Taxonomic structure and key species of ascidian fauna in the seas of the Russian Arctic

The taxonomic structure and quantitative representation of marine benthic invertebrates belonging to the ascidian class (Ascidiacea) in the seas of the Eurasian Arctic have been studied. Using the Paliy-Kownacky significance index, which accounts both the frequency of occurrence and the abundance of taxa, we have identified the most significant orders, families, genera, and species of ascidians. The similarity in the importance of taxa at different levels across various seas shows the strongest correlation with the geographical location of the water bodies at the species and genus level. The results, based on more than 3000 catalogued finds from the Zoological Institute of the Russian Academy of Sciences and literature data for 69 species inhabiting the region, indicate that the order Stolidobranchia is everywhere dominant, while Phlebobranchia and Aplousobranchia can be considered less significant. At the family level, members of the Styelidae and Molgulidae families dominate, and at the genus level, Molgula, Rhizomolgula, and Styella are the most common. The key species that shape the appearance of the ascidian fauna in the Arctic as a whole are Boltenia echinata (Linnaeus, 1767), Molgula retortiformis Verrill, 1871, Pelonaia corrugata Goodsir et Forbes, 1841, Rhizomolgula globularis (Pallas, 1776), Styela coriacea (Alder et Hancock, 1848) and Styela rustica Linnaeus, 1767. Among these, R. globularis and P. corrugata are more common and occur in large numbers in different seas. The adaptive features of these two ascidian species allow them to occupy distinct, albeit adjacent, ecological niches, avoiding strict competitive exclusion. As a result, their distributions in the Arctic seas show little overlap.