Mormyrids occupy an ecological niche largely similar to that of other large group of freshwater weakly electric fishes, the ostariophysan South American gymnotiforms (Lowe-McConnell 1987). Fishes of both groups, with some exceptions, are nocturnal benthic invertebrate-feeders and have adapted to a number of different types of freshwater habitats. Interestingly, the widely separate phylogenetic positions of these two groups among non-electroreceptive teleost clades indicates independent evolution of their electrosensory systems (see Bass 1986c, Kramer 1990). Mormyrids are much more abundant and diverse in river and stream habitats than in lakes (in marked contrast to the African cichlids). Some form large schools near the bottom of pools, others are adapted for life in and near rapids (Roberts & Stewart 1976) smaller streams, marginal habitat, or swamps (Lowe-McConnell 1987). Rainy season spawning migrations from river mouths to upriver breeding habitats have been reported for some taxa (Daget 1957, Blake 1977). Little information exists regarding the reproductive behavior in mormyroids, although male Gymnarchus niloticus and Pollimyrus isidori are known to construct and guard elaborate floating nests in which larvae remain for some time after hatching (see Hopkins 1986).