Asian Date Mussel—Arcuatula senhousia (Benson, 1842): On-going Invasion in the Azov-Black Sea Basin

The morphology, biological traits, sex and size structure, and genetic diversity of the recent invader—the Asian date mussel, Arcuatula senhousia—in the Kerch Strait, Taman Bay, and the southern part of the Sea of Azov were described. The Sea of Azov population exhibits a high level of genetic diversity, comparable to that of other invasion regions. The surveyed population showed significant similarity to invasive populations from both Italy and the United States. Arcuatula senhousia specimens were found on soft bottoms, rocky substrates, and macrophytes as fouling. The mussels were observed attached to other mollusks and forming cocoons with byssus and filamentous algae on soft bottoms. The highest abundance in the study area on soft bottoms reached 190 ind/m2, which was an order of magnitude lower compared to the abundance in native regions and other invaded areas. Dense mats of A. senhousia were not observed. Specimens of varying sizes were found in the study areas from 2019 to 2021, with shell lengths ranging from 0.5 to 28 mm. The largest specimens were of a size comparable to adults from both invasive and native regions. The gonads of specimens with shell lengths of 7–21 mm were mature and in the pre-spawning phase, indicating the formation of a self-sustaining population in the Sea of Azov. The main limiting factors for the formation of a dense and widespread population were high wave action and predation pressure. Further expansion of the species could be expected in the Caspian basin.