First record of the foraminiferal species Ammonia confertitesta in southeastern Baltic sea.

Early identification of non-indigenous species is important, as they have the potential to disrupt biogeochemical cycles, ecosystem stability, and trophic energy transfer in fragile ecosystems. Non-indigenous species may impact the functioning of the Baltic Sea ecosystem, the largest brackish water body in the world. This study presents the first record of the non-indigenous Asian foraminiferal species Ammonia confertitesta in the southeastern Baltic Sea. The species was recorded at 6 out of 14 sampling stations in the Gulf of Gdańsk, Poland, with one station sampled several times throughout the year, where A. confertitesta was consistently present. The species was also found in Puck Bay, a shallow western branch of the Gulf of Gdańsk. Our finding marks the first modern record of the genus Ammonia in the southeastern Baltic Sea. It provides new evidence for the rapid spread of A. confertitesta in European waters, highlighting its ability to thrive in environments where other calcareous foraminiferal species face difficulties. Morphometric comparisons with a population of the same species found in the western Baltic Sea shows that the Gulf of Gdańsk specimens are smaller. This may indicate that the species has reached its limits of environmental tolerance. However, given the increasing abundance of A. confertitesta in the Gulf of Gdańsk, it is important to monitor its further progression and investigate its long-term ecological impact on native species and the Baltic Sea ecosystem.