Abstract <p>While monitoring the polar bears in the “Medvezh’i Ostrova” (Bear Islands) Nature Reserve in October 2023, a number of mainland water bodies were surveyed: Nesolenoe Lake and an unnamed lake, the Krestovaya River, an unnamed watercourse on Chetyrekhstolbovyi Island, and the marine bay of Kolyma Bay, all in the East Siberian Sea. Representatives of the zoobenthos were found in each of the water bodies studied, but the fauna was not very diverse. A total of only seven species of benthic invertebrates were revealed in the surveyed water bodies of the Medvezh’i Ostrova Nature Reserve, of which three species were found in sea water: <i>Pacifoculodes pallidus</i> (G.O. Sars 1892) (Amphipoda), <i>Yoldiella nana</i> (M. Sars 1865) (Bivalvia), and <i>Enchytraeus albidus</i> Henle 1838 (Oligochaeta). In the water bodies feeding the Galgavaam River, two species from the family Chironomidae and three species of eurytopic Oligochaeta were recorded. Their abundance ranges from 200 to 1700 ind./m² and 0.10 to 3.70 g/m² biomass in freshwater bodies, compared to biomass not exceeding 600 ind./m² and 1.90 g/m² in the sea. Seasonal cohorts are formed there, 37% of which are merohydrobionts (Сhironomidae) and amphibiotic Оligochaeta, and 13% each are neritic amphipods and bivalves. The formation of seasonal cohorts is determined by the short growing season and the extended ice formation on island and mainland watercourses and reservoirs.</p>