1. Morphic detritus, including coarse particulate organic matter such as terrestrial tree leaves and wood, is consumed by few fishes in temperate stream systems but is ingested by abundant and diverse groups of specialized fishes in tropical rivers; physiological assimilation and partitioning of morphic detritus by fishes remain poorly understood.
2. This study examines seven species of Neotropical suckermouth-armored catfishes (Loricariidae) that live among and feed on coarse woody debris. Five species represent two unrelated evolutionary lineages showing convergent morphological specializations for gouging into and eating wood, small particles of which fill their guts. Two morphologically distinct species unrelated to wood-eaters and to each other forage along the surface of wood.
3. We examined six jaw functional morphological characteristics of each loricariid species as well as C and N stable isotope ratios of blood plasma, red blood cells and fin tissue of three wood-eating species and muscle tissues of all seven species. Consumer isotopic signatures were compared among species and with isotopic signatures of potential food resources, including biofilm, seston and both bulk wood and holocellulose extracted from bulk wood.
4. Wood-eating species had robust jaws specialized for gouging wood, δ13C signatures consistent with assimilation of cellulosic wood carbon (not bulk wood carbon or lignin) and elevated δ15N values (>5·8‰) relative to wood that were consistent with assimilation of N from intermediate microbial decomposers in the environment rather than direct assimilation of N from wood or from endosymbiotic N-fixers. Two non-wood-eating species occupied divergent regions of jaw functional morphospace, and isotopic signatures were consistent with assimilation of C from biofilm and seston, respectively, and N from enriched sources such as microbes, macroinvertebrates or seston.
5. Food resources associated with the surfaces of coarse woody debris in Neotropical rivers are partitioned among at least three guilds of loricariid consumers with divergent jaw morphologies specialized for wood gouging, surface grazing and macroinvertebrate probing. Direct consumption of morphic detritus by specialized Neotropical fishes constitutes a potentially important but poorly understood component of detritus processing and nutrient cycling in tropical rivers.