ИСТИНА

Nudibranch Onchidoris muricata (O.F. Muller, 1776) is a well-known widespread dorid species in the northern Hemisphere. It is characterized by an oval body of white or whitish-yellow colors. The entire surface of the notum is abundantly covered with clavicular and rounded tubercles; rhinophores with two rhinotubercles are located at the anterior end, and unipinate gills are located at the posterior end. Onchidoris muricata lacks a shell like other nudibranch, but the body is strengthened by a large number of subepidermal spicules. In the present work, we study morphological and genetic variability of O. muricata from different locations, including the White Sea, the Barents Sea and the Sea of Japan. The samples were examined by light, scanning electron microscopy, and morphological dissections. For all specimens the mitochondrial COI and 16S markers were obtained to reveal the intraspecific population structure. We confirm species identity of all studied populations, however we detected a distinct genetic divergence between Arctic-Atlantic and Pacific populations and presence of three haplogroups. We also determined conservative and variable morphological characters. Individuals from all studied areas have a reduced plate-like central tooth of the radula, the radular formula (21-30) 1: 1: 1: 1: 1. Internal lateral teeth have a wide base from which the coracoid process departs with or without teeth. The spicule composition is also universal within the species: tetraxon spicules predominate in the body of O. muricata; mono-, di- and thriaxon spicules are also found. Features of the reproductive system morphology are variable traits. Significant differences were found in the shape of the ampulla and bursa copulatrix and in the thickness of the prostate.