Shane's World Right Species Right Parasitic Catfishes in the Aquarium

Article © Shane Linder, uploaded January 01, 2002.

The call came early on a Saturday morning.

"Hey Shane, this is Rhonda at the pet store. We received some strange catfishes in our shipment today. Actually, we are not even sure if they are catfish. Can you come take a look at them?" Telling me there is a new unidentified catfish around is probably the fastest way to get me moving on a Saturday morning. I grabbed a couple of books and headed down to the pet store.

Once I saw the fish I knew the trip had been worth my time. Nearly lost in a fifteen gallon tank, sat half a dozen tiny catfish. They were each just over an inch long and less than the diameter of a pencil. "Are you sure they are catfish? They look more like loaches." - "Yes, they are catfish. Remember that all loaches have scales and all catfishes do not. These fish are scaleless."

I was looking at the first member of the catfish family Trichomycteridae (pronounced trick-oh-mick-TARE-id-dee) that I had ever seen. I explained that these catfish were members of the same South American family as the infamous candiru. The candiru are a group of parasitic trichomycterids that enter the gill cavities of other fish. Once inside the gill cavity, the candiru expand sharp spines (known as odontodes) on their gill covers, anchor themselves in the host's gills, and feast upon the soft gill tissue or suck blood from their unlucky host.

I wondered if the trichomycterids in the store were parasites or not. Only a few members of the family are actually parasites. The majority of species live secretive lives hidden among the leaf litter or burrowed into the sand or mud substrate. Most trichomycterids also feed on what we would consider a normal diet of insects, small worms and the like. Of course, the next question was if I wanted these fish. I thought about it a minute but was not too sure. Then I heard the magic words, "You can have them for free." The pet store was obviously not as impressed with these interesting little catfish as I was. Along with the trichomycterids, I brought home a large feeder goldfish. I had no idea if these fish were a parasitic species, but I wanted to place them with potential prey and see if they reacted.

For my newest charges I prepared a ten gallon tank with a sponge filter. I was, it turned out necessarily, afraid that the catfish might be attracted to the current near the intake of any other type of filter and lodge themselves in the uptake. The sponge filter's return was kept above the water line so that the fish could not enter it. The tank was set up with a substrate of fine sand. The trichomycterids immediately buried themselves in the sand alligator fashion with only their eyes showing. The temperature was kept at 75°F and the water soft (2°GH) and slightly acidic (pH 6.8).

After hours of observation, I was quite sure that the fish burrow as a defensive measure, to hide from predators, and not as an offensive measure, to ambush prey. After a few weeks the fish must have realized there were no predators and stopped burying themselves. Many catfishes lose their natural instincts in aquariums. Corydoras, for example, will stop schooling, a natural defensive measure, in the aquarium. However, when Corydoras perceive danger, such as during a water change, they revert back to their natural instincts. During a water change, Corydoras will form a tight school, but within a few hours they will once again be spread out all over the aquarium. These trichomycterids behaved in a similar manner and after a few days only burrowed into the sand when they were frightened.

Henonemus taxistigmaHenonemus taxistigma, another scale eating trichomycterid and found in the Rio Tinaco, Venezuela.   Henonemus taxistigmaThe Rio Tinaco, Venezuela.

A few large rocks and a piece of driftwood covered with Java moss completed the artificial habitat. After the trichomycterids stopped burying themselves, they would spend their days perched upon the rocks or driftwood. They remained inactive for long periods of time but would occasionally dart to the surface. Because the fish are so small, and there is no coloration on the underside, it is possible to see through their skin and into their stomachs. I often noted air bubbles visible in their gut and believe that this species is capable of breathing atmospheric air in a manner similar to that practiced by Corydoras.

The next morning I took a close look at my new set up and was very surprised to see that the goldfish was now completely white. Closer inspection revealed that, with the exception of a few scales on the caudal peduncle, the goldfish was completely scaleless. The catfish sat listless on the bottom, their translucent bellies were glowing bright gold as a result of their recent meal. The reflective gold scales were clearly visible inside the trichomycterids. Well, I thought, there is your answer. These are definitely parasites. A few members of the family are known to be scale-eaters or mucus-eaters. The mucus-eaters feed on the body slime of other fishes similar to how baby Discus feed from their parents. The scale and mucus-eaters have not received the infamy of their blood-sucking relatives.

I soon learned that in complete contrast to their normal lethargic behavior was their activity at feeding time. Every two to three days I added a two inch feeder goldfish. I never used larger feeder fish for fear that the catfish might become the feeders. Shortly after a goldfish was added, the catfish became very active. The catfish swam around until they found their prey, aligned themselves parallel to the goldfish, and then quickly grabbed a mouth full of scales. The catfish always gorged themselves until their bellies glowed orange from the goldfish scales inside. By the next morning, the goldfish would be almost completely without scales and usually still alive. The trichomycterids showed a preference for the scales around the gill and eye and any remaining scales on the goldfish the following morning were inevitably on the caudal peduncle. As an experiment, I placed some neon tetras and some white cloud minnows in the tank. The trichomycterids never bothered these small fish. Perhaps there is a minimum prey size that these trichomycterids find suitable.

Although their manner of feeding may sound horrible it is actually more humane and resource conserving than just eating other fishes. In the wild, a trichomycterid might eat a few scales from its prey and then the irritated prey swims away. In a short time the prey regenerates the lost scales, and the trichomycterid can feed from the same fish again. This feeding habit only becomes a problem in the closed aquarium where the prey is unable to escape. An interesting experiment (that I have yet to try) would be to buy a dozen or more feeder goldfish and rotate them through the trichomycterid tank for an hour or so once every two weeks. This would give the goldfish time to regenerate their lost scales and ideally the goldfish would provide an inexhaustible food source.

My observations have also led me to believe that chemical compounds, such as ammonia and/or urea, attract at least this trichomycterid to their prey. Despite various currents from the filter, which was eventually lowered, and the siphon (when refilling the tank) the fish never seemed attracted to either source. When prey was added, the trichomycterids remained still for a few moments and then simultaneously rose to seek out their meal. It appeared to me that this small amount of time was needed for the catfish to "smell" that a meal was near. Perhaps it is necessary for the various chemicals to reach a certain concentration before the prey can be detected. Once these chemicals reach this concentration all of the trichomycterids in the area are able to sense that prey is near. As I noted before, the catfish were kept with neon tetras and white cloud minnows for a time and never attacked either species. I have no explanation for how the trichomycterids are able to determine which fish were potential prey and which were not. Winemiller and Yan (1989:511) theorized that the mucus-eating trichomycterid species, Ochmacanthus alternus, fed only on larger hosts. My observations support this, but I am unable to explain how the catfish differentiate. Perhaps they prey on larger fishes because large fish are an easier target or maybe the reason is that more mucus or scales can be obtained from a single attack on a larger fish. Since scale and mucus-eating catfishes depend on a renewable resource, it would be to the predator's advantage that the prey does not die from an attack. Evolution may have conditioned these catfish not to attack any fish so small that the victim could not recuperate from an attack.

Unfortunately, I was never able to identify these fish to species or even genus. The family Trichomycteridae contains more than 175 species and most are difficult, if not impossible, to identify with the naked eye. I have since seen a half dozen or so species of trichomycterids imported and have collected another half dozen species in South America myself. All others imported for the aquarium trade that I have seen have turned out to be non-parasitic catfish that were happy on a diet of frozen and live foods and could be kept with appropriate tankmates such as tetras, dwarf cichlids, and other small catfishes. Whether parasitic or non-parasitic, we know little about this fascinating catfish family. Trichomycterids are becoming increasingly available in the aquarium trade, and hopefully as they do so, we will learn more about the habits, behavior, and reproduction these bizarre little catfish.

Burgess W. 1989. An Atlas of Freshwater and Marine Catfishes: A Preliminary Survey of the Siluriformes. Neptune City, N.J.: TFH
Ferraris C. 1991. Catfish in the Aquarium. Morris Plains, N.J.: Tetra
Winemiller K. & Hong Yong Yan. 1989. Obligate Mucus-feeding in a South American Trichomycterid Catfish (Pisces: Ostariophysi). Copeia 1989(2): 511-514.

There is further information on this species on the Cat-eLog page.

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