Do fish get thirsty?
Well yes, at least some of them do, so long as we leave aside the subjective human feeling of 鈥渢hirst鈥�. There is also a substantial difference between fish in seawater and fresh water, and we also need to consider the possibility of the thirsty shark.
Bony fish, known as , have a salt concentration in their bodies that is not dramatically different from that of land-dwelling vertebrates. This means that the teleosts of the sea 鈥� marine fish 鈥� live in an environment with a much higher salt concentration than is present in their blood. Their relatives in fresh water are in the opposite position.
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Water tends to move along concentration gradients through water-permeable biological membranes like those shielding most organisms from their environment 鈥� a process known as . Therefore, marine fish, which have a low salt concentration compared with that of seawater, will constantly leak water through their body wall 鈥� especially through the thin and permeable gill epithelia. To replenish the lost water, marine fish need to drink, so it would be easy to argue that they become thirsty. The surplus salt they ingest by drinking seawater is excreted by specialised cells located in the gills.
鈥淢arine fish, with a low salt concentration, will leak water through their body wall鈥�
Freshwater fish, on the other hand, are unlikely to become thirsty. Because they live in a more dilute environment, they have the opposite problem: water flows inwards and dilutes their blood. The freshwater fish therefore need to excrete excess water, which they do in much the same way we do, via a dilute urine.
So we can see that marine fish get thirsty and drink, while freshwater fish avoid drinking but pee a lot.
Finally, sharks, dogfish, rays and skates 鈥� which are cartilaginous rather than bony and are called 鈥� are also marine fish (with a few Central and South American freshwater exceptions). Although the concentration of inorganic salts in their blood is not dramatically different from that of marine teleosts, they have little or no osmotic gradient between blood and seawater. This is because they retain organic molecules, the main ones being urea (carbamide) and trimethylamine oxide (TMAO). In this way, the cunning sharks avoid an osmotic water flow from their body surfaces, and may not be very thirsty.
Stefan Nilsson, Professor of Zoophysiology, G枚teborg University