Aspects of osmotic and lonic regulation in two Baltic teleosts: Effects of salinity on blood and urine composition

Hydromineral regulation was studied by examining the response to different environmental salinities in two Baltic brackish-water (BW) teleosts—a species of marine ancestry, Myoxocephalus scorpius (L.), and a glacial relict, M. quadricornis (L.). M. scorpius tolerated fresh water (FW) and M. quadricornis sea water (SW) for only about 24 h, but the survival time of M. scorpius in SW and M. quadricornis in FW was one to several weeks. M. scorpius seems able to balance plasma ionic concentrations in salinities down to about 2 to 3. Death of M. scorpius in FW was associated with partial haemolysis, increased volume of red blood cells (RBC), increased plasma K⁺ concentration, and decreased concentrations of Na⁺, Cl^- and Mg²⁺ in plasma and, to a lesser extent, in urine. Death of M. quadricornis in SW was associated with increased plasma osmolality, and Na⁺, Cl^- and Mg²⁺ concentrations, but the renal excretion of ions approached that generally found in marine teleosts. In most cases, RBC volume followed the changes in plasma osmolality or Na⁺ and Cl^- concentrations. Both species showed an ability to increase tubular Mg²⁺ secretion much over that needed in BW, and increased secretion was associated with high urine Cl^- concentration. M. quadricornis, but not M. scorpius, reabsorbed Na⁺ effectively in SW also. Differences between Oceanic and Baltic specimens of M. scorpius are discussed.