Revision of the Arctic group of species of the family Elpidiidae (Elasipodida, Holothuroidea)

Species of the family Elpidiidae are the only representatives of elasipodid holothurians in the Arctic. To date, four species have been recognized in the Arctic basin: Elpidia glacialis Theel, 1876, Elpidia heckeri Baranova, 1989, Irpa abyssicola Danielssen & Koren, 1878 and Kolga hyalina Danielssen & Koren, 1879. The taxonomy of these species is not well known: Elpidia glacialis and Kolga hyalina are regarded tentatively as a combined species and Elpidia heckeri and Irpa abyssicola were known only from single localities. Three known species are recognized in the examined material: Elpidia glacialis, Elpidia heckeri and Kolga hyalina. One species appears to be new to science: Elpidia belyaevi sp. nov. Irpa abyssicola was not found. A lectotype has been designated for Kolga hyalina. Diagnoses of all examined species have been revised. Elpidiids from the deeper parts of the Arctic Ocean, determined previously as Elpidia glacialis, are identified as Elpidia heckeri or Elpidia belyaevi sp. nov. All Arctic specimens of Kolga are referred to Kolga hyalina. Distribution patterns and vertical ranges of all examined species have been clarified. Zoogeographical aspects are discussed.