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.