Specialized diets, "Leopard Frogs", and re-thinking assumptions...

I'm fascinated by the dietary preferences of fishes. How they've evolved over eons to consume various items found in their environments; how many fishes became "specialists" as an adaptation to the habitats in which they live.

And, 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 seunderlying feeding strategies fascinating. One of the more interesting 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, having maintained maybe a couple dozen or so species during a lifetime of aquarium keeping. However, 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 our fishes' diets!

And of course, I'm equally 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 by these fishes is equally cool and fascinating! 

It's thought by ichthyologists 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, (Pterygoplichthys 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. Like, they're taking in wood to get other stuff out of the deal... And detritus is comprised of stuff like macrophytes, algae, and particulate organic carbon.


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 Pt. 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.

Oh, that's weird.

Yet, anatomical studies of these fishes showed that the so-called "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?



And to make it even weirder, check out this passage from a study by Lujan, Winemiller, and Armbruster:

"Loricariids have a dense endoskeleton and are covered with dermal plates composed primarily of calcium phosphate, giving them a high physiological demand for dietary phosphorus. Paradoxically, the rivers and streams inhabited by loricariids as well as the detritus and biofilm that most loricariids consume tend to be highly Phosporus deficient."

The same study noted that, "Loricariids as a whole are largely unable to digest lignocellulose, and instead derive most nutrients and energy from easily digestible breakdown products (e.g., disaccharides and dipeptides) that are produced during microbial degradation of submerged, decomposing wood."

Oh, interesting!

I think it's yet another case of us as hobbyists drawing innocent conclusions based on anecdotal or superficial observations. I mean,"... they're munching on my wood, therefore, they must be 'eating' it!"


Now, to the point of the argument that most loricariids are primarily detritivores, consuming a matrix of biofilm, algal growth, microorganisms, and (for want of a better word) "dirt"- what does this mean to us as hobbyists? Well, for one thing, this has made them remarkably adaptable fishes in the aquarium. They will definitely rasp at wood", but according to the studies I just cited, they are not "eating" it, per se.

Now, my personal experience with Loricariidae is nothing like many of yours, and an observation I've made over the years is at best anecdotal- but interesting:

If you follow "The Tint", you know I've had a years-long love affair with Peckolotia compta aka "L134 Leopard Frog"- a beautiful little fish that is filled with charms. Well, I recall, are years back, that my first specimen seemed to have vanished into the ether following a re-configuration/rescape of my home blackwater/botanical-method aquarium. I thought somehow I either lost the fish during the re-scape, or it died and subsequently decayed without my detecting it... Pretty upsetting either way, but I couldn't find any trace of 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 formed 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 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 fuck was she feeding on during this time?

Well, I suppose it's possible that some bits of frozen food (I fed frozen almost exclusively at that time) 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 mid-water-column feeders! To think that any appreciable amount got away from the hungry hoard was a bit hopeful. I believed at the time (and now am fully convinced) that it was more likely the biofilms, fungal growth, and perhaps some of the compounds from surface tissues of the "Spider Wood" I used in the hardscape that she was feeding on.

"Spiderwood" (aka Azalea root) stuff does recruit significant biological growth on it's surfaces when submerged , and curiously, in this tank, I noticed that, during the first few months, 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.

I'm sure that was a valid observation, but they were actively taking prepared foods as the bulk of their diet, so I have a hard time that they solely were responsible. 

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. Since they are rather "durable", they do accumulate a lot of fungal growth and biofilms on their surfaces.

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 don't think I ever saw my L134 consuming prepared food. When I did observe her activities, she was seemingly "grazing" away at the wood surfaces and on botanicals...That's all the proof that I needed to confirm my theory that she's pretty much 100% detritivorous, and that the botanical-method aquariums she's resided in provide a sufficient amount of this material for her to consume.

To this day, I've never seen her eat prepared foods!

I have since acquired three captive-bred specimens from my friend, master breeder Sumer Tiwari, and this group has been seen to take prepared food on occasion. At the very least, adding some pellets or frozen foods seems to initial some kind of response in the fish, wether they appear to eat it or not.

So, back the the whole "xylophore thing"... After reading the studies I mentioned, I think that in the aquarium, as well as in the wild, much of what we think is actually "consumption" of the wood by the fishes is simply incidental- as in, the fishes are trying to eat the biocover and detritus on the surface tissues of the wood, and perhaps obtain some nutrition from the compounds contained in the softer portions of the wood. They apparently do a pretty good job (with their specialized mouthparts) of rasping away the surface tissues of the wood!

So, yeah- apparently, 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! WTF do I know about chickens!)...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."



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? 

Well, for one thing, once again- detritus/biofilm/fungal growths = good. Don't loathe them. Love them.

Your fishes apparently do.

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-method 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; "Skyfruit" pods, Cariniana pods, Mokha pods, bark, etc.) tend to recruit significant fungal growths and 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 consumption.

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-method aquariums, and the accumulation of biofilms and microbial growth is something that we've discussed for a long time. By their very nature, the structure and decomposition 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!" 

Now, while while we're on the subject of loricariids, a further scan of scientific literature revealed some interesting things about what these fishes are actually taking in when they "graze" in the wild. It's kind of eye opening, to me. One study revealed that loricariids consumed five principal items: sponges, organic detritus, bryophytes, bryozoans and sediment.

Wood is definitely part of the equation somewhere, but for the species examined in one of the studies I found (Rhinelepis aspera, Hypostomus regani, H. ternetzi, H. maragaritifer, H. microstomus, and Megalancistrus aculeatus) the gut content analysis was quite revealing:

The food spectrum of R. aspera is primarily "organic detritus and small quantities of sediment"; with few periphytic organisms. Although H. regani was found to consume large quantities of organic detritus as well, it also consumed "plant detritus, various sediment, and periphytic organisms" (i.e.; bryozoans, sponges and aquatic insect larvae). Bryozoans and sponges, huh?

Wow! Freshwater sponges...

The study indicated that bryozoans and organic detritus were the main food food of H. ternetzi, which, according to the gut contents of a number of individuals,  tended to consume more sediment, rotifers, chironomids (i.e.; "Bloodworms'), gastropods and harpacticoids than the other species.

Harpactoids...you mean, like "copepods?" Stuff we as reefers feed all the time? H. margaritifer was found to ingest plant material. Other periphytic organisms such as insect larvae, and those bryozoans and sponges contributed to the diet of H. margaritifer.

And it gets more interesting still...

Sponges- I can't let that go.

Sponges were the principal food resource of H. microstomus and M. aculeatus, along with a healthy does of chironomids, various gastropods, Trichoptera (insects), and some bryozoans also consumed. Diets of these two fishes were composed of larger-sized items, with the finer organic detritus and such being less important than it was to the other species in the study. 

This kind of information is tantalizing. It's compelling. 

And what really gets me going is learning that some of our favorite, most beloved fishes are consuming large quantities of materials that I doubt any freshwater aquarist adds to his/her arsenal of foodstuffs. We're really good at feeding our catfishes baby vegetables and stuff, while typically overlooking many species' surprisingly high dietary dependency on items like insects, bryozoans, harpactoid copepods, and interestingly...sponges!

While we kind of always knew that these fishes ingested wood and "stuff", it's interesting to see what they're actually eating in the wild...especially the "stuff"- and configuring our aquariums and the supplemental and primary feeding opportunities available to the fishes accordingly.

We have some interesting, yet perhaps overlooked possibilities to provide some of these items. 

In fact, there are a number of marine aquarium-purposed foods (typically targeted at certain marine angelfishes, many of which consume significant quantities of sponge) which contain sponges in their formulation. One of my favorite is Ocean Nutrition's "Angel Formula." Granted, these foods contain stuff like mussels, and other marine foods, and the sponges included are marine sponges, but I can't help but wonder if these are that morphologically or nutritionally different/palatable to the fishes than a freshwater/tree sponge would be?

Could the next great frozen Loricarid food include sponges? And we DO have harpactoid copepods available live, and in a variety of other formats intended for marine fishes and corals...Interestingly, I remember that the big "knock" by us reefers, for a long time, about some of these copepods was that they were "freshwater" varieties, and therefore didn't have the "correct" nutritional profile for marine organisms.

Hmm. We're talking about freshwater fishes here, right? Yeah. 

So, like, why the hell haven't we been feeding these foods to our freshwater fishes all of these years?

Try some of these foods with your loricariids..and other fishes as well. What's to lose?

Oh, I can hear the objections: 


Is it?. Online ordering is really cool. It might just catch on.

"Too much work!" 

Really? C'mon. Ever cultured Grindal Worms or wingless fruit flies?  THAT is "too much work" by definition.

"This is ridiculous; No need to experiment with these wacky foods. We're doing just fine now with Zucchini and stuff! Stupid."  

Urghhhhh. "If man was meant to fly, he'd have wings..."

To not experiment is stupid, IMHO. 

Don't be stupid. And I mean that in the kindest way possible. Don't just accept "what works" as "the way." 

Push forward. Experiment. Fail quickly, or move forward rapidly with success. Play a hunch or two. Try something different. This is how advances in the hobby are made. This is how breakthroughs happen.

You gotta try.

Stay studious. Stay curious. Stay observant. Stay engaged. Stay resourceful...

And Stay Wet.


Scott Fellman

Tannin Aquatics 

Scott Fellman
Scott Fellman


Leave a comment