ИСТИНА

The Kuril Islands are elongated in latitude and are washed by the Sea of Okhotsk and the Pacific Ocean. The hydrological regime of the archipelago’s coastal waters is highly diverse. The Kuril Basin, the Kuril–Kamchatka Trench, and small-scale underwater relief features complicate the seabed structure, creating a wide range of environmental conditions that support a rich sponge fauna. This study analyzes the diversity and biogeography of marine demosponges of the Kuril Islands. The material examined was derived from the museum collection of the NSCMB. Three new species were recorded in the waters of the Kuril Islands: Histodermella kagigunensis Lehnert, Stone & Heimler, 2013; Stelodoryx oxeata Lehnert, Stone & Heimler, 2006; and Geodia sp. nov. For 26 species, new depth ranges and distributions were identified. The integration of published and newly obtained data indicates the presence of 183 sponge species. Of these, 139 species belong to Demospongiae (76%), representing 2 subclasses, 12 orders, 30 families, and 62 genera. A checklist of species was compiled. The most species-rich order is Poecilosclerida, comprising 90 species (65%). The number of endemic species is 20 (14%). The relatively low level of endemism is likely due to the limited isolation of the Kuril Islands’ waters and their strong connections with adjacent seas and the Pacific Ocean. Based on Bray–Curtis similarity analysis, the species composition of demosponges can be divided into four faunal complexes: the Kuril–Kamchatka Trench; the North Kurils; the Middle Kurils + Urup; and the South Kurils (excluding Urup) + the Lesser Kuril Chain. The formation of the Middle Kurils + Urup group corresponds to the concept of a cold-water fauna of Urup, distinguishing it from the other islands of the southern group. Indicator species of the cold-water fauna (from the Arctic to Urup inclusive) are Mycale (Carmia) helios (Fristedt, 1887) and Tetilla sibirica (Fristedt, 1887). Indicator species of the warm-water fauna (Iturup and southward) are Aulospongus villosus (Thiele, 1898) and Forcepia japonica Koltun, 1959. More than 130 species of demosponges are known from the Aleutian Islands, and 434 species have been recorded from Japan. The Kuril Islands, located between these two regions, represent a transitional zone that supports cold-water, warm-water, and ecologically eurytopic species. These patterns indicate that the currently recorded 139 species is an underestimation of the species richness of the Kuril Islands, highlighting the need for further targeted research in this region.