something we wondered earlier:
do fish pee?
from www.nanfa.org/archive/nanfa/nanfamay02/0186.html
Bruce Stallsmith (fundulus_at_hotmail.com)
Tue, 14 May 2002 10:21:25 -0400
Yeah, fishes have very active osmoregulatory systems that involve the formation and excretion of urine. In short, freshwater fishes are constantly producing a large volume of dilute urine because they are passively taking up freshwater from the environment. Saltwater fishes produce small amounts of very rich urine, because they face the opposite environmental stress: they are constantly losing water to the surrounding environment by passive
osmosis. The various sharks and rays have a different strategy for osmoregulation that involves trying to conform to the surrounding waters, in part by suffusing their body tissues with urea (a nitrogenous compound). Even so, they still have active kidney systems for ionic regulation via urine formation.
–Bruce Stallsmith
Huntsville, AL, US of A
from: oceanlink.island.net/ask/fishy.html
Fish do actually pee. Marine fish excrete very low volume of concentrated urine that is formed in the kidneys, as in other vertebrates. For the most part though, their nitrogenous wastes are excreted through their gills during respiration.
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