Tuesday, July 12, 2016

[Ichthyology • 2012] Trophic Diversity in the Evolution and Community Assembly of Loricariid Catfishes

Figure 1 Representative morphological diversity within Loricariidae.
Inset: CT reconstructions of upper and lower jaws of (A) Leporacanthicus (an insectivore); (B) Panaque (a wood-eater); and (C) Chaetostoma (a detritivore-algivore). Photos (scaled to approximate relative size): 
(a) Pseudancistrus pectegenitor (Ancistrini), (b) Panaque armbrusteri (Ancistrini), (c) Pseudohemiodon sp. (Loricariini), (d) Hemiancistrus subviridis (Ancistrini), (e) Hypancistrus contradens (Ancistrini), (f) Spatuloricaria sp. (Loricariini), (g) Parotocinclus eppleyi (Hypoptopomatinae), (h) Hemiancistrus pankimpuju (Ancistrini), (i) Oxyropsis acutirostra (Hypoptopomatinae), (j) Chaetostoma sp. (Ancistrini), (k) Farlowella sp. (Farlowellini), (l) Leporacanthicus triactis (Ancistrini).
Fish photos by N. K. Lujan (b,g,h,i,j,k) and M. H. Sabaj Pérez (a,c,d,e,f,l).
DOI: 10.1186/1471-2148-12-124 

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 δ13C and δ15N stable isotope signatures from 649 specimens representing 32 loricariid genera and 82 species from 19 local assemblages distributed across South America. We calculated vectors representing the distance and direction of each specimen relative to the δ15N/δ13C 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 δ15N assemblage centroid deviation spanning 4.9‰, which is within the tissue–diet discrimination range known for Loricariidae, indicating that they feed at a similar trophic level and that δ15N largely reflects differences in their dietary protein content. Total range of δ13C 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 δ15N and δ13C 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.

Figure 3 Evolutionary patterns of loricariid trophic diversification revealed by assemblage centroid-standardized isotope vector analysis (ACSIVA) of C and N stable isotope data (Figure3; See Additional file 4 : Figures 3–10 for isotope biplots of all local assemblages examined). Cladograms A and B represent the hypothesized ancestral distributions of lineages along a vertical axis of 15 N-enrichment relative to assemblage centroid (pink to red being 15 N-enriched relative to centroid, light green to green being 15 N-depleted relative to centroid). Cladogram A represents a phylogenetic regression of 649 individual vectors (Figure 3A) grouped by genus irrespective of site (see Additional file 2: Figure S1 for a full size version of this phylogeny), and cladogram B represents a regression of 120 mean genus x site vectors (Figure 3 C). Circle plots illustrate the direction of mean genus x site vectors for each genus and statistical significance of the mean vector is indicated by * (Rayleigh’s Z test: P < 0.05) or † (0.10 > P > 0.05; Tables 1, 2). Cladogram C resulted from the same analysis as B, but is color-coded to reflect hypothesized distributions of ancestral lineages along a horizontal axis of 13 C-enrichment relative to assemblage centroid (see Additional file 3: Figure S2 for a full size version of this phylogeny). Oral disk photos by N. K. Lujan or M. H. Sabaj Pérez. 

Our study introduces the ACSIVA method of visualizing a consumer’s trophic position relative to sympatric taxa in isotope biplot space, and uses this method to integrate isotopic data both spatially across landscapes and evolutionarily across a phylogeny. Our analysis suggests that Loricariidae should be seen not only as a highly diverse phyletic radiation, but also as an ecological radiation that has diversified along trophic niche dimensions that were heretofore cryptic, yet consistent with previously observed jaw morphological diversity. Current understanding of ecological radiation has been heavily influenced by studies of plants and vertebrates that diversified among island archipelagos and lakes, but there are few prominent examples of ecological radiations in river basins or among the detritivores and herbivores that dominate food webs in tropical rivers and virtually all other ecosystems. The frequently amorphous appearance and low taxonomic resolution achievable for gut contents of most herbivores and detritivores may account for our currently poor understanding of niche relationships within this important trophic guild. Detritivores and herbivores appear to select food items based more on chemical and nutritional qualities than taxonomy or morphology. By estimating molecular patterns of food resource assimilation over time, stable isotope, fatty acid signature analysis, and nutritional physiological approaches provide powerful tools for investigating herbivore and detritivore niche diversification and partitioning.

Nathan K. Lujan, Kirk O. Winemiller and Jonathan W. Armbruster. 2012. Trophic Diversity in the Evolution and Community Assembly of Loricariid Catfishes.
BMC Evolutionary Biology. DOI: 10.1186/1471-2148-12-124