How many of you have purchased a group of new long sort after Corys, only to find that they have all died in the bag on the way home.
Self-poisoning in Corydoradinae Catfishes has been known for quite a long time, I first became aware of the problem more than twenty years ago, after loosing a group of newly purchased Corydoras trilineatus. All were dead in the bag when I got home and at the time I just thought the fish were poor quality, although they actually looked very good in the shop. I contacted the shop and the owner willingly offered to replace them, so I returned taking the fish with me. We were both at a loss as to what had happened as the rest of the stock in the shop looked in perfect condition. The replacement fish were in a bag on the counter and we were talking about other fishy things, when he noticed one of the new fish just roll over in the bag, shortly followed by a second. We quickly opened the bag and instinctively put them all in a container with fresh clean water, within a few minutes all were looking just fine and showed no sign of any problems at all and made the trip home without a recurrence of the of the problem.
So why does this happen? Well, from my observations over the years I have discovered that many if not all Corydoradinae catfish release toxins; at first it was not known from where or how the toxins were released. It was only many years after the first encounter that the mucus like toxic substance were seen to be exuded from the base of the gills and was being triggered when the fish were put under stress. The strength of the substance also seems to vary in strength according to species with Corydoras trilineatus probably being the strongest. The term “poor travellers” was often put to several species before this phenomenon was known.
I believe this to be a natural defensive mechanism and probably present in many other genera of fish as well as Corydoradinae. I believe it is primarily a defence against predation, which is released when in this case a Cory, is grabbed by a larger predator and hopefully causing it to release its grip and affording the Cory a chance to escape.
Putting this theory into hobby terms, with we the aquarist being the predator, catching the Cory in our net and putting it into a bag or small container. The Cory naturally is put under stress and instinctively releases its toxic fluid. Here we come to a slightly different situation for the Cory, although it has been released from the net it is still under stress and in a state of shock. In the confines of a small space such as a plastic bag or show tank it cannot escape from the poisonous toxins it has released and in a very short space of time the fish quickly starts to be affected by the poisonous fluids that have now fully dispersed into the water. The effect on the Cory is such that it seams to stop breathing and literally within the space of a few minutes the fish is dead. It was a pair of Corydoras sterbai that were being exhibited at the Catfish Study Group Open Show in 2002, where I was able to witness the actual substance being released by a Cory, this was from the female of the pair and only one image of it actually being exuded is known (fig1).
This problem can and does happen whenever we transport Corys, be they new fish from a shop, a group of young you are taking to a friend or a shop to sell, or even specimen fish you wish to exhibit at a show. The first indication we have that there is a problem are small bubbles forming at the edge of the water surface in the bag or container and the fish may be breathing rapidly, although it is usually the very opposite and their breathing slows down or even stops altogether. This may be an instinctive defensive reaction by the fish to stop itself from breathing in the toxins.
As long as a few basic procedures are followed it is quite a simple matter to avoid loosing Corys through self-poisoning. The first thing to do when catching your fish is to take enough water for transporting the fish from the host tank and put it into a bag or container. Then take another larger container, also containing host tank water, catch the Cory/s and put them into the second larger container, now disturb the Cory/s a few times over the next few minutes, this can be simply done by chasing them with a small net, enough to make them skittish, but not enough to cause them to panic and jump. This should create the necessary amount of stress to ensure that they release their toxins. The Cory/s should then be removed from the second container and placed into a bag containing water from the first container; the water from the second container should then be discarded.
A further tip and one that will possibly cause a comment or two from the animal rights fraternity, is when buying Corys, to try and ensure that they are stressed enough by the person catching them to ensure they have released their toxins before being put into a bag or transporting container. Sometimes it is the inexperienced shop assistant that is the best person to catch your new Corys, mainly because they tend to chase them around a little more than an experienced person, creating that extra little bit of stress.
I know I am repeating what I said earlier but it is important to spot the first signs of the potential problem, which will be small bubbles forming around the edge of the surface of the water, the water may also start to take on a yellowish tinge and to cloud up. Quick action is required, which is to re-bag the fish in new clean water, most if not all aquatic shops will gladly re-bag fish if you think there is a problem.
The species I have found to be most susceptible to self-poisoning are C. adolfoi, C. arcuatus, C. melini, C. metae, C. panda, C. rabauti, C. sterbai, and C. trilineatus. However to avoid any transporting problems care should be taken with all Cory species.
fig1. Female C. sterbai releasing mucus from the gills.