Additional osmotic challenges placed on some elasmobranchs are concerned with feeding habits. For example, the European Lesser Spotted dogfish, Scyliorhinus canicula, feeds primarily on invertebrates, and tends to gorge feed. This imposes a high salt load on the animal introducing an acute osmotic challenge.

Elasmobranchs adapted to seawater employ a hyperosmotic strategy, in which the plasma osmolality is maintained slightly higher than that of the surrounding medium. Elevation of blood plasma osmolality is maintained by the retention of urea and methylamine compounds such as trimethylamine oxide (TMAO). However, an concentration gradient, from seawater to blood plasma, for sodium and chloride ions still exists.

Gill: Elasmobranch gills are extremely impermeable to urea and indeed, the permeability coefficient for urea in the elasmobranch gill epithelia is reported to be one of the lowest in the animal kingdom. The mechanism behind this impermeability is unclear and it has been suggested that this effect is due to a ‘physical barrier’ to urea. Despite this apparent physical barrier the gills are still the greatest site for net loss of urea. As with the kidney the gills are another important site for the regulation of sodium and chloride in the elasmobranch. However, unlike teleosts the elasmobranch gill does not have the capacity to produce a greater efflux than influx of sodium and chloride ions. Therefore, during periods of osmotic challenge the gill epithelia alone cannot efficiently maintain a constant balance of salts and water in the animal.

Kidney: The length and complexity of the functional unit of the elasmobranch kidney, the nephron, has almost certainly hindered scientific understanding of renal function. However, the retention of urea at the nephron is undoubtedly one reason for its huge complexity. Whether this process is entirely active or passive has yet to be determined. Using an in situ perfused renal preparation we have demonstrated renal effects of a number of peptide hormones, including arginine vasotocin (AVT), angiotensin II (Ang II) and C-type natriuretic peptide (CNP). AVT and Ang II have similar effects, causing a reduction in urine flow rate and glomerular filtration rate (GFR). This reduction in GFR was due to a decrease in the filtering population of nephrons. CNP caused an increase in urine flow rate and GFR, but no increase in the filtering population of nephrons. However, elasmobranch fish do not have the capacity to produce a concentrated urine. Therefore, in the face of a sudden osmotic challenge the elasmobranch cannot maintain osmoregulatory balance through kidney function alone.

Gut: The role of the gut in elasmobranch osmoregulation was thought to be relatively insignificant. However, recent research by FERG has demonstrated that this may not be the case, especially during adaptation to different environmental salinities. Classically elasmobranchs were thought not to drink due to the hyperosmotic strategy that they employ. Work carried out by FERG in collaboration with Prof. Yoshio Takei in Tokyo demonstrated that transfer of the Japanese dogfish, Triakis scyllia, from 75% to 100% salinity induced a drinking response. Recent work has demonstrated a similar effect in S. canicula.