Behaviour of Lake Baikal amphipods as a part of the night migratory complex in the Kluevka settlement region (south-eastern Baikal)

INTRODUCTION The nocturnal migration processes that take place in the coastal pelagic zone of large water bodies are considerably different from those that occur in the open- and deep-water pelagic zones. In these open- and deep-water pelagic zones the most active and mobile groups of pelagic species are commonly observed undertaking diel vertical migrations (Vinogradov, 1970; Ringelberg, 2010, among others). By contrast, the migratory complex in the water column above shallower areas at night is composed of a different group of organisms. The migratory complex in marine ecosystems typically includes pelagic organisms approaching the coast, for example, the amphipod family Hyperiidae (Shulenberger, 1977; Williams & Robins, 1981; Elder & Seibel, 2015, among others), euphausiids (Iguchi & Ikeda, 2004), calanoids and other copepods (Vinogradov, 1970), as well as typical benthic organisms that temporarily rise into the pelagic zone, such as cartilaginous and bony fishes (Gauthier & Rose, 2002; Sims et al., 2006), amphipods, isopods, cumaceans, tanaidaceans (Saigusa & Oishi, 2000; Fernandez-Gonzalez et al., 2014, among others), chaetognaths (Nishihama & Hirakawa, 1998), and the free-swimming larvae of benthic invertebrates (e.g., sponges, coelenterates, polychaetes and echinoderms) (Mileykovsky, 1977, among others). All of these organisms have previously been identified as transiting this zone (meroplanktonic organisms). Although Lake Baikal is one of the most unique freshwater ecosystems in the world, it has not been extensively studied. It was previously believed that in this massive lake, the migratory complex was composed only of pelagic species; however, it is now understood that some benthic organisms also can join the nocturnal migratory complex (Melnik et al., 1993; Takhteev et al., 2014). Among the typical pelagic crustaceans in the lake are endemic copepods, Epischura baicalensis Sars, 1900; pelagic Cyclops species (e.g., Cyclops...