Discussion
Diet profile
The DNA metabarcoding method provided a clear taxonomic resolution of
the partially digested stomach content and potentially detected highly
diverse food taxa. This high taxonomic resolution is in line with those
of previous dietary studies based on DNA-based approaches (e.g.,
Harms-Tuohy et al. 2016; Pan et al. 2021; Rees et al. 2020; Sakaguchi et
al. 2017). The results of this study indicate that the diet ofCampylomormyrus and Gnathonemus species is composed mainly
of three types of prey items, i.e., benthic invertebrates, allochthonous
invertebrates, and macrophyte material. The most dominant prey taxa
found in the gut contents of these species belong to benthic
invertebrates, especially aquatic insects. In particular, dipterans
(Chironomidae, Simuliidae, Drosophilidae, and Tephritidae), coleopterans
(Zopheridae, Carabidae, Histeridae, and Scarabaeidae), trichopterans
(Hydropsychidae, Lycaenidae, and Elateridae), ephemeropterans
(Leptophlebiidae, Baetidae, and Ephemerellidae), and odonatans
(Coenagrionidae, Chlorocyphidae, and Chlorogomphidae) are important diet
constituents of all the species. The larvae of these insects live
usually in holes and interstitial spaces of the riverbed. Although the
DNA approach used here cannot tell the stage of the aquatic insects
found in the diet, it is reasonable to assume that the aquatic larvae,
rather than the terrestrial imagines, were targeted byCampylomormyrus and Gnathonemus .
Beside aquatic insects, annelid worms (such as Glossoscolecidae,
Naididae, and Megascolecidae) were also found in the diet of allCampylomormyrus and Gnathonemus species in this study.
Similar to the insects’ larvae, the annelid worms hide in mud and among
aquatic vegetation in the substrate of the riverbed. Other benthic
invertebrates found were freshwater snails (Gastropoda, orders
Pachychilidae and Stylommatophora), and crustaceans (Malacostraca,
orders Decapoda, Copepoda, Cladocera, and Amphipoda).
The second group of food items found in the diet ofCampylomormyrus and Gnathonemus species is allochthonous
invertebrates. The most abundant prey taxa from this group are
Hymenoptera (including Formicidae, Mymaridae, and Braconidae) and
Lepidoptera (including Nymphalidae, Lycaenidae, and Hepialidae).
Additionally, Araneae (Arachnida) were frequently found in the diet.
The third group of food items is plants, including grasses, such as
Poaceae of the Poales order, and flowering plants, such as Fabaceae and
Asterales.
It must be noted that we cannot exclude some of these taxa having
derived from the diet of the primary prey (secondary predation; Sheppard
et al. 2005) or comprise small organisms and plant debris
unintentionally ingested during grasp suction. However, the stomach
contents of Campylomormyrus and Gnathonemus species found
in this study using a DNA metabarcoding approach are compatible withCampylomormyrus mainly (about 90 %) feeding on aquatic insects
(Nwani et al., 2008; Roberts & Stewart, 1976). A previous study, based
on morphological observation, reported that stomach contents of someCampylomormyrus species contain larvae of chironomids, Povilla,
trichopteran, ephemeropterans and odonates, dead plant debris, and
decomposing animal debris (Roberts & Stewart, 1976). This study
reported also that the stomach content of a specimen of C.
rhynchophorus had Chironomidae, Simuliidae, and trichopterans, and a
few small ephemeropterans. Another study on the stomach contents ofC. tamandua using morphological observation reported similar food
taxa (Nwani et al., 2008). The few available dietary studies on other
fish species inhabiting the Congo River such as Schilbe
intermedius (Dirat et al. 2019) and Distichodus antonii ,D. affinis and D. lusosso (Zebe et al. 2010) showed
similar prey spectra.
Diet comparison among
species
The dietary compositions of the Campylomormyrus andGnathonemus species suggest that their feeding behavior is
opportunistic, hence depending on food availability and accessibility.
Based on our results, all species exploit diverse food niches in their
habitats. For instance, the prevalence of benthic invertebrates, such as
larvae of dipterans and coleopterans and annelid worms, in the diet of
all species suggests that these species exploit the bottom of the
riverbed, while the occurrence of allochthonous invertebrates, such as
Formicidae, Nymphalidae and Arachnida spiders, may indicate a certain
degree of surface feeding. Further, the diets also include food items
from the water column, such as copepods. Accordingly, these species may
exhibit high trophic flexibility and diverse feeding behaviors.
However, the RRA results revealed significant differences among the
dietary compositions among the species, potentially associated with EOD
and snout length. For example, the diet of some species, such asC. compressirostris , C. curvirostris , and G.petersii , contains more dipterans, while the diet of C.
tshokwe contains more coleopterans. The diet of C. numenius was
exceptional, as it contained large amount of grass. Unfortunately, this
latter finding remains anecdotal, as we had only one sample of this
species available.
Species with long EOD had preferentially fed on other taxonomic groups
(i.e., coleopterans, ephemeropterans, spiders, annelids), compared to
the species with short EOD, where dipterans dominated in the diet (Fig.
S6.1 in the Supplementary File S6). Similar differences were found
according to snout length (Fig. S6.2 in the Supplementary File S6).
Note, however, that species with long EOD often also exhibit a long
snout (Fig. 4; cf. Lamanna et al. 2016).
Dietary analysis and the radiation
scenario
Our study was motivated by potentially providing further support for the
hypothesis that radiation of Campylomormyrus is caused by an
adaptation to different food sources, associated with diversification of
EOD. The results proved that all the species tested in the current study
are able to exploit diverse niches, especially the bottom fauna,
regardless of their snout shape and EOD.
The current study still provides some evidence that different snout
morphologies and the associated divergence in the EOD translate into
different prey spectra. As a different morphology of the feeding
apparatus among the species may constitute a functional adaptation to
exploiting different substrates (Amen et al. 2020), the different prey
spectra could simply reflect differential availability of different prey
in the respective microhabitats. Unfortunately, no data on
substrate-specific benthofauna are available for the Congo River.
However, it has been argued that the duration of the EOD plays a crucial
role during food detection by determining the prey items that can be
detected best (Harlan Meyer 1982). In fact, prey detection via
electrolocation is a frequency-based process. Our results cannot
establish a causal link between EOD characteristics and prey spectra,
yet they are compatible with the hypothesis that the divergence in EOD
could be of adaptive value during feeding as well, beside its proven
function as a prezygotic isolation mechanism. In this case, EOD would be
a ‘magic trait’ triggering both adaptation and reproductive isolation
(Feulner et al. 2009). One approach to further investigate a potential
dual function of the EOD divergence (feeding specialization and
reproductive isolation) would be to expand on the choice experiments
performed by Amen et al. (2020) by presenting a variety of food items.
This will though still be confounded by the association of long EOD with
long snout. Ultimately, one would like to know prey-specific detection
probabilities, relative to the physical properties of the different
discharges. To achieve such knowledge remains a challenge.
In general, the snout morphology of Campylomormyrus may enhance
their grasp suction mode of feeding (Marrero & Winemiller 1993). This
enhanced mode enables them to obtain the aquatic insects that burrow
into the different substrate structures. This functional foraging
specialization allow an efficient exploitation of the rich bottom fauna
of benthic invertebrates, especially aquatic insect larvae, which other
fishes may not reach. Such trophic specializations may have triggered
the observed radiation of Campylomormyrus in the Congo river.