On balance this is a metabolic shortcut for sharks. A detailed examination
of their kidney tissue structures which are relatively simple has lead some
researchers to conclude that sharks must have evolved in a freshwater
environment, which I doubt. To deal with buildups of internal salts sharks
have developed the rectal (or salt) gland, a fingerlike organ just inside
the cloaca that can secrete salt externally. The kidneys themselves are not
very substantial strips of tissue built into the back of their abdominal
And we haven't even talked about any gill epithelium cells with their
various cool bundled-protein ion pumps...
Huntsville, AL, US of A
>Are you sure? I just read about this today, in Fish Anatomy, Physiology,
>Nutrition, p. 61, "The combination of salts at concentrations siimilar to
>those in bony fishes and nitrogenous waste products increases the osmotic
>concentration of the blood so that it is slightly hypertonic to the
>----- Original Message -----
>From: "Bruce Stallsmith" <fundulus_at_hotmail.com>
>Sent: Sunday, January 20, 2002 7:43 PM
>Subject: Re: NANFA-- Osmoregulation in freshwater sharks
> > >FYI, Jeremy, unlike marine bony fishes, sharks have a higher blood ion
> > >concentration 2 percent, as opposed to about .2 percent in the bony
> > >This higher concentration is due to urea, which would kill bony fishes
> > >concentration so high.
> > Urea is present in high concentration, but so is another organic salt,
> > which is usually found in a concentration of 2:1 relative to the urea.
> > enables sharks to be isotonic (roughtly same salts concentration)
> > to sea water and eases osmoregulatory pressures.
> > (10 years ago I could have told you off the top of my head what TMAO
> > for...).
> > --Bruce Stallsmith
> > Huntsville, AL, US of A
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