There are various different catfish species in the world, all belonging to the order Siluriformes. In June 2005, the 37th catfish family was added to Siluriformes. Examples of catfishes that are commonly kept by aquarists include the immensely popular Plecos. In this group you will for instance find the Suckermouth Catfish (Hypostomus plecostomus) and the Bristlenose Catfish (Ancistrus dolichopterus). These two species can be successfully kept even by beginner fish keepers and are popular choices for the first aquarium.
While Plecos stay comparatively small, there are many species of Catfish that can not be kept in normal aquariums as adults. Be careful when offered a catfish for your aquarium; you never know how large it will become. Always try to find out the species of the fish, or at least to which genus it belongs, and do some reading regarding its maximal size, preferences in the aquarium etcetera. Wells Catfish can, for instance, reach a length of 5 meters (16 feet), while the heavy Mekong Catfish from Asia can weigh in at nearly 300 kilograms (661 lbs).
Catfish are not only found all over the world; they are also found in very dissimilar waters and ecological niches. A majority of the world’s catfish species live only in freshwater but there are notable exceptions, such as certain members of the families Plotosidae and Ariidae. Even though the many catfish species varies immensely when it comes to size, look and behavior, they do have a few things in common, including a hollow leading ray that excretes a dangerous protein, barbells, and the fact that they are all scaleless.
A hollow leading ray that excretes dangerous protein is not found in all catfish species; those of the family Malapteruridae will instead use electricity to defend themselves. Members of this family are therefore commonly known as Electric catfishes and can deliver a potent electric shock. For the rest of the world’s catfish species, the potency of the protein sting varies greatly. If you handle catfish from the genus Heteropneustes in the family Plotosidae, you should be especially careful since their protein sting is strong enough to seriously injure even an adult. If you get stung by one of these catfishes, it is recommended to seek medical attention.
As mentioned above, all catfish species have barbells. Barbells are tactical organs that protrude from around the mouth are highly useful when light is scarce, just like the whiskers on a cat. Despite folk etymological claims to the contrary, catfish is not called catfish because it is especially popular among cats or only useful as cat food. A catfish uses its barbells when hunting in dark and murky waters where eyes are not really useful. The barbells are equipped with taste buds that enable catfish to taste the surrounding environment without putting anything in its mouth. This is a highly useful skill, and several other fish families have developed similar solutions. You can, for instance, find barbells on quite a few carps and on some sharks.
One example of a much-feared catfish that is not commonly kept by aquarist is the Candirú (Vandellia cirrhosa). This species is also known as the “Vampire Fish of Brazil” since it survives by drinking the blood of other fishes. Since the Candirú wants to attach itself to the gills of a fish, it is always on the lookout for something that excretes ammonia or blood particles. The source does not necessarily have to be a fish; it might just as well be a urinating human. If you urinate while swimming in a South American river where the Candirú lives, it can mistake your urinal tract for the gill of a fish and swim in an attach itself. The Candirú is very small and have a transparent body, so it is really difficult to spot in the water.