Dolioletta advena sp. nov., a New Species of Doliolid (Tunicata, Thaliacea) from the Adriatic Sea

The Adriatic Sea, as a part of Mediterranean, is one of the best investigated areas in the world regarding zooplankton. Nevertheless, in the last decade four new species of gelatinous zooplankton were described from the Adriatic Sea. Whether these species are newcomers or they were simply overlooked is still under investigations. Here we provide a description of a new species of Doliolida from the genus Dolioletta, Dolioletta advena sp. nov., found in the Adriatic Sea in August 2021, in a period of high sea temperatures and salinities, suggesting its thermal predilection. Its blastozooids dominated the studied doliolid blastozooid communities in the South Adriatic, except in the 50–100 m depth layer at a coastal Lokrum station. Blastozooids of D. advena sp. nov. possess unique morphological features which easily distinguish it from other doliolid species: the most prominent feature being the curved digestive tract where the intestine does not form a tight coil as in other Dolioletta species. The placement of this species in the genus Dolioletta is corroborated by COI phylogenetic analysis which showed that D. advena sp. nov. forms a well-supported monophyletic clade with Dolioletta gegenbauri (81% bootstrap support). In addition to D. advena sp. nov. COI sequence, we provide COI sequences of five doliolid and one pyrosomatid species, which will greatly improve the availability of thaliacean sequences for metabarcoding studies. The origin of D. advena sp. nov. is unknown, but given the fact that doliolids are well investigated in the Mediterranean Sea, it is likely that it arrived with sea currents either through the Suez Canal or the Strait of Gibraltar. Future investigations will confirm or reject this hypothesis.