Limited occurrence of the profunda morph of the quagga mussels (Dreissena rostriformis bugensis) in the Volga River reservoirs of Russia and limited genetic differences between morphs in Russia and North America

The quagga mussel Dreissena rostriformis bugensis is a highly invasive species. It plays an important role in benthic communities, influencing their structure and functioning. Two morphs of this mussel have been described: a shallow-water (i.e. the typical) morph and a deep-water morph, profunda. Currently, profunda has been found in several water bodies only within the nonnative range of D. r. bugensis. In North America, the profunda morph is widespread and abundant in the Laurentian Great Lakes. In Europe, profunda was found for the first time in 2009 in the Cheboksary Reservoir, which is located on the central part of the Volga River. A 2016 search for profunda in the four deepest Volga reservoirs (Cheboksary, Kuybyshev, Saratov and Volgograd) failed to find this morph even at the site where it was found in 2009. Traditional and outline-based morphometric analyses showed that only the shallow-water morph individuals inhabited studied sites at depths of 25–33 m. The present study revealed that morphological differences between the typical and profunda morphs from both the European and North American ranges were accompanied by limited genetic differences. Microsatellite analysis showed only minor differences in allele frequencies between morphs sampled in 2019 from the Cheboksary Reservoir; these differences were not significant after correction for null alleles. High phenotypic and ecological plasticity of D. r. bugensis may facilitate its invasion success.