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stock tank. Within a few minutes they were all looking just fine, showing no
sign of any further problems. All made the trip home without any recurrence of
the problem.
Self-poisoning
So why does this happen? Well, from my observations over the years I have
discovered that many, if not all, catfishes of the subfamily Corydoradinae
(Aspidoras, Brochis, Corydoras, and Scleromystax) release toxins when under
extreme stress. At first it was not known from where or how the toxins were
released. It was only several years after my original encounter with the problem
that I first noticed a mucus-like substance being exuded by a Corydoras
under stress—in this case, from under the base of the left-hand gill plate. The
fish releasing the mucus was the female of a pair of Corydoras sterbai (Sterba’s
Cory) that were being exhibited at the Catfish Study Group Open Show in
Wigan, England, in 2002. I immediately took a photo (reproduced at left),
and this is the only photographic record of this phenomenon that I am aware
of. In this particular case, the water in the show tank was replaced, and the
fish quickly made a full recovery and went on to win its class.
The strength of the substance seems to vary according to the species, with
Corydoras trilineatus apparently being the most toxic. The term “poor travelers”
was frequently applied to several species before the phenomenon now
known as self-poisoning was understood.
I believe this behavior to be a natural defensive mechanism that is probably
present in many other genera of fishes, and not just the Corydoradinae.
I suspect it is primarily a defense against predation, with the toxin being
released when the fish is grabbed by a larger predator, an attempt to cause the
latter to release its grip and affording the prey a chance to escape.
How does this theory translate into hobby terms? The aquarist represents
the predator, catching the cory in a net and putting it into a bag or small
container. The fish is, of course, put under stress and instinctively releases its
toxic fluid. Here we deviate from the predator/prey situation because, although
the cory has been released from the net, it is still under stress and in a
state of shock; in the wild it could simply swim away in the copious toxin-free
water of its native river, but in a plastic bag or show tank it cannot escape
from the poisonous fluid it has released. As a result, in a very short space of
time the fish starts to be affected by the poison that has now fully dispersed
into the water. The cory appears to stop breathing, and the fish is dead within
a few minutes.
This problem can and does happen whenever we transport corys, be they
new fishes from a shop, a group of young you are taking to a friend or a shop
to sell, or even specimen fishes you wish to exhibit at a show. The first indication
of the existence of a problem is small bubbles forming at the edge of
the water’s surface in the bag or other container; the fishes may be breathing
rapidly, although usually the opposite is the case—their breathing slows down
or even stops altogether.
Mucus being released from
the gills of a female Corydoras s
sterbai.