of Tatarocyclops dayanae gen. et sp. nov. includes: female antennule 11-segmented; first to fourth swimming legs with two-segmented exopods and endopods; endopod of the fourth leg with two apical spines; and fifth leg two-segmented, not fused to the somite. The new genus shares a two-segmented fifth leg with Reidcyclops Karanovic, 2000, but differs from it and from other genera of the Mixocyclops Kiefer, 1944 group by the following combination: mandibular palp with one seta; maxilliped armature formula 2.2.1.2; first swimming leg basis with inner spine; spine formula of the second exopodal segments of the first to fourth swimming legs 2.3.3.3; third swimming leg without sexual dimorphism; fourth swimming leg endopod with two apical spines; and distal segment of the fifth leg with two setae. Among Cyclopidae with 9–11-segmented antennules, Tatarocyclops gen. nov. is distinctly separated by its unique combination of characters, especially the two-segmented fifth leg. The discovery of a stygobiotic cyclopid genus in the Middle Volga region aligns with recent findings of endemic amphipods in the same area and highlights a previously unexplored diversity of groundwater fauna, which is important for understanding the biogeography of Palearctic freshwater crustaceans.