OSMOTIC, IONIC AND NITROGENOUS-WASTE BALANCE | Osmoregulation in Fishes: An Introduction

Abstract

Because fishes arose from marine ancestors and entered freshwater (FW) early in their evolution, the ionic composition and total osmotic concentration of their blood is intermediate between seawater (SW) and FW. Since they have permeable gills, marine fishes (except the hagfishes, which are isotonic to SW) face dehydration and diffusive influx of salts (predominantly NaCl). FW fishes face an osmotic influx of water and diffusional loss of ions. Marine teleosts osmoregulate by drinking the medium, using the water gained to balance the osmotic loss of water, and excreting the excess NaCl across the gill epithelium, because the kidney cannot excrete urine that is hypertonic to the blood. Marine elasmobranchs are slightly hypertonic to SW because their blood contains high concentrations of the organic solutes urea and TMAO. They actually gain water by osmosis and therefore have relatively high urine flows (compared with marine teleosts). Excess salt is secreted by the rectal gland, which empties into the cloaca. FW fishes balance the osmotic influx of water by producing copious volumes of dilute urine, and extract needed NaCl from the environment by active transport across the gill epithelium.