First Record of the Alien Tintinnid Ciliate IRhizodomus tagatzi/I Strelkow and Wirketis 1950 in the Adriatic Sea

The tintinnid ciliate Rhizodomus tagatzi has spread rapidly in recent decades in neritic areas of the Mediterranean and adjacent seas, where it is classified as a non-indigenous species. The present study documents the first finding of the species in the Adriatic Sea, in the transitional waters of the Neretva River estuary. Zooplankton material was collected with 5 L Niskin bottles in three layers (1, 5 and 7 m) at two stations, at the mouth of the Neretva River and 16 km upstream, in the period from May 2022 to January 2023. In addition to the morphological characteristics of R. tagatzi, we discuss the state and variability of the populations based on the abundance of the species and the influence of abiotic environmental factors (temperature and salinity) and possible pathways of entry into the Adriatic Sea. The maximum abundance of R. tagatzi in the Neretva River estuary of ~2 × 10[sup.3] cells L[sup.−1] is among the highest in the Mediterranean Sea, and the species has a significant contribution to the tintinnid community (>80%), indicating successful establishment of the population in the estuary and intense influence on ecosystem functioning until the strong river flow completely displaces the wedge from the seabed. Statistical methods confirm the differences between stations in the total abundance of tintinnids and differences in their vertical distribution at the upstream station.