Because wood is composed almost entirely of structural polysaccharides (primarily cellulose) few animals can digest it, let alone thrive on it. Most animals that do subsist on wood require endosymbiotic microorganisms in their digestive tracts to aid in the digestion of structural polysaccharides. Several species of catfishes (family Loricariidae) from the Amazonian basin (South America) were recently described as being “wood-eating”, or “xylivorous”. However, beyond some cursory analyses of gut contents and digestive enzyme activities, little is known about these animals and whether they can digest wood. In this dissertation I explored the structure and function of the digestive tract in four species of loricariid catfishes: three xylivorous species ( Panaque cf. nigrolineatus “Marañon”, P. nocturnus, and Hypostomus pyrineusi) and one detritivorous species (Pterygoplichthys disjunctivus) that represents the common feeding mode of the family. Thus, I was able to examine whether the xylivorous species had specializations of the digestive tract affording them the ability to digest wood in comparison to the non-xylivorous, detritivorous species.
I measured several aspects of the fishes' gut morphology, including intestinal folding patterns, microvilli surface area, pH and redox potentials, and microbial diversity in different regions of the gut. I also measured the activity levels of 14 digestive enzymes in the guts of the fish, and determined the sources of these enzymes: endogenous (produced by the fish) vs. exogenous (produced by microbes). These data were then compared to concentrations of microbial fermentative end products, called short chain fatty acids (SCFAs), and soluble carbohydrate profiles in the intestinal fluids of the fish to determine where microbes might be most active, and where nutrients were being hydrolyzed and absorbed. And finally, I measured the ability of captive fish to digest wood, to grow on it, and how quickly the fish pass wood through their digestive tracts.
The results of these analyses suggested that, unlike termites, the alleged xylivorous catfishes of the Amazon cannot digest the fibrous components of wood in their digestive tracts. I found no evidence that they harbored endosymbionts in their guts capable of digesting the structural polysaccharides of wood. And, the laboratory feeding trials showed that the fish could not assimilate significant amounts of fiber or energy from wood, resulting in the fish losing weight on a wood diet. The fishes' entire feeding strategy, ranging from intake, to gut passage rates, digestive enzyme activities, intestinal morphology, soluble carbohydrate profiles, and levels of SCFAs throughout the gut suggest that the fish eat as much as they can, pass it through the digestive tract quickly, and assimilate the soluble, non-fibrous components available to them. Unlike many other wood-eating animals (e.g., termites, beavers), these fishes consume decaying wood in aquatic systems; decaying wood is in the process of being degraded by microbes, which produce soluble degradation products (e.g., β-glucosides) that the fish can actually digest and assimilate. Thus, rather than harboring endosymbiotic microorganisms to digest wood fiber within their guts, the fish rely on microbial decomposition occurring in the environment. In this vein, the wood-eating catfishes are actually detritivores like so many other loricariid catfishes.