> How does the poison of a pufferfish protect the pufferfish or
> its closest kin? Why is the poison concentrated in the liver and
> ovaries?
>
> The poison of a pufferfish concentrates in its liver and its
> ovaries. However, if the predator gets that far then the pufferfish is
> clearly dead. Whether the animal dies, or gets sick, is of no
> relevance to the individual. Why is the poison concentrated in these
> organs?
While the liver and ovaries have famously high concentrations of
tetratotoxin in puffer fish species, the toxin is also found in eggs and
the skin. With the skin in mind, see:
Kodama, M., Sato, S., Ogata, T., Suzuki, Y., Kaneko, T., Aida, K.,1986.
Tetrodotoxin secreting glands in the skin of puffer fishes.Toxicon 24(8),
819-829
(
http://dx.doi.org/10.1016/0041-0101(86)90107-8)
Abstract:
"Unique exocrine glands or gland-like structures were found in the skin of
several species of puffer fishes of the genus Takifugu. The glands of T.
pardalis and T. vermiculare porphyreum consisted only of secretory cells
with a large vacuole. These cells were completely enclosed by epithelial
cells with developed microfilaments, except at their opening to the lumen.
The contents of the large vacuole in the peculiar secretory cell were
forced out when the puffer was stimulated. Exocrine glands or gland-like
structures with peculiar secretory cells were also found in the skin of T.
poecilonotus, T. niphobles and T. vermiculare radiatum. A high
concentration of TTX was detected in the gland contents collected directly
from live specimens of T. pardalis. We therefore conclude that these glands
are TTX secreting glands"
This is pretty old, but the idea seems to still be considered valid.
"TTX plays an apparent defensive role against predators in many species
possessing it, from the expulsion of toxinfrom specialized skin glands in
puffer fish"
(Lehman et al 2004 p244
http://137.111.107.167/~chris/lit/refs/LBBr01.pdf)
>
> One possibility that I was thinking is that the poison may be
> distributed throughout the body, but the bacteria that make the poison
> happens to like the environment of these organs. So ,the regions of
> greatest concentration are not in themselves adaptive from the
> pufferfish point of view. Just the bacteria.
It's probably a combination of factors. The presence of glands on the skin
which can expell TTX when the fish is frightened is certainly adaptive -
presumably the toxin can be absorbed through the skin or perhaps even gills
of potential predators. It is interesting that the highest concentrations
are found in the liver and ovaries, however, since as you pointed out the
toxin in these organs is unlikely to protect the individual from being
eaten. However, as you mentioned there are two possibilities:
1. the concentration in the liver and ovaries is incidental to the presence
of the skin glands - either due to the accumulation of systemic toxin or
due to the bacteria liking the environment.
2. in the event the skin toxicity isn't adequate to protect the fish, a
potential predator might be kind enough to die after consuming the levels
of TTX found in the liver. While not much good to the individual, it would
certainly have survival value for the school.
One thing that has interested me lately is the fact that TTX is highly
concentrated in the ovaries, but not in the testes. Apparently the idea
that TTX levels are higher during the fugu spawning season hasn't been
supported by experiment - in fact the opposite has been found to be true in
a couple of Takifugu species:
Yu & Yu "Are puffer fish more toxic in their spawning seasons?" Marine
Biology 140(5):1432-1793, May 2002
http://www.springerlink.com/content/mnatxrbyywtldcbp/
This seems to follow: if spawning season triggered higher TTX retention,
then surely one would expect the testes - which are only produced for
spawning - to have a high level of toxin; Yu & Yu report that the testes
are non-toxic (less than 10 MU/g). This seems to suggest that rather than
being adaptive, the concentration of TTX in the liver and ovaries is
incidental.
> Or maybe the liver just
> happens to collect the toxin for disposal, like it does many toxins.
Exactly. But what about the ovaries? I'm stumped on that one.
--
Bon nou mujin sei gan dan
