Thick-lipped mouth and trophic divergence
Although sympatric generalized and thick-lipped ecomorphs were clearly divergent in lip size in all six pairs, not all have partitioned trophic resources as might be expected from their phenotypic divergence. Generally, the diet of lipped ecomorph was enriched with benthic invertebrates compared to that of the generalized ecomorph with normally developed lips. However, the difference in amount of consumed benthic invertebrates between lipped and non-lipped ecomorphs varied significantly in different rivers. As one might expect, the sympatric pairs with larger differences in diet also showed significant divergence in SI signatures (e.g., in the Didessa and Gojeb) that confirmed usage of SI composition as diet proxy. Lipped ecomorphs had higher δ15N values compared to the generalized ecomorphs in all six comparisons (extra 0.2-1.9‰). Significant enrichment of the lipped ecomorph in δ15N (4 pairs: extra 0.9-1.9‰) was usually accompanied by significant enrichment in δ13C (3 of 4 pairs: extra 1.3-1.7‰). Nevertheless, SI Bayesian ellipses showed great trophic niche widths and overlaps in half of the comparisons of pairs of sympatric ecomorphs (Fig. 4). When thick-lipped ecomorph partitioned trophic resource with co-occurring generalized ecomorph, it was more specialized to feed on aquatic invertebrates similar to the feeding mode of thick-lipped cichlids that provides more examples of convergent evolution among fishes. The variation in diet divergence between sympatric pairs of thick-lipped and generalizedLabeobarbus ecomorphs is similar to other cases of sympatrically co-occurring thick-lipped and thin-lipped cichlid fish such as ecomorphs/species of the genus Amphilophus Agassiz, 1859 from various Nicaraguan lakes (Elmer et al., 2010; Kautt et al., 2012; Manousaki et al., 2013).
Thus, hypertrophied lips in the Labeobarbus as a phenotypic trait is not sufficient to predict the diet of fish. The same was found not only for riverine populations but also for lacustrine. For instance, the thick-lipped ecomorph in Lake Tana had almost the same food spectrum as sympatric generalized ecomorph (Sibbing & Nagelkerke, 2001). This is within Liem’s paradox, i.e., that even species with specialized trophic morphologies have dietary flexibility (Liem, 1980). Many examples corroborate Liem’s paradox suggesting it is a common phenomenon among fishes (e.g., Sturmbauer et al., 1992; Robinson & Wilson, 1998; Binning et al., 2009; Wagner et al., 2009; Golcher-Benavides et al., 2019). This phenomenon reduces the prediction of diet by phenotype and provides evidence for the greater trophic plasticity of specialists (see also recent examples from scraping feeders in Komarova et al., 2021; 2022). Previous experimental studies on various diets exposed to Neotropical and East African cichlid species showed significant phenotypic plasticity provoked by diet (Meyer, 1987; Muschick et al. 2011, Schneider et al., 2014). It suggests that not only certain phenotypes considered as trophically specialized may be plastic in relation to diet but that phenotypic plasticity may also rise from a different diet. This coincides with a flexible stem hypothesis on the origin of adaptive radiations from ancestral flexible stem (West-Eberhard, 2003; Wund et al., 2008; Schneider et al., 2014; Schneider & Meyer, 2017; Gibert, 2017).
What conditions can enable a thick-lipped mouth to function? Both abiotic, (e.g ., type of bottom substrate etc .), or biotic, (e.g ., food resources availability), factors might promote the adaptive value of the thick-lipped phenotype in some studied localities. Unfortunately, information on the phenotype-environment correlation is hard to collect for thick-lipped Labeobarbus . Moreover, existing data on association of thick-lipped phenotype with bottom substrate are unclear. Some studies reported stony habitats as preferred (Groenewald, 1958; Matthes, 1963) while another study (Kisekelwa et al., 2021) reported that species L. longifillis with thick-lipped phenotype (Congo basin) appears to be linked to muddy substrates without pebbles, cobbles and boulders but in warmer localities with a relatively low electrical conductivity compared to sympatrically co-occurring species with a “generalized” mouth. The presence of the thick-lipped ecomorph on muddy substrate is reported by F.N. Shkil (personal communication) for the Lake Tana basin. Due to an unstable hydrological regime in mountain rivers of the Ethiopian Highlands, the environment varies significantly from season to season. Our assessment of diet and trophic position using SI analyses was done based on the material collected during the dry season while the conditions may change during the wet season. One may suggest that thick-lipped trophic morphologies may serve as trade-off between consumption of commonly available food (detritus) and benthic invertebrates in unstable riverine environment, e.g., when seasonally common prey is rare or even absent in some seasons. Additional ecological (natural and experimental) studies need for the clarification of the mechanisms of ecological functionalization of thick-lipped phenotype.