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SynopsisAnatomical and ecological studies of the gymnotiform Rhabdolichops troscheli (Kaup 1856), one of the weakly electric fish of South America, indicate it to be unique among gymnotiforms in possessing certain characteristics found in characins. These include post-temporal fossae, well developed gillrakers, a planktivorous feeding mode, and diurnal feeding activity . We use these findings to speculate about the evolution of gymnotiform fishes, by assuming that these character states in this most primitive gymnotiform convey information about the group's evolutionary history . We suggest that the electric organs in this group of ostaryophysans have evolved primarily as a means of locating and capturing prey in river habitats of South America whose turbid waters result in low light penetration, ruling out visual sensing systems as an effective means of receiving information . Other aspects of morphology and behavior such as the absence or reduction of scales, and the nocturnal feeding activities characteristic of other gymnotiform, can be explained as consequences of food resource availability and predation pressures in these riverine environments .

Deep-water sampling of the Orinoco River main channel resulted in the collection of an unexpectedly high abundance and diversity of specialized fishes. Twenty-eight of the more than 60 species collected belong to the Gymnotiformes(New World electric or knife fishes). One of the more numerous of these, a recently described species of the genus Rhabdolichops, consumes large numbers of very small planktonic Crustacea and insect larvae. These items are captured in the very swift, turbid, and deep waters of the Orinoco. Although the strong dependence of the river food web on terrestrial and floodplain food sources is well known, the specialized capabilities of Rhabdolichops and of other fishes that occur with it indicate a significant extension of the river food web into the main channel.

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