Re: NANFA-- Osmoregulation in freshwater sharks

Bruce Stallsmith (fundulus_at_hotmail.com)
Sun, 20 Jan 2002 23:48:49 -0500

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Yeah, the nitrogenous waste products are urea, which in bony fishes (as well
as ourselves) is found in elevated levels pretty much only in kidney
(nephron) tissues and structures. The other salts would be various potassium
and sodium compounds that are ionic in solution. The overall balance is
indeed slightly hypertonic to seawater, so that water tends to flow into the
shark (or other elasmobranch...). Getting rid of what becomes a dilute urine
is much easier for the fish than the physiological processes of
concentrating urine to a higher osmolarity than either blood (like
ourselves) or the external environment (most notably bony fishes).

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
cavity.

And we haven't even talked about any gill epithelium cells with their
various cool bundled-protein ion pumps...

--Bruce Stallsmith
Huntsville, AL, US of A

>Are you sure? I just read about this today, in Fish Anatomy, Physiology,
>and
>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
>seawater."
>----- Original Message -----
>From: "Bruce Stallsmith" <fundulus_at_hotmail.com>
>To: <nanfa_at_aquaria.net>
>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
>fishes.
> > >This higher concentration is due to urea, which would kill bony fishes
>in
>a
> > >concentration so high.
> > Urea is present in high concentration, but so is another organic salt,
>TMAO,
> > which is usually found in a concentration of 2:1 relative to the urea.
>This
> > enables sharks to be isotonic (roughtly same salts concentration)
>relative
> > to sea water and eases osmoregulatory pressures.
> >
> > (10 years ago I could have told you off the top of my head what TMAO
>stands
> > for...).
> >
> > --Bruce Stallsmith
> > Huntsville, AL, US of A

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