On the taxonomic classification of Spio (Annelida, Spionidae) species from the Sea of Azov – Black Sea basin

Three polychaete species of the genus Spio Fabricius, 1785 have been found in the Black Sea: Spio decorata Bobretzky, 1870; Spio filicornis (Müller, 1776); Spio multioculata (Rioja, 1918). Only one was found in the Sea of Azov – Spio filicornis (Müller, 1776). S. multioculata is known to occur only in the near-Bosporus region and off the coast of Romania. The species S. decorata was first described in 1870 by N. V. Bobretsky using materials collected in the Sevastopol area. S. filicornis was first found near the coast of Crimea in the Karadag region in 1931 by K. A. Vinogradov. Later, polychaetes of the genus Spio, found in the southwestern part of the Black Sea, off the coasts of Crimea, Caucasus and Bulgaria, and in the Sea of Azov, were classified as Spio filicornis. The species S. decorata was forgotten despite the fact that it was registered in the Mediterranean Sea and off the European coast. To clarify the species identity, polychaetes from the Sea of Azov and Black Sea macrozoobenthos collected during research cruises on RV “Professor Vodyanitsky” and from the Sevastopol Bay (the usual natural habitat of Spio decorata) were used. The bottom sediments were sampled using bottom grabs “Okean-25” (with the capture area of 0.25 m²). The sediments were washed through sieves with the smallest mesh size of 1 mm. In the Sevastopol Bay, macrozoobenthos was collected using a scuba-diver hand-held grab (S = 0.1 m²). Polychaete larvae were collected monthly in the Sevastopol Bay using the Juday net. The live material was processed under a binocular microscope MBS-9; Spio larvae were set aside and let grow until characteristic species attributes appeared. In this work, a morphological description of the collected Spio species as well as photographs and drawings of their characteristic attributes are given. The morphological characteristics of the considered polychaetes of the genus Spio are found to match those of Spio decorata Bobretzky, 1870. It is noted that S. decorata is widespread in the northwestern part of the Black Sea, where it is found at depths up to 38 m. The species occurs in different types of sediments, but prefers slightly silted coquina with sand. The largest occurrence of S. decorata (38 %) is observed at depths of 20–30 m, and the largest density of specimens is at a depth less than 20 m. The maximum density of S. decorata (556 ind. per m²) was registered in 2010 in the northwestern part of the Black Sea at a depth of 19 m. Larvae of S. decorata are found in plankton from March to October at water temperature of +8…+26 °C. Consequently, breeding of this species in the Black Sea occurs in the spring and summer seasons, and the environmental characteristics of its habitat point out the thermophilic character of this species, as opposed to S. filicornis dwelling in Arctic waters. It can be assumed, that in the previous reports on finding the polychaete S. filicornis in the Sea of Azov – Black Sea basin, the found species was actually S. decorata.