Simultaneous measurements of flow rates and major electrolyte composition of urine and rectal fluid were made on five southern flounders, Paralichthys lethostigma, averaging 1,19 kg body weight. Rectal fluid is a residue of seawater swallowed by the fish, isosmotic to the blood plasma (336 mosmoles/l), and composed principally of magnesium (average 180 mmoles/l), sulfate (average 105 mmoles/l), and chloride (average 126 mmoles/l). Only 15.5% of the magnesium and 11.3% of the sulfate swallowed was absorbed from the intestine; all absorbed was probably excreted entirely by the kidney. The rate of seawater ingestion calculated from the total (renal + rectal) excretion of magnesium and sulfate yielded estimates averaging 4.6 ml/h × kg body weight. Of this volume, 24% (average 1.1 ml/h × kg) was eliminated as rectal fluid, 4% (average 0,18 ml/h × kg) was excreted as urine and 72% (average 3.3 ml/h × kg) was lost osmotically across the body surface. Nearly 99% of the sodium, 98% of the potassium, and 93% of the chloride swallowed was removed by an extrarenal route, presumably the gills. Of the calcium swallowed, 32% remained in the rectal fluid residue, 8% was excreted in the urine, and the rest (60%) was removed by unknown extrarenal pathways.
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Cleveland P. Hickman (1968) studied this question.