Phylogenetic comparative analysis of electric communication signals in ghost knifefishes (Gymnotiformes: Apteronotidae)

Abstract

The evolution of communication behavior, like that of all traits, is shaped both by external forces of natural and sexual selection and by internal forces of development and physiology. Complexity in both the function and production mechanisms of communication signals makes them a powerful model for understanding evolutionary interactions within and across levels of biological organization (Ryan, 2005).Electrocommunication in weakly electric fish is an outstanding model for integrating mechanistic and historical biology (sensu Autumn et al., 2002) because these behaviors are diverse across species, are easily recorded and analyzed, and are controlled by a well-characterized neural circuit. The family Apteronotidae is particularly well-suited for a comparative approach because it has the highest species diversity among Neotropical electric fish (Crampton and Albert, 2006). Apteronotids continuously emit a quasi-sinusoidal voltage signal or electric organ discharge (EOD) that has two functions. Nearby objects locally distort the electric field generated by the EOD, and by detecting these distortions with their electroreceptors, fish can electrolocate. The fish can also use their EODs to communicate by detecting the interactions of their own EOD with those of other fish. Individual apteronotid fish maintain an extremely stable EOD frequency (EODf) (Bullock, 1970;Moortgat et al., 1998) and waveform (Rasnow and Bower, 1996). In addition to using the baseline EOD frequency and waveform as communication signals, fish also modulate the frequency and amplitude of the EOD during social interactions Electrocommunication signals in electric fish are diverse, easily recorded and have well-characterized neural control. Two signal features, the frequency and waveform of the electric organ discharge (EOD), vary widely across species. Modulations of the EOD (i.e. chirps and gradual frequency rises) also function as active communication signals during social interactions, but they have been studied in relatively few species. We compared the electrocommunication signals of 13 species in the largest gymnotiform family, Apteronotidae. Playback stimuli were used to elicit chirps and rises. We analyzed EOD frequency and waveform and the production and structure of chirps and rises. Species diversity in these signals was characterized with discriminant function analyses, and correlations between signal parameters were tested with phylogenetic comparative methods. Signals varied markedly across species and even between congeners and populations of the same species. Chirps and EODs were particularly evolutionarily labile, whereas rises differed little across species. Although all chirp parameters contributed to species differences in these signals, chirp amplitude modulation, frequency modulation (FM) and duration were particularly diverse. Within this diversity, however, interspecific correlations between chirp parameters suggest that mechanistic trade-offs may shape some aspects of signal evolution. In particular, a consistent trade-of...

Evolution and hormonal regulation of sex differences in the electrocommunication behavior of ghost knifefishes (Apteronotidae)

“…A discriminant function analysis based on EOD parameters in 12 apteronotid species was able to predict species identity in most cases (Turner et al, 2007). EOD waveform and frequency thus contain information that might allow fish to discriminate conspecific from heterospecific individuals.…”

“…These adaptations allow apteronotids to generate EODs at higher frequencies than those of other electric fishes. EOD waveform and frequency vary across apteronotid species (Table1) (Crampton and Albert, 2006;Kramer et al, 1981;Turner et al, 2007), which may allow EODs to be used as species-identification signals. All apteronotids modulate their EODs during social interactions to produce chirps and rises, and the structure of chirps also differs across species (Turner et al, 2007).…”

“…The within-species variation in EODf, however, can be quite large. EODf in A. bonapartii spans 600Hz (1000-1600Hz), and EODf commonly spans 200-300Hz in other species (Table1) (Kramer et al, 1981;Turner et al, 2007). Large within-species variation in EODf and the species richness of apteronotids at many sites result in extensive overlap of EODf across sympatric species, which limits the utility of EODf alone as a species-identification signal.…”

“…Unlike other wave-type gymnotiforms, which produce monophasic EODs, apteronotids produce EODs with complex waveforms that vary across species (Table1) (Crampton and Albert, 2006;Kramer et al, 1981;Turner et al, 2007). EOD waveform complexity in apteronotids results from a different mechanism than waveform complexity in pulse-type electric fish, which have muscle-derived electric organs.…”

Endocrine Regulation of Dynamic Communication Signals in Gymnotiform Fish

“…Short-term frequency modulations of wave species are often used during courtship and aggressive interactions (Hagedorn and Heiligenberg, 1985;. EOD waveform and frequency are highly variable across apteronotid species and might be used in species recognition (Turner et al, 2007). The function of amplitude modulation, however, is not clear.…”