Morphological differentiation in African weakly electric fish (genus Campylomormyrus) relates to substrate preferences

Rahma Amen,Frank Kirschbaum,Ralph Tiedemann,Rebecca Nagel,Maximilian Hedt

doi:10.1007/s10682-020-10043-3

Abstract

Under an ecological speciation scenario, the radiation of African weakly electric fish (genus Campylomormyrus) is caused by an adaptation to different food sources, associated with diversification of the electric organ discharge (EOD). This study experimentally investigates a phenotype–environment correlation to further support this scenario. Our behavioural experiments showed that three sympatric Campylomormyrus species with significantly divergent snout morphology differentially react to variation in substrate structure. While the short snout species (C. tamandua) exhibits preference to sandy substrate, the long snout species (C. rhynchophorus) significantly prefers a stone substrate for feeding. A third species with intermediate snout size (C. compressirostris) does not exhibit any substrate preference. This preference is matched with the observation that long-snouted specimens probe deeper into the stone substrate, presumably enabling them to reach prey more distant to the substrate surface. These findings suggest that the diverse feeding apparatus in the genus Campylomormyrus may have evolved in adaptation to specific microhabitats, i.e., substrate structures where these fish forage. Whether the parallel divergence in EOD is functionally related to this adaptation or solely serves as a prezygotic isolation mechanism remains to be elucidated.

Highlights

In the absence of allopatric separation, disruptive selection on the feeding apparatus can reduce competition by divergent adaptation with regard to resource acquisition
The present results support the hypothesis that Campylomormyrus species have undergone ecological speciation
The diversification of the feeding apparatus may be triggered by niche partition, to reduce interspecific competition

Summary

Introduction

In the absence of allopatric separation, disruptive selection on the feeding apparatus can reduce competition by divergent adaptation with regard to resource acquisition. The definite geographic origin of these species remains unclear, most occur in sympatry (Feulner et al 2009a, b) They exhibit pronounced divergence in the feeding apparatus, i.e. the length, thickness, and curvature of their snout (Feulner et al 2008; Lamanna et al 2016), as well as their electric signals (Electric Organ Discharge, EOD) (Tiedemann et al 2010). While the role of the EOD in electrocommunication [as a prezygotic isolation mechanism (Feulner et al 2009a, b; Nagel et al 2018a, b)] and electrolocation [for object location and foraging (von der Emde and Bleckmann 1998; von der Emde 1999)] is well established, the adaptive value of divergent snout morphology has remained enigmatic so far (Feulner et al 2008). It is further not known, whether the divergence in EOD, beyond its proven function as a prezygotic isolation mechanism, is of adaptive value during feeding as well, under which circumstances EOD would constitute a ‘magic trait’, triggering both adaptation and reproductive isolation (Feulner et al 2009a, b)

Methods

Results

Discussion

Conclusion