Osmotic and ion regulation in different species of acipenserids (acipenseriformes, acipenseridae)

Species specificity of osmotic and ion regulation in five species of acipenserids connected in varying degrees with sea conditions is investigated. The objects of study were freshwater middle-Volga sterlet Acipenser ruthenus; Siberian sturgeon A. baerii, inhabiting the Lena River and migrating to the sea near the river mouth; and the following migratory acipenserids of the Volga-Caspian Basin (regular diadromous migrations): the Russian sturgeon A. gueldenstaedtii, the great sturgeon Huso huso, and the starred sturgeon A. stellatus. The study was carried out on immature specimens (of age 2+). The level of osmolarity and concentrations of cations (Na+, K+, Ca2+, and Mg2+) were determined in blood serum, urine, and liquid from the cavity of spiral valve in fish from the fresh water (control) and after their adaptation (period of seven to ten days) to sea water with salinity 12.5–12.7 and 18‰. The increase was found in the functional activity of the mechanism of osmotic and ion homeostasis in sturgeons according to the increase of the environmental salinity of a particular species. The freshwater sterlet adapted to the sea environment as osmoconformer. The other studied species were capable of maintaining a relative stability of osmolarity and ion concentration in blood serum at different levels while adapting to the sea environment; they changed from hyperosmotic (in fresh water) to hypoosmotic type of regulation. The kidney plays a more significant role in the removal of excessive Na+ in sturgeons than in bony fish. An increase in the sodium-reabsorption and magnesium-secretion functions of the kidney as well as the sodium-absorption function of the intestines was observed in sturgeons, relative to the increase of environmental salinity.