The amphipod <i>Amathillina cristata</i> Sars, 1894 is an overlooked native species in the Black Sea basin: insights from molecular data, historical records and SEM imaging

Ponto-Caspian gammarids comprise dozens of endemic taxa. Many are invasive throughout the world, being exceptionally adaptable thanks to the dynamic geological history of the region where they have evolved. Many species described from the Caspian basin were found in the Pontic (Black Sea) basin only in the last century. It remains unclear whether they are native or non-native in this region. Herein, we investigate whether Amathillina cristata, originally described from the Caspian Sea and occasionally listed as an invasive species in the Black Sea, is in fact native to the Black Sea basin. To achieve this goal, we analyse molecular data from Ukraine and – for the first time ever – from Turkey; we also review the entire geographical distribution of the species. For descriptive purposes, we document for the first time the cuticle ultrastructure throughout the body, appendages and mouthparts using scanning electron microscopy. Our results show that A. cristata is distributed in the NW-W part of the Black Sea basin and all investigated Black Sea drainages contain unique haplotypes that appear to have been isolated since the Late Pleistocene, demonstrating that this species is native throughout this basin. Ultrastructure investigation revealed a uniform pattern of pores and sensilla throughout all body regions except the urosome. At least two colour morphs were observed in living specimens. We consider that this species is vulnerable, in particular the Ukrainian populations, due to the high anthropogenic pressure in the region. The destruction of the Kakhovka reservoir in June 2023 most likely affected the known populations in the lower reaches of the Dnieper and Southern Bug rivers.