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

Abstract

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.