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.