Plecs — it's all in the mouth!


The mouths of loricariids are an evolutionary marvel, says Dr Michael Hardman. And they can tell us so much about the diets of the different suckermouth catfish species we keep.

In April 2009, there were 782 valid species of suckermouth. That’s more than a quarter of all catfishes and almost 3% of all vertebrate species! What a diversity for a family that occurs from Costa Rica to Argentina...

Their mouths are an evolutionary marvel. They must provide a firm attachment to an irregular surface in fast flowing water — and do this without forming a perfect seal because they still have to breathe!

They also have to taste the surface and scrape, dig, manipulate and pass backwards food that include detritus, algae, insect larvae, seeds, macrophytes, sponges, snails, worms, clams and even wood.

They also have to settle territorial disputes, dig holes, claim mates and carry or clean a clutch of eggs.

Ask an ichthyologist what suckermouths eat and, if they have checked the stomach contents of museum specimens, they will usually answer ”brown mush”.

Because most suckermouths have combs of fine teeth, for when they scrape or brush food into their mouths it becomes very difficult to make out what it was beforehand.

One line of research relies on analysis of suckermouth tissues. When food is digested and becomes part of the animal, its chemical signature is incorporated. Different foods leave different signatures and by measuring their proportions we can work out what the animal has been feeding on.

Fishkeepers quickly figured out that many of the striking suckermouths first seen in the 1990s needed a more meaty diet than bristle-nosed relatives. Diversity in diet seemed matched by a diversity in mouth morphology. Yet how far does this match go? Can we look at a mouth and know what its owner eats? To a large extent, yes.

Suckermouths have very different jaws to humans. They don’t have mobile tongues and their jaws are not attached to the skull. Their jaws sit in a ring of flexible but tough connective tissue that keeps them from dislocating while giving them freedom of movement.

Many suckermouths are generalist feeders and their mouth must handle items that pose different problems. Those such as Pterygoplichthys and Ancistrus are generalists, having moderate-sized jaws filled with numerous fine teeth. The jaw muscles contract to scrape soft foods such as algae, biofilm, fruits and soft seeds.

Generalists can scrape decomposing wood or handle small shrimp or aquatic insect larvae, so it’s not uncommon to see Ancistrus rasping bogwood in the community aquarium. Generalists also quickly adapt to taking flake and tablet foods.

South American freshwater fishes have evolved to fill an ecological niche that in the northern hemisphere is filled by insects; detritivory. Feeding on detritus has been perfected by curimatids (Prochilodus) but suckermouths such as Rineloricaria have capitalised on this resource too.

Higher animals such as suckermouth catfishes can’t digest cellulose in detritus, but can handle the fungi and bacteria that live on it. So feeding on decomposing detritus is like us eating a sponge soaked in chicken soup.

Detritivores typically have lightly built jaws with a few strong teeth for scraping, rolling and chopping their food into swallowable pieces.

As the Andean mountains began to rise, high gradient streams began to form in which few fishes had the nerve or equipment to spend any time. As such, these were virtually predator free and prime real estate for small catfishes.

Enter the bulldog plecs (Chaetostoma). These have run riot with the sucking feature of their mouths by developing huge lips. Their jaws have become broad and filled with hundreds of fine teeth, perfect for brushing algae in a crashing stream.

Algae substitute

Unfortunately, they rarely thrive in captivity because home aquaria typically don’t grow enough algae. I’ve got round this by blending hot spinach, broccoli, peas and shrimpmeat with boiling agar. I coat pebbles or make cubes with the agar paste while still molten and place them in the fridge or freezer until needed.

Many fancy suckermouths in the hobby hail from north-flowing tributaries of the Amazon of north-eastern Brazil. Algae doesn’t grow well in these nutrient-poor systems, so most plecs that live there have found alternative ways to make a living. One of the most common has plugged into aquatic insect larvae.

Several species of Hypancistrus and other small, brightly patterned plecs have been collected with stomachs bulging with mosquito and stonefly larvae. These are attached to surfaces in clean, clear water so evolution has placed them squarely on the suckermouths’ dinner plate.

Suckermouths specialising in aquatic insect larvae have neat, small mouths with a few strong teeth. What’s more, they rarely exceed 10cm/4” because they need to access the tight places where they find their prey.

In those same streams of low algal productivity, suckermouths exist that are capable of handling more sizeable prey than larvae.

The vampire plecs (Leporacanthicus) have long and narrow upper jaws that contain a few chisel-like teeth. The lower jaw is broad, heavily reinforced and contains three to five shorter and more robust teeth.

Similarly, Scobinancistrus, believed a close relative of Panaque, has strong jaws with spoon-shaped teeth to crack open or winkle out meaty foods from protective shells.

One group of suckermouths are the bane of external filters everywhere — the wood feeders. Suckermouths such as Panaque and Panaqolus have short, reinforced jaws set in a diamond shape that bear several strong spoon-shaped teeth.

Wood nutrition?

Fishkeepers have long known the importance of wood in the suckermouth aquarium, but only  recently have researchers tried to test whether it is the main source of nutrition or just incidentally ingested.

In the animal kingdom, only insects have really managed to digest cellulose and lignin — the main components of wood — and only with the aid of some special animals that live in their guts.

Results suggest that Panaque, although it eats a lot of wood, obtains little if anything from it. Possibly, by scraping deeper into the material, more decomposing fungi or biofilm is ingested. So feed your wood eaters more than a lump of bogwood.

Did you know?

Loricariid jaws are incredibly variable, with almost every species having a different configuration. Specifics of the jaw typically correspond to the preferred food and a close look can often give the aquarist clues about what to feed a new plec.

Baryancistrus (above) is a classic generalist. With multi-purpose jaws filled with fine but tough teeth, these suckermouths can handle anything from bogwood to boiled spinach.

The award for weirdest mouth surely goes to Planiloricaria cryptodon (pictured above). This species lives deep in the Peruvian Amazon and its fimbriate mouth with almost no teeth is specialised for detecting and inhaling invertebrates.


Add extra foods, medicines or supplements to a suckermouth diet by blending them with a preferred food and then using molten agar to bind them together before feeding.