Physiology of Finfish and Shellfish

2.4.7. Osmoregulatory problem in hag fishes

In hag fishes (marine cyclostomes), the blood is nearly isoosmotic (equal concentrations) to seawater, so that there is practically no movement of water across the semi permeable membranes. That is osmotic work is avoided, as the concentration of Na⁺ and Cl^- ions is similar to seawater. The hag fishes do not drink seawater and their requirement of water for urine formation is met from the blood of the host.