Proximate and ultimate causes of signal diversity in the electric fishGymnotus

Abstract

SummaryA complete understanding of animal signal evolution necessitates analyses of both the proximate (e.g. anatomical and physiological) mechanisms of signal generation and reception, and the ultimate (i.e. evolutionary) mechanisms underlying adaptation and diversification. Here we summarize the results of a synthetic study of electric diversity in the species-rich neotropical electric fish genus Gymnotus. Our study integrates two research directions. The first examines the proximate causes of diversity in the electric organ discharge (EOD) -which is the carrier of both the communication and electrolocation signal of electric fishes -via descriptions of the intrinsic properties of electrocytes, electrocyte innervation, electric organ anatomy and the neural coordination of the discharge (among other parameters). The second seeks to understand the ultimate causes of signal diversity -via a continent-wide survey of species diversity, species-level phylogenetic reconstructions and field-recorded headto-tail EOD (ht-EOD) waveforms (a common procedure for characterizing the communication component of electric fish EODs). At the proximate level, a comparative morpho-functional survey of electric organ anatomy and the electromotive force pattern of the EOD for 11 species (representing most major clades) revealed four distinct groups of species, each corresponding to a discrete area of the phylogeny of the genus and to a distinct type of ht-EOD waveform. At the ultimate level, our analyses (which emphasize the ht-EOD) allowed us to conclude that selective forces from the abiotic environment have had minimal impact on the communication component of the EOD. In contrast, selective forces of a biotic nature -imposed by electroreceptive predators, reproductive interference from heterospecific congeners, and sexual selection -may be important sources of diversifying selection on Gymnotus signals.

Electric Imaging through Evolution, a Modeling Study of Commonalities and Differences

“…Among the studied species, G. coropinae shows the largest complexity. In this species the duration of the different components is shorter in relation to the delay between regions, therefore facilitating overlapping between generators of opposite sign [36], [70]. Furthermore, as a consequence of EO complexity, synchronous but opposed field generators in different regions of the fish cancel out relatively close to the fish's body.…”

“…G. coropinae and other members of the clade 1 [36], [44], [70] are small sized fish exhibiting multiple asynchronous sources, yielding a very complex near field and a much less extended far field (Figure 4). This feature allows G. coropinae to use its complex discharge mainly for active electrolocation while the same complexity reduces the far field potentials, allowing this fish to cryptically hide from predators.…”

Fish biogeography in the “Lost World” of the Guiana Shield: Phylogeography of the weakly electric knifefish Gymnotus carapo (Teleostei: Gymnotidae)

“…While not the focus on this study, these results provide insight regarding taxonomic and evolutionary aspects of the Gymnotus carapo species. Albert and Crampton () defined Gymnotus carapo sensu stricto, but phylogenetic studies using both morphological and molecular datasets have failed to resolve G. carapo as a monophyletic lineage (Albert et al., ; Lovejoy et al., ; Brochu, ; Crampton et al., ). The present study, which includes increased molecular sampling of G. carapo populations, confirms that there is a complex relationship between geographical isolates of this species and their close relatives.…”

“…Crampton, Lovejoy, and Albert () defined G. carapo sensu stricto and we included samples from several allopatric populations delineated by these authors, including from the western and central Amazon, and the Orinoco. Phylogenetic studies based on morphological and molecular data have failed to support the monophyly of G. carapo , and have placed other described and undescribed Gymnotus species within G. carapo (Albert, Crampton, Thorsen, & Lovejoy, ; Brochu, ; Crampton, Rodríguez‐Cattáneo, Lovejoy, & Caputi, ; Lovejoy, Lester, Crampton, Marques, & Albert, ; Maxime, ). We included representatives of as many of these species as possible (generally corresponding to members of “ G. carapo clades” B, C, and D from Crampton et al., ).…”

“…Phylogenetic studies based on morphological and molecular data have failed to support the monophyly of G. carapo , and have placed other described and undescribed Gymnotus species within G. carapo (Albert, Crampton, Thorsen, & Lovejoy, ; Brochu, ; Crampton, Rodríguez‐Cattáneo, Lovejoy, & Caputi, ; Lovejoy, Lester, Crampton, Marques, & Albert, ; Maxime, ). We included representatives of as many of these species as possible (generally corresponding to members of “ G. carapo clades” B, C, and D from Crampton et al., ). As outgroups, we included G. obscurus , G. varzea , G. curupira and G. chaviro (members of the “ G. carapo clade A” from Crampton et al., ).…”