Depth-Dependent Phenotypic Plasticity Differs Between Two Deep-Freshwater Amphipod Scavengers of the Genus Ommatogammarus Despite Similarly Low Genetic Diversity in Ancient Lake Baikal

Although deep-water environments make up the world’s largest ecosystem, they are poorly characterized. Lake Baikal, the only freshwater reservoir possessing rich deep-water fauna, offers unique opportunities to examine the evolutionary processes that occurred independently and concurrently with adaptation to these environments in the ocean. Here, we focus on amphipods as one of the dominant elements of Baikal deep-water fauna. This study examines the genetic diversity across broad vertical (~1 km) and horizontal (~500 km) ranges, as well as depth-related traits in two deep-water scavengers, Ommatogammarus flavus (Dybowsky, 1874) and Ommatogammarus albinus (Dybowsky, 1874). Our results revealed low intraspecific diversity of the cytochrome c oxidase subunit I gene marker fragment across locations and depths, indicating the absence of significant barriers in the distribution of either species and a bottleneck event in their evolutionary histories. At the same time, we found depth-related stratification in carotenoid-based body coloration and eye shape in O. flavus, as well as in eye color for both species. These findings suggest partial isolation between vertically stratified populations and help to characterize the ecological differences between the two studied species.