June 20, 2019

"Wood" you believe this?

As an aquarist who derives great pleasure from seeing his fishes "live off the land" and consume foods from the aquarium environment in which they reside, I really find some of the underlying feeding strategies fascinating. One of the best examples is the consumption of wood by various species of fishes.

We read a lot about fishes which eat wood and wood-like materials. Of course, the ones that come immediately to mind are the Loricariidae, specifically, Panaque species. Now, I admittedly am the last guy who should be authoritatively discussing the care of catfishes, but I do understand a little bit about their diets and the idea of utilizing wood- and botanical materials- in the aquarium for the purpose of supplementing their diets!

And of course, I'm fascinated by the world of biofilms, decomposition, microorganism growth and detritus...And this stuff plays right into that!

Now, the idea of xylophagy (the consumption and digestion of wood) is of course, a pretty cool and interesting adaptation to the environment from which these fishes come from. And as you'd suspect, the way that wood is consumed and digested is equally cool and fascinating!

It's thought that the scraping teeth and highly angled jaws of the Loricariidae are a perfect adaptation to this feeding habit of scraping wood. And of course, it's even argued among scientists that these fishes may or may not actually digest the wood they consume! While scientists have identified a symbiotic bacteria which is found in the gut of these fishes that helps break down wood components, it's been argued by some the the fishes don't actually digest and metabolize the wood; indeed deriving very little energy from the wood they consume!

In fact, a lab study by Donovan P. German was described in the November, 2009 Journal of Comparative Physiology, in which several species were fed wood and found to actually digest it quite poorly:

"...in laboratory feeding trials, (P. cf. nigrolineatus and Hypostomus pyrineusi) lost weight when consuming wood, and passed stained wood through their digestive tracts in less than 4 hours. Furthermore, no selective retention of small particles was observed in either species in any region of the gut. Collectively, these results corroborate digestive enzyme activity profiles and gastrointestinal fermentation levels in the fishes’ GI tracts, suggesting that the wood-eating catfishes are not true xylivores such as beavers and termites, but rather, are detritivores like so many other fishes from the family Loricariidae."

Did you see that? Detritioves.

Hmm...

And this little nugget from the same study: "...The fishes consumed 2–5% of their body mass (on a wet weight basis) in wood per day, but were not thriving on it, as P. nigrolineatus lost 1.8 ± 0.15% of their body mass over the course of the experiment, and Pt. disjunctivus lost 8.4 ± 0.81% of their body mass."

Yet, anatomical studies of these fishes showed that the "wood-eating catfishes" had what physiologists refer to as "body size-corrected intestinal lengths" that were 35% shorter than the detritivore species. What does this mean? Could they have perhaps had at one time- and subsequently lost- their ability to digest wood?

Maybe?

Arrgh!

To the point of the argument that they are primarily detritivores, consuming a matrix of biofilm, algal growth, microorganisms, and (for want of a better word) "dirt"- what does this mean? In fact, many species in the Loricariidae are known to be detritivores, and this has made them remarkably adaptable fishes in the aquarium.

Now, my personal experience with Loricariidae is nothing like many of yours, and an observation I made not too long ago is at best anecdotal- but interesting:

If you follow "The Tint", you know I've had an almost two year love affair with my Peckolotia compta aka "L134 Leopard Frog"- a beautiful little fish that is filled with charms. Well, my specimen seemed to have vanished into the ether following a re-configuration/rescape of my home blackwater/botnanical-style aquarium. I thought somehow I either lost the fish during the escape, or it died and subsequently decayed without my detecting it...

For almost three months, the fish was M.I.A., just....gone.

And then one, day- there she was, poking out from the "Spider Wood"thicket that forms the basis of my newer hardscape! To say I was overjoyed was a bit of an understatement, of course! And after her re-appearance, she was been out every day. She looked just as fat and happy as when I last saw her in the other 'scape...which begs the question (besides my curiosity about how she evaded detection)- What the @#$% was she feeding on during this time?

Well, I suppose it's possible that some bits of frozen food (I feed frozen almost exclusively) got away from my population of hungry characins and fell to the bottom...However, I'm pretty fastidious- and the other fishes (characins) were voracious! I think it was more likely the biofilm, fungal growth, and perhaps some of the surface tissues of the "Spider Wood" I used in the hardscape that she was feeding on.

This stuff does recruit some biological growth on it's surfaces, and curiously, in this tank, I noticed during the first few months that the wood seemed to never accumulate as much of this stuff as I had seen it do in past tanks which incorporated it.

I attributed this to perhaps some feeding by a population of Nanostomus eques, which have shown repeatedly in the past to feed on the biofilm or "aufwuchs" accumulating on the wood.

And as an interesting side observation, this wood (so-called "Blonde" Spider Wood) actually darkens and develops a sort of "patina" of biofilms and such over time, which probably facilitates the feeding, right?

There was also a layer of Live Oak leaves distributed throughout the booth of the wood matrix, which, although they break down very slowly compared to other leaves we use, DO ultimately soften over time and break down over time.

Interestingly, in this tank, I was finding little tiny amounts of very broken-down leaves, which I attributed to decomposition, but thinking back on it, looks more like the end product of "digestion" by someone!

I must admit, I don't think I've ever seen my L134 consuming prepared food. I've always seen her rasping away at the wood surfaces and on botanicals...That's all the proof I needed to confirm my theory that she's pretty much 100% detritivorous, and that the botanical-style aquariums she's resided in provide a sufficient amount of this material for her to consume.

SO, back the the whole "xylophore thing"...I think that in the aquarium, as well as in the wild, much of what we think is actually "consumption" of the wood is simply incidental, as in, the fishes are trying to eat the biocover and detritus on the surface tissues of the wood, but do a pretty good job (with their specialized mouthparts) of rasping away the surface tissues as well.

Some of the wood may pass through the digestive tract of the catfishes, but it's passed without metabolizing much from it...perhaps like the way chickens consume gravel, or whatever (don't they? City boy here!)...or the way some marine Centropyge angelfishes "nibble" on corals in their pursuit of algae, detritus, and biofilms.

Again, my perusal of German's scientific paper seems to support this theory:

"Catfishes supplement their wood diet with protein-rich detritus, or even some animal material to meet their nitrogen requirements. Although I did not observe animal material in the wood-eating catfish guts, Pt. disjunctivus did consume some animal material (including insects parts, molluscs, and worms), and all three species consumed detritus."

And finally, the "clincher", IMHO: "The low wood fiber assimilation efficiencies in the catfishes are highly indicative that they cannot subsist on a wood only diet."

Boom.

I mean, it's just one paper, but when he's talking about isotopic tracing of materials not consistent with digestion of wood in the guts of Loricariids, I think that pretty much puts the "eats wood" thing to bed, right? His further mention that, although some cellulose and lignin (a component of wood and our beloved botanicals!) was detected in the fish's fecal material, it was likely an artifact of the analysis method as opposed to proof that the fishes derived significant nutrition from it.

So what does all of this stuff mean to us?

Detritus/biofilm/fungal growths = good. Don't loathe them. Love them.

I think it means that, as hobbyists probably knew, theorized, and discussed for a long time- that the Loricariids consume detritus, biofilms, and prepared foods when available. This is not exactly earth-shattering or new. However, I think understanding that our botanical-style aquariums can- and do- provide a large amount of materials from which which these and other fishes can derive significant nutrition furthers my assertion that this type of system is perfect for rearing and maintain a lot of specialized feeders.

Materials like the harder-"shelled" botanicals (ie; Cariniana pods, Mokha pods, Sterculia pods, Catappa bark, etc.) tend to recruit significant biofilms and accumulate detritus in and on their surfaces. And of course, as they soften, some fishes apparently rasp and consume some of them directly, likely passing most of it though their digestive systems as outlined in the cited study, extracting whatever nutrition is available to them as a result. This is likely the case with leaves and softer botanicals as well.

Incidental.

The softer materials might also be directly consumed by many fishes, although the nutrition may or may not be significant. However, the detritus, fungal, and microorganism growth as a result of their decomposition is a significant source of nutrition for many fishes and shrimps.

Detritivores (of which the amount of species in the trade is legion), have always done very well in botanical-style aquariums, and the accumulation of biofilms and microbial growth is something that we've discussed for a long time. By their very nature, the decomposition and accumulation of botanical materials make the "functional aesthetics" of our aquariums an important way to accommodate the natural feeding behaviors of our fishes.

So, the answer to the question (literally!), "Who has the (literal) guts for this stuff?" is quite possibly, "everyone!"