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How old are the weakly electric fishes of Africa and the Neotropics?

New study investigates evolutionary ages and convergence of freshwater elephantfishes and neotropical knifefishes…

Among teleosts some of the more bizarre-looking belong to the superfamily Mormyroidea (freshwater elephantfishes) and order Gymnotiformes (neotropical knifefishes), but both these groups are perhaps most notable for their shared ability to generate and receive weak electric fields used in communication and navigation.

© Lavoué et al.

They’re native to Africa and South America, respectively, and exhibit marked similiarities in some aspects of morphology, swimming and reproductive behaviour, ecology, patterns of nocturnal activity, electrical signals, and even the patterns of nerve activity used to avoid jamming of electrolocation and communication signals.

They thus display aspects of both convergent and parallel evolution, with the former term applicable to the fact that the two groups evolved from unrelated, different-looking ancestors, and the latter to significant similarites in structure and function of the organs used for sending and receiving electric signals.

The question of how novelty arises in evolution has been disputed since the time of Darwin and is viewed as one of the more fundamental issues in biology, and the parallel origins of the complex traits shared by weakly electric fishes provides an opportunity to investigate. Studies have been hampered, however, by a lack of hypotheses for the timing of the origin and early diversification of the two groups.

A new paper published in the online, open access journal PLoS ONE seeks to provide such a theory by examining the timeframe over which the Mormyroidea and Gymnotiformes developed and diversified in a phylogenetic sense and discussing how this may assist in understanding their evolution.

Surprisingly, the two groups were found to have originated independantly from unrelated ancestors at about the same time, more-or-less corresponding to the final separation of western Gondwana, and that a comparable period of time elapsed between the origins of electroreception and the more complex electrogenesis in these fishes.

Origin and diversification of weakly electric freshwater fishes therefore commenced around the beginning of the histories of their respective continents, a fascinating fact in itself but also one which enhances their comparative value for future investigations of evolutionary novelty and historical species radiation.

For further information please refer to the full, open access paper: Lavoué, S., M. Miya, M. E. Arnegard, J. P. Sullivan, C. D. Hopkins, and M. Nishida. 2012. Comparable Ages for the Independant Origins of Electrogenesis in African and South American Weakly Electric Fishes. PLoS ONE 7(5) e36287

Image legend: Mormyroid African electric fishes (left column) are facing gymnotiform South American electric fishes (right column) with similar aspects of morphology (such as elongate bodies, extended tube-like snouts, reduced eyes, and/or small mouth sizes). Anterior portion of body shown above small image of whole body (except for Petrocephalus sullivani); electric organ discharge waveform shown for every species (each trace 5 ms in total duration with head-positivity plotted upwards).

(A) Mormyrops zanclirostris, 175 mm standard length (SL), Ivindo River, Gabon, (B) Sternarchorhynchus oxyrhynchus, 220 mm total length (TL), Rio Negro, Brazil; (C) Mormyrus proboscirostris, 232 mm standard length, Ubundu, Congo River, D.R. Congo; (D) Rhamphichthys sp., 305 mm TL, Rio Negro, Brazil; (E) Mormyrops anguilloides, 195 mm SL, Yangambi, Congo River, D.R. Congo; (F) Gymnotus sp., 195 mm TL, Rio Negro, Brazil; and (G) Petrocephalus sullivani, Ogooué River, Gabon; (H) Eigenmannia sp., Apure River, Venezuela. Species A–D feed on benthic invertebrates, species E, F are piscivorous, and G, H feed on pelagic invertebrates.

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