The Neotropical catfish family Loricariidae contains over 830 species that display
extraordinary variation in jaw morphologies but nonetheless reveal little interspecific
variation from a generalized diet of detritus and algae. To investigate this paradox, we
collected 13C and 15N stable isotope signatures from 649 specimens representing 32
loricariid genera (82 species) from 19 local assemblages distributed across South America.
We calculated vectors representing the distance and direction of each specimen relative to the 15N/13C centroid for its local assemblage, and then examined the evolutionary diversification of loricariids across assemblage isotope niche space by regressing the mean vector for each genus in each assemblage onto a phylogeny reconstructed from osteological characters.
Loricariids displayed a total range of 15N assemblage centroid deviation spanning 4.9,
which is within the tissuediet discrimination range known for Loricariidae, indicating that
they feed at a similar trophic level and that 15N largely reflects differences in their dietary
protein content. Total range of 13C deviation spanned 7.4, which is less than the minimum
range reported for neotropical river fish communities, suggesting that loricariids selectively
assimilate a restricted subset of the full basal resource spectrum available to fishes.
Phylogenetic regression of assemblage centroid-standardized vectors for 15N and 13C revealed that loricariid genera with allopatric distributions in disjunct river basins partition basal resources in an evolutionarily conserved manner concordant with patterns of jaw morphological specialization and with evolutionary diversification via ecological radiation.
Trophic partitioning along elemental/nutritional gradients may provide an important
mechanism of dietary segregation and evolutionary diversification among loricariids and
perhaps other taxonomic groups of apparently generalist detritivores and herbivores.
Evolutionary patterns among the Loricariidae show a high degree of trophic niche conservatism, indicating that evolutionary lineage affiliation can be a strong predictor of how basal consumers segregate trophic niche space.
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