Ion and Water Balance in Elasmobranch Fish

Abstract

Marine elasmobranchs maintain their plasma slightly hyperosmotic to that of the surrounding environment primarily due to the retention of the nitrogenous compound urea, the toxic effects of which are counterbalanced by the presence of trimethylamine oxide (TMAO). Although plasma sodium (Na) and chloride (CI) concentrations are generally higher than those found in marine teleosts they are nevertheless lower than that of seawater (SW) and the fish face a continuous influx of NaCl across semi-permeable membranes in particular the gills. Due to the plasma hyperosmolarity some influx of water will occur and urea will be lost to the environment along a concentration gradient. The lesser spotted dogfish, Scyliorhinus canicula, feeds mainly on invertebrates while the spiny dogfish, Squalus acanthias, tend to feed sporadically on Euphausiids and other fish (Livingston, 1987, Tanasichuk et al., 1991). Salt loading is presumably greatest during feeding when the ionic content of the food together with SW imbibed during feeding adds to the basal salt influx. Typically S. canicula tend to gorge food taking in large quantities over a short period of time following an interval of reduced feeding activity, and therefore the salt load associated with feeding is be intermittent. S. canicula must therefore regulate salt and water balance in the face of acute sodium and chloride challenges associated with large changes in dietary intake on an intermittent basis. The kidneys possess the capacity to produce a urine that is hyposmotic to SW and are not a major site of NaCl excretion; renal retention may be more a important function.