The expansion of invasive species to the East: new sites of the bullheads (genus Ameiurus Rafinesque 1820) in Ukraine with morphological and genetic identification.

This study confirms the extended distribution of two invasive species of the genus Ameiurus in Ukraine. Specifically, A. melas is recorded for the first time in the Southern Buh basin and A. nebulosus has expanded further eastward within the Dnipro basin. Material collected in 2019 and 2022 was identified by morphological features and confirmed by molecular genetic analysis. The most reliable morphological characters for distinguishing these two species include anal-fin membrane pigmentation (light or black), gill raker count (fewer or more than 16), and serrations on the pectoral-fin spine (well-developed along the full length or small, absent near the tip). The analysis of the cytochrome oxidase subunit I barcoding marker identified all samples from the Dnipro Basin (Tnia and Velykyi Luh localities) as A. nebulosus, while all specimens from the Vinnytsia region within the Southern Buh basin (Sotskoho and Vyshenske lakes) were attributed to A. melas. The maximum-likelihood analysis revealed clearly separated clades with high bootstrap support (>75%), strongly supporting the presence of the two separate species. This study suggests the potential for further eastward expansion of both species within Ukraine: A. nebulosus in the northern direction and A. melas in the southern direction.