The Cichlid Fishes of Lake Malawi, Africa

Abstract of Doctoral Dissertation

Matthew Erin Arnegard
Cornell University, Ithaca, New York, 2005
Adviser: Carl D. Hopkins  

Signal divergence and design in sympatric morphs of an electric fish species flock  

Mechanisms by which 'species recognition signals' diverge in animal lineages without extrinsic barriers to gene flow remain poorly understood. Empirical cases lag behind theory in this arena of evolutionary biology. Promising places to explore sympatric signal diversification are animal groups of recent origin, in which species-typical signals are distinct and easy to quantify. An excellent example is a species flock of mormyrid fishes from Gabon (genus Brienomyrus). All mormyrids produce discrete electric organ discharges (EODs) to detect objects in their environment and to communicate. These fishes have been difficult to study in the wild due to their nocturnal habits and inaccessibility. Among mormyrids, the Brienomyrus species flock exhibits particularly striking EOD waveform diversification. In Chapter 1, I describe electric signaling and group predation by mormyrids using the first video recordings of any freely-behaving species (Mormyrops anguilloides) in its natural habitat (Lake Malawi). In Chapter 2, I introduce the reader to Gabon's Brienomyrus species flock, and I apply quantitative methods to characterize EOD waveform variation among morphologically-distinct species. In Chapter 3, I use new genetic markers to show that members of the species flock with unique appearances and distinctive EODs are reproductively isolated. Given this context, I describe an atypical system of Brienomyrus morphs belonging to the flock's magnostipes complex. I show that EODs of these morphs have diverged in the absence of extrinsic barriers to gene exchange. In Chapter 4, I assay EOD discrimination behavior using electrical playbacks, and I make physiological recordings of Knollenorgan electroreceptors. These experiments demonstrate that EODs serve as functional displays for the magnostipes complex, in which distinct signals are linked to body size differences between morphs. In Chapter 5, I construct theoretical simulations of potential relevance to the magnostipes complex. These simulations describe conditions in which sympatric speciation is expected to occur by strong mating preferences, alone. Some mode of natural or sexual selection apparently underlies the occurrence of multiple EOD dimorphisms within the magnostipes complex. EODs may signal alternative behavioral or life history strategies within lineages, or selective influences driving sympatric speciation may be recruiting EODs for behavioral isolation between incipient species. Regardless of the mechanism, my results set the stage for future work on this potentially informative model of sympatric divergence in animal recognition signals.




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