Copepod Arctodiaptomus laticeps (Crustacea, Calanoida) in Large Reservoirs of the Upper Volga (Russia)

The zooplankton of two large reservoirs of the Upper Volga has been studied: Rybinsk Reservoir in May–July 2016–2023 (13–43 stations) and Sheksna Reservoir in July 2017 (23 stations). Breeding populations of the northern species Arctodiaptomus laticeps (Sars, 1863), which is the most widespread and numerous (890 ind./m3 of adults) in the Sheksna Reservoir, are identified for the first time in both water bodies. A description of the structure of the species is provided with photographs of important identifying characteristics. It has been established that, since 2017, A. laticeps has formed a small (less than 100 ind./m3) self-reproducing population in the Main Reach of Rybinsk Reservoir. In these water bodies, the species is monocyclic and has a short breeding period during June and, in cold years, when the water temperature is below 18°C, until the first ten days of July. The structural features of the species and its life cycle are discussed.