> 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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>
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