Vigorous SO42– influxviathe gills is balanced by enhanced SO42– excretion by the kidney in eels after seawater adaptation

Abstract

SUMMARYSulfate (SO42–) is maintained at ∼1 mmol–1 l–1 in teleost fishes that are exposed to media of varying SO42– concentrations. We first measured plasma SO42– concentration in euryhaline fishes that adapt to both SO42–-poor freshwater (<0.5 mmol l) and SO42–-enriched seawater (30 mmol l–1). Unlike Mozambique tilapia and chum salmon, Japanese eels maintained higher plasma SO42– concentration in freshwater (6.2±2.3 mmol l–1) than in seawater (0.7±0.1 mmol l–1). We then analyzed the whole-body SO42– budget using 35SO42–. 35SO42– influx in seawater-adapted eels occurred by 84.5% via body surfaces and 15.5% via digestive tracts. The SO42– influx was higher in seawater eels (1.55 μmol kg–1 h–1) than in freshwater eels (0.09 μmol kg–1 min–1), but it was facilitated in freshwater eels when the difference in SO42– concentrations between plasma and environment was taken into account (freshwater eels, 6.2 vs 0.3 mmol l–1; seawater eels, 0.7 vs 30 mmol l–1). One hour after injection of 35SO42– into the blood of seawater eels, the kidney excreted ∼97% of the ionized form, whereas the radioactivity increased gradually in the medium and the rectal fluid more than 3 h after injection. As the radioactivity was poorly adsorbed by anion-exchange resin, 35SO42– in the blood may be incorporated into cells and excreted by the intestine, gills and skin, probably as mucus. These results show that freshwater eels take up SO42– actively from the environment, but seawater eels cope with the obligatory influx of SO42– through the gills by excreting excess SO42–via the kidney and in mucus.