Differences in Depth-Related Phenotypic Plasticity Between Two Deep-Freshwater Ommatogаmmarus Amphipod Scavengers over Similarly Low Genetic Diversity Throughout Ancient Lake Baikal

Deep-water environments constitute the largest ecosystem on Earth and are insufficiently characterized. Lake Baikal, the only freshwater reservoir possessing rich deep-water fauna, provides unique opportunities to investigate the evolutionary processes that happened to such fauna independently and in parallel to adaptation to these environments in the ocean. Here, we focus on amphipods as one of the dominant elements of Baikal deep-water fauna and study the genetic diversity across a wide 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 data revealed low intraspecific diversity of the cytochrome c oxidase subunit I gene marker fragment, at both vertical and horizontal scales, indicating no significant distribution barriers and a bottleneck event in the evolutionary history of both species. At the same time, we found depth-related stratification in carotenoid-based body coloration and eye shape in O. flavus and eye color over stable opsin expression in both species. These findings might indicate partial isolation between vertically stratified populations and characterize the ecological distinctions between the two studied species.