Proximate and ultimate causes of signal diversity in the electric fish Gymnotus.

A 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 head-to-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 organ discharge diversity in the genus Gymnotus: anatomo-functional groups and electrogenic mechanisms.

Previous studies describe six factors accounting for interspecific diversity of electric organ discharge (EOD) waveforms in Gymnotus. At the cellular level, three factors determine the locally generated waveforms: (1) electrocyte geometry and channel repertoire; (2) the localization of synaptic contacts on electrocyte surfaces; and (3) electric activity of electromotor axons preceding the discharge of electrocytes. At the organismic level, three factors determine the integration of the EOD as a behavioral unit: (4) the distribution of different types of electrocytes and specialized passive tissue forming the electric organ (EO); (5) the neural mechanisms of electrocyte discharge coordination; and (6) post-effector mechanisms. Here, we reconfirm the importance of the first five of these factors based on comparative studies of a wider diversity of Gymnotus than previously investigated. Additionally, we report a hitherto unseen aspect of EOD diversity in Gymnotus. The central region of the EO (which has the largest weight on the conspecific-received field) usually exhibits a negative-positive-negative pattern where the delay between the early negative and positive peaks (determined by neural coordination mechanisms) matches the delay between the positive and late negative peaks (determined by electrocyte responsiveness). Because delays between peaks typically determine the peak power frequency, this matching implies a co-evolution of neural and myogenic coordination mechanisms in determining the spectral specificity of the intraspecific communication channel. Finally, we define four functional species groups based on EO/EOD structure. The first three exhibit a heterogeneous EO in which doubly innervated electrocytes are responsible for a main triphasic complex. Group I species exhibit a characteristic cephalic extension of the EO. Group II species exhibit an early positive component of putative neural origin, and strong EO auto-excitability. Group III species exhibit an early, slow, negative wave of abdominal origin, and variation in EO auto-excitability. Representatives of Group IV generate a unique waveform comprising a main positive peak followed by a small, load-dependent negative component.

Gymnotus capanema, a new species of electric knife fish (Gymnotiformes, Gymnotidae) from eastern Amazonia, with comments on an unusual karyotype.

Gymnotus capanema n. sp. is described on the basis of cytogenetic, morphometric, meristic and osteological data from nine specimens (one male and eight females) from the municipality of Capanema, Pará, in the eastern Amazon of Brazil. Later, three additional specimens were found in museums and regarded as nontypes (not cytogenetically analysed). Gymnotus capanema, which occurs in sympatry with Gymnotus cf. carapo cytotype 2n = 42 (30m/sm + 12st/a) exhibits a novel karyotype for the genus, with 2n = 34 (20m/sm + 14st/a). Gymnotus capanema can be unambiguously diagnosed from all congeners on the basis of a combination of characters from external anatomy, pigmentation and osteology. The constitutive heterochromatin, rich in adenine-thymine (A-T) base pairs [4',6 diamidino-2-phenylindole dihydrochloride (DAPI) positive], occurs in the centromeric region of all of the chromosomes, and in the pericentromeric and the entire short arm of some chromosomes. The nucleolar organizing region (NOR), stained by silver nitrate, chromomycin A(3) (CMA(3)) and 18S ribosomal (r)DNA fluorescence in situ hybridization (FISH), occurs in the short arm of pair 15. FISH, with telomeric probes did not show interstitial telomeric sequences (ITS), despite the reduced 2n in comparison to the karyotypes of other species of Gymnotus. The karyotype of G. capanema, with a reduced 2n, is strikingly different from all other previously studied congeners.