The maintenance of this osmotic balance is energetically expensive , as it involves the action of several ATP-fuelled transporters. The most important of these, responsible for directly or indirectly driving most of the ion and water movements, is the Na+, K+-ATPase (NKA). For example, in muscle cells NKA can account for up 80% of the resting cellular metabolic rate . Measurements in isolated osmoregulatory tissues have shown that the cost of NKA pumping represents between 25 and 60% of the total tissue VO2 in the dogfish Squalus acanthias. At a whole animal level the cost of fuelling NKA is likely to be much lower than that in isolated muscle cells or tissues , but as there are many other transporters and processes involved in osmoregulation it is still likely to be a relatively expensive process. It has therefore been hypothesised that it would be energetically advantageous for a fish to remain in salinities close to its internal osmolality (i.e. near the isosmotic point) to minimise osmoregulatory costs . At salinities lower or higher than the isosmotic point the osmoregulatory costs are likely to increase, mainly owing to processes of ion pumping, but also due to the initiation and maintenance of the remodelled osmoregulatory epithelia.