Catfish: The Vacuum Cleaners of the Freshwater Fish World

Although almost absent from temperate waters, there is a huge diversity of catfishes in the lakes and rivers of the tropics. Their diet is just as diverse as the catfish themselves, but among them is a large group of specialist algae-eaters. These include a number of long-standing aquarium favorites, particularly members of the Loricariidae family such as the pleco (Hypostomus plecostomus), the bristlenose catfish (Ancistrus ancistrus), and the whiptail catfishes. All are incredibly well adapted for their life of rasping off the rich algal film that forms on fallen wood and stones.

How are the algae-eating catfishes adapted for their grazing lifestyle?

Perhaps the most remarkable feature of these fish is the mouth. In each case, it is positioned ventrally, beneath the fish's body. The belly is flat, creating a highly streamlined profile. This is an advantage for fish that move along the substrate in their home waters, which in many cases are fast-flowing streams. But it is not just their hydrodynamic shape that allows these catfish to prosper in Whitewater habitats. Their mouths also provide powerful suction, fitting snugly and forming a seal on the surface that they feed from. While the
fleshy lobes of their mouths do the holding, highly adapted jaws rasp at their holdfast. The teeth vary between species: those that eat soft algae, including Ancistrus sp., have broad flat teeth, whereas Panaque sp. have spoon-shaped teeth that enable them to gouge tougher substances, such as fallen wood.

Is the catfish's armor enough on its own to protect the fish?

Although many catfish carry armored plates on their back and flanks to protect them from attack, fish such as the whiptailed catfish are also extremely well camouflaged. As they feed on roots and the submerged branches of trees, they blend perfectly into their environment because their elongated bodies resemble the twigs that surround them.

So are these fish almost like vacuum cleaners for the home aquarium?

Although it tends to live in still or slowly flowing waters, the Chinese algae-eater (Gyrinocheilus aymonieri) fills a niche similar to the armoured catfish of Africa and South America. It is often bought as a scavenger for the community aquarium at home because it is cheap and believed to remain much smaller than the tank-busting plecs. Of course, the reality is that sucking loaches are not scavengers, but algae-eaters. What is more, they can grow to an impressive size — some reaching 10 inches (25 cm). Added to this, they are occasionally known to graze on the sides of their tankmates — the mucus presumably making a change from the regular diet of greens. The solution for algae in an aquarium is balance: efficient filtration to remove excess nitrates, a sensible light regime and no direct sunlight. The various kinds of algae-eaters deserve a place in the aquarium in their own right, not just to work as a maintenance team.

How do algae-eaters distribute themselves to get the most food?

Grazing animals, such as the plec, live in a world where food is everywhere, because algae takes hold wherever a surface faces the sun. Nevertheless, their food is only spread thinly so the fish distribute themselves carefully, going where the food is. More than this, they seek an area where they can obtain the most food with the least amount of competition. It is pointless simply to go where the richest algal mat is, if that means constantly fighting off others. A study on plecs in their Amazon home showed that although there were fewer fish in the deeper waters where the algae is not as rich, neither were they all crowded into the shallow pools at the margins where the algae is at its most succulent Instead, the fish distributed themselves perfectly throughout their environment so that each maximized its food intake.