Ingestion, intestinal absorption, and elimination of seawater and salts in the southern flounder, Paralichthys lethostigma

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Abstract
Simultaneous measurements of flow rates and major electrolyte composition of urine and rectal fluid were made on 5 southern flounders, P. lethostigma, averaging 1.19 kg body weight. Rectal fluid is a residue of seawater sawllowed by the fish, isomotic to the blood plasma (336 mosmoles 1), and composed principally of Mg (average 180 mmoles/1), sulfate (average 105 mmoles/1), and chloride (average 126 mmoles/1). Only 15.5% of the Mg 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 Mg and sulfate yielded estimates averaging 4.6 ml/hr. X kg body weight. Of this volume, 24% (average 1.1 ml/hr. X kg) was eliminated as rectal fluid, 4% average 0.18 ml/hr. X kg) was excreted as urine and 72% (average 3.3 ml/hr. X kg) was lost osmqtically across the body surface. Nearly 99% of the Na, 98% of the K, and 93% of the chloride swallowed was removed by an extrarenal route, presumably the gills. Of the Ca swallowed, 32% remained in the rectal fluid residue, 8% was excreted in the urine, and the rest (60%) was removed by unknown extrarenal pathways.