Metabolic Tolerance to Atmospheric Pressure of Two Freshwater Endemic Amphipods Mostly Inhabiting the Deep-Water Zone of the Ancient Lake Baikal

Simple Summary Deep-water habitats are the largest ecosystem on the planet: over half of the Earth's surface is covered with a water layer deeper than 200 m and remains poorly explored. Lake Baikal is the only freshwater body inhabited by animals adapted to the deep-water zone independently from their marine counterparts. Comparing these convergently evolved freshwater and marine animals is invaluable for revealing the basic mechanisms of adaptation to high hydrostatic pressure. However, laboratory experiments on deep-water organisms still usually require lifting them to the water's surface and exposing them to potentially hazardous decompression, while endemics from Lake Baikal are poorly studied in this regard. Here, we compared metabolic reactions to such pressure decreases in two Baikal deep-water amphipods (shrimp-like crustaceans) from the genus Ommatogammarus: one species is known to tolerate pressures close to atmospheric levels, while the second was only observed at the pressures from 5 atm and above. We expected that the energy metabolism of the shallower-dwelling species would function better under the atmospheric pressure but found no substantial differences. Thus, despite some difference in long-term survival at atmospheric pressure, both species are suitable for laboratory studies as freshwater model objects adapted to large pressure variations. Lake Baikal is the only freshwater reservoir inhabited by deep-water fauna, which originated mostly from shallow-water ancestors. Ommatogammarus flavus and O. albinus are endemic scavenger amphipods (Amphipoda, Crustacea) dwelling in wide depth ranges of the lake covering over 1300 m. O. flavus had been previously collected close to the surface, while O. albinus has never been found above the depth of 47 m. Since O. albinus is a promising model species for various research, here we tested whether O. albinus is less metabolically adapted to atmospheric pressure than O. flavus. We analyzed a number of energy-related traits (contents of glucose, glycogen and adenylates, as well as lactate dehydrogenase activity) and oxidative stress markers (activities of antioxidant enzymes and levels of lipid peroxidation products) after sampling from different depths and after both species' acclimation to atmospheric pressure. The analyses were repeated in two independent sampling campaigns. We found no consistent signs of metabolic disturbances or oxidative stress in both species right after lifting. Despite O. flavus surviving slightly better in laboratory conditions, during long-term acclimation, both species showed comparable reactions without critical changes. Thus, the obtained data favor using O. albinus along with O. flavus for physiological research under laboratory conditions.