Do some catfish really eat wood?

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Have you watched the wood in your aquarium gradually being turned into sawdust by your plecs? Dr Jonathan Armbruster tells us about wood-eating catfish.

Which genera or species of loricariid catfish are known to eat wood?
Panaque (including Panaqolus) and the Hypostomus cochliodon group are the only fish known to consume wood almost exclusively. Wood found in the guts of other loricariids is probably incidental.

Both seem to like swift sections of otherwise lowland streams. Some can be found in slower water, but rarely in very swift movement. There have to be dead trees in the water, and you can usually tell if there are wood-eaters around by looking at the wood - the fish leave scrape marks.

Do we know how and why this wood-eating has evolved and how has it changed the bone structure of the fish and its mouthparts over time?
Based on recent publications by Donovan German and his coauthors, it seems that they are not digesting the wood as was speculated in previous papers by Nelson and coauthors. 

They don't seem to have a gut anatomy designed to support the microorganisms that would be needed to break down the wood and, like other loricariids, have a very fast gut passage time.

If you look at gut contents, it looks like little wood chips at the front part of the intestine and the same at the back end of the intestine. 

However, Nelson and his colleagues recently recently received a grant to look at wood digestion in loricariids, so I guess the jury is still out.

If they aren't digesting the wood, then what are they doing?

Put it this way, I had a Hypostomus hemicochliodon in an aquarium with a chalky rock. That rock supported a lot of algae, and I noticed that the H. hemicohliodon was slowly converting it into sand. Clearly the rock was of no nutritive value, so the fish had to be feeding on the algae.

Loricariids as a whole seem designed to pass as much material through their gut as quick as possible and extract whatever digestible material they can out of it. 

In this way, the wood-eaters are not different.  They may be consuming the microorganisms that feed on the wood or just on the molecules that those microorganisms are extracting from the wood, but not the wood itself. 

There aren't as many arthropods that perform a similar function in South America, so the loricariids have moved into that niche.  The wood was probably a pretty safe place for a microorganism or fungus to survive until the wood-eaters turned it into lunch.

How has it changed the bone structure of the fish and its mouthparts?
In the skeleton, there have been some rearrangements.

A former graduate student of mine, Nathan Lujan, has been studying some aspects of jaw anatomy and how they might work to increase strength or speed. There is actually quite a bit of variability of jaws in the wood-eaters, so they may be attacking the wood in different ways.

When compared to relatives that do not eat wood, the jaws form an acute angle (less than 90°) and are thicker.  This may serve to strengthen the jaws. The muscles seem to be thicker, and the bones that support the jaws are more firmly attached to one another. In the true Panaque (like P. nigrolineatus), this reaches an extreme, and the fishes are very solidly built.

When you look at the distributions of the wood-eaters, what you can notice is that when there is less competition between a wood-eater and other loricariids, the wood-eater may have teeth more like a normal loricariid. For example, in Guyana, there are not many Hypostomus that like relatively slack water, so the teeth of H. taphorni are actually more similar to H. plecostomus than they are to other members of the H. cohliodon group.

Do we have any idea how much wood one of these catfish can go though in a given period?
Not really.  I have watched the wood in my aquaria slowly dissolve from loricariids, though!