Do fish get thirsty?
Do fish get thirsty?
Well yes, at least some of them do, so long as we leave aside the subjective human feeling of "thirst". 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 teleosts, 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 - marine fish - 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.
Water tends to move along concentration gradients through water-permeable biological membranes like those shielding most organisms from their environment - a process known as osmosis. Therefore, marine fish, which have a low salt concentration compared with that of seawater, will constantly leak water through their body wall - 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.
(New Scientist 24 November 2007)
