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). They use the EOD in
social communication and pair formation as well as for object location
and foraging. Even closely related species markedly differ in their
pulse duration (up to 100-fold difference) and/or waveform shape
(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). The diversifications in these species-specific traits makeCampylomormyrus 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. 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). Currently, 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).
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 five species of Campylomormyrus with markedly different EODs
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 traits (EOD and snout morphology) 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). High Throughput Sequencing (HTS)-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. A pilot study revealed that this approach is not
working for our samples (Lamanna & Tiedemann, unpublished), indicative
of low DNA quality. This lowered DNA quality may be attributed to
logistical constraints during field work in Congo, i.e.,Campylomormyrus are known to be nocturnal feeders, while fish
catching activities were only possible at day-time, leaving time for
progressed digestion which may have prevented effective direct PCR
amplification with standard protocols. 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/HTS
shotgun approach to quantify diet composition in gut contents taken from
wild-caught African electric fish. We specifically test the hypothesis
that the diversification in EOD characteristics and snout shape
translates into differences in the diet compositions ofCampylomormyrus. Thereby, we aim at contributing to our
understanding of the adaptive radiation in this genus.