Typical gut contents of a loricariid catfish.
There has historically been little evidence to suggest that diverse assemblages of herbivorous and detritivorous fishes partition resources, or that resulting specializations may be evolutionarily conserved. To investigate this, I gathered tissues for stable isotope analysis from several species-rich assemblages of loricariid catfishes distributed across Amazonia. I then examined evolutionary trends in the relative position of species in assemblage isotope space (i.e., relative to their assemblage mean, see vector plots at top). Results of this study revealed deep conservatism in the relative position of species in assemblage isotope space, and large-scale patterns of dietary specialization that were consistent with species jaw diversity. My recently published DNA based phylogeny is now being used to reexamine these trends and explicitly test for correlations between expanded isotope and jaw morphological datasets.
EXAMPLE 2: Lujan, N.K., D.P. German, and K.O. Winemiller. 2011. Do wood-grazing fishes partition their niche?: morphological and isotopic evidence for trophic segregation in Neotropical Loricariidae. Functional Ecology 25:1327–1338.
Wood-eating is a particularly intriguing dietary specialization that, among fishes, is only known to occur within the Loricariidae. Phylogenetic research that I completed in 2015 revealed that within the Loricariidae, the specialized morphology and diet of wood-eaters has originated three times. This earlier study in Functional Ecology combined data on functionally relevant jaw morphological characteristics with isotope data describing the position of wood-grazing species relative to food resources associated with the wood surface, including purified cellulose itself. We found that wood-eating catfishes did indeed assimilate wood cellulose, but that assimilation had to have occurred via an intermediate pathway (e.g., fungi and bacteria living on the wood). We also found clear evidence of trophic partitioning among sympatric wood-grazing species.
The trophic community ecology of algivorous and detritivorous fishes has historically received little attention, in part because of the difficulty of detecting dietary specialization and resource partitioning among fish primary consumer assemblages using gut contents data alone. Because algivore and detritivore gut contents often consist largely of unidentifiable, amorphic detritus and inorganic substrates (see right), and because these fishes would be predicted to selectively feed based more on the nutrient ratios of their food items than on visually identifiable characteristics, chemical tracers of foods that have been assimilated (e.g., stable isotopes) may be much more informative of dietary differences than traditional gut contents analysis. A goal of my research is to integrate phylogenetic and isotope analyses to
better understand dietary diversification in these fishes.