Introduction
African weakly electric fish (Mormyridae) comprise a species rich group
of freshwater fish endemic to Africa with more than 200 described
species in 20 genera (Lavoué et al. 2003). Within the genusCampylomormyrus , 15 species are described native to the Congo
River and its tributaries (Feulner et al. 2007). Each species exhibits a
species-specific electric organ discharge (EOD). Even closely related
species markedly differ in their waveform shape and/or pulse duration
(Tiedemann et al. 2010). Further, they exhibit species-specific
morphological traits in their feeding apparatus, i.e. the snout,
regarding the snout’s length, thickness, and curvature (Feulner et al.
2008). This makes Campylomormyrus a prime model system to study
the role of ecology in driving an adaptive radiation.
The adaptive radiation within the genus Campylomormyrus has been
studied with regard to molecular genetics (Canitz et al. 2020; Lamanna
et al. 2016), electrophysiology (Feulner et al. 2006), morphometry
(Feulner et al. 2007; Lamanna et al. 2016), and behavior (Amen et al.
2020; Nagel et al.et al. 2018a,b). In combination, these studies suggest
an ecological speciation scenario that Campylomormyrus radiation
is caused by an adaptation to exploit different microhabitats and/or
food sources, associated with diversification of the EOD. Indeed,
behavioral experiments using sympatric Campylomormyrus species
revealed an association between differing snout morphologies and
preferences for certain types of substrate structure (Amen et al. 2020).
Specifically, in a choice experiment, the short snouted species
(C. tamandua ) favored a sandy substrate, while the long snouted
species (C. rhynchophorus ) preferred a stone substrate for
feeding.
While these trait-specific substrate preferences appear plausible from a
mechanical point of view (i.e., longer snouts allow for probing further
into interstitial between stones, Amen et al. 2020), there is currently
no information available as to whether different trunk shapes are
associated with different diets. Furthermore, it is still not known
whether the diversification of EOD serves as a prezygotic isolation
factor only (Nagel et al. 2018 a,b) or also is related to foraging
specialization (Feulner et al. 2009).
Information on diet composition of Campylomormyrus is so far
limited to only two studies on single species: Roberts & Stewart (1976)
reported briefly, based on field observations, some food items found in
the stomach of a single specimen assigned to C. rhynchophorus . At
that time, phylogeny and species delimitation of Campylomormyruswas not well established. Nwani et al. (2008) reported the diet
composition of C. tamandua based on morphological determination.
Hence, so far, to the best of our knowledge, there have been no
controlled studies which compare the diet composition amongCampylomormyrus species under natural conditions. Our study aims
at contributing to fill that knowledge gap by performing a dietary study
for some species of Campylomormyrus with markedly different EOD
and snout morphologies. Our purpose is not only to document the dietary
ranges and components for these species, but also to infer whether
species with specific morphological traits prefer specific food items.
Direct observation of feeding in the natural habitat, i.e., the Congo
river, seems unfeasible and microscopic examination of gut contents may
yield incomplete results, as food items may be digested to various
degree, compromising their microscopic identification (Pompanon et al.
2012). Therefore, we used DNA-based dietary analysis using hybrid
capture and subsequent next generation sequencing (NGS) to analyze
stomach contents of wild-caught fish. NGS-based DNA metabarcoding has
been successfully used to investigate the DNA extracted from highly
degraded diet samples (Deagle et al. 2006; Jarman et al. 2004), however,
an initial PCR amplification necessitates DNA fragments of a certain
length to allow both for primer annealing and a large-enough
species-specific target sequence. As an alternative, DNA hybridization
capture (target enrichment) has been successfully applied to enrich
low-concentration and highly degraded DNA-fragments from environmental
DNA (Shokralla et al. 2012) or ancient sedimentary DNA (Krueger et al.
2022). Here, we apply a combined hybrid capture/NGS shotgun approach to
quantify diet composition in gut contents taken from wild-caught African
electric fish to contribute to our understanding of their adaptive
radiation.