The First Record of Obesogammarus crassus (G.O. Sars, 1894) (Amphipoda) in Lake Ilmen

In 2024, the Ponto-Caspian amphipod Obesogammarus crassus (G.O. Sars, 1894) was first discovered in Lake Ilmen in the Novgorod Region. It is assumed that it was introduced into the lake from the mouth of the Dnieper River in the early 1980s, together with other species of crustaceans. Previously, in the Northwest of Russia, this species of crustacean was recorded only in the Kaliningrad Region. In Lake Ilmen, this invading species was found in the littoral zone on hard substrates in thickets of higher aquatic vegetation. The morphology of Lake Ilmen specimens of O. crassus has been studied in detail by light and scanning electron microscopy. The specimens studied revealed deviations in a number of morphological characters compared to the original description of the species, which may be due to better preservation of the material and the higher resolution of electron microscopy, or may reflect existing intraspecific variability.