See "Descriptions" tab above for a key to mormyrid genera.
Introduction to the Mormyridae
With more than 200 species in 21 genera, the Family Mormyridae is a modern radiation within the Osteoglossomorpha, an ancient lineage of teleost fishes in which most other living groups are species-poor. Mormyrids are found only in Africa, in freshwater habitats over most of the continent with the exception of the Sahara, northernmost Mahgreb and southernmost Cape provinces. Mormyrids reach their highest diversity in the river systems of Central and West Africa and are often the most abundant kind of fish in riverine habitats.
Mormyrid fishes have long served humans as an important food source along Africa's inland waterways. Ancient Egyptians accurately depicted mormyrids on the walls of their tombs and even worshipped Mormyrus in the temple of Oxyrhynchus. 
However, scientists only discovered mormyrids' most unusual characteristic in the latter half of the 20th Century: an active electric sense by means of which individuals orient to their environment and communicate with others (see "Electric Organ Discharge" below). Mormyrid fishes have since become a model system for research into vertebrate sensory biology, behavior, and communication. They are also popular in the tropical fish hobby where they are known as "elephant-nose fish" and "baby whales."
Adult mormyrids range from about 4 centimeters to 1.5 meters in length and vary considerably in morphology. Most species of genera Petrocephalus, Pollimyrus and Stomatorhinus are short, laterally compressed, deep-bodied fishes with blunt, rounded snouts and small, often inferior to subinferior mouths. Others, such as species of Mormyrops and Isichthys, are elongate with more cylindrical bodies, with terminal mouths. Species of Campylomormyrus and some Mormyrops and Mormyrus have long tubular snouts used for extracting invertebrates from sediment and root masses (Marrero & Winemiller, 1993). Others in genera Marcusenius, Gnathonemus, and Genyomyrus possess a variously developed fleshy protuberance on the chin that functions in electrolocation of prey organisms. In all mormyrids, the mouths are non-protrusible and small cycloid scales cover all but the head. The head (including the eyes), dorsum and belly are covered by a thin layer of skin beneath which lie electroreceptors of different varieties (see below). All mormyrids retain a full complement of paired and unpaired fins. The dorsal and anal fins lack spiny rays and are variable in length among the different genera. In many genera these fins are positioned far back on the body and are more or less symmetrically opposed about the midline. The caudal fin in mormyrids is deeply forked and has a distinctive rounded V-shape with symmetrical, scaled and fleshy dorsal and ventral lobes; it emerges from a narrow, cylindrical peduncle within which lies the electric organ.
In addition to specializations for electroreception which include an enlarged cerebellum, electroreceptors on the body surface, an electric organ in the caudal peduncle and an ortholog of the potassium channel gene expressed only in electric organ tissue, mormyrids have other specializations for acute audition: a gas-filled tympanic bladder coupled to the sacculus in each ear (Fletcher & Crawford, 2001). Males of the genus Pollimyrus communicate not only electrically, but also acoustically, with elaborate courtship songs generated by muscles that vibrate the swim bladder (Crawford, 1997). Little is known about the role of acoustic communication in other genera.
Most mormyrids are nocturnal invertebrate-feeders. However, some species of the genus Mormyrops are piscivores. At night, Mormyrops anguilloides from Lake Malawi engage in a form of semi-cooperative "pack hunting" of sleeping cichlids (Arnegard & Carlson, 2005).
The sister-group to the Mormyridae is the monotypic family Gymnarchidae. Gymnarchus niloticus has a nilo-sudanic distribution and is also electrogenic, but its EOD resembles a continuous wave unlike like the pulsatile EODs produced by mormyrids. Together, the Mormyridae and the Gymnarchidae make up the Superfamily Mormyroidea. Gymnarchus and mormyrids share numerous anatomical characteristics—both related and unrelated to active electrolocation—and they are also the only vertebrates known to posses aflagellate sperm (Morrow, 2004)..
Based on osteological characters, Taverne (1972) divided the Mormyridae into two subfamilies, the Petrocephalinae, containing only the genus Petrocephalus, and the Mormyrinae, containing the remaining genera. Molecular phylogenetic studies (Lavoué, 1999; Sullivan, 2000; Lavoué et al., 2003) have supported this division.
Data of fish populations in African freshwaters are scarce and there are no mormyrid species known to be threatened and with extinction and none are CITES-listed. This is not to say that particular species are not under threat of local extinction in areas impacted by human activity, including over-fishing, and development.
Unlike the electric eel Electrophorus and the electric catfish Malapterurus, mormyrids cannot produce strong electric discharges for defense or to immobilize prey. Instead, by means of a specialized organ near the tail these fishes generate a relatively weak electric field around their body that they monitor using cells embedded in their skin called electroreceptors. Using active electroreception they are able to calculate the size, position and electrical properties of nearby objects in the water and can be active at night when vision is of little use. The sensory receptors used for electroreception are termed mormyromasts. They are numerous and distributed underneath a thick epidermis found on the head, upper back and belly of the fish. Alongside the mormyromasts, a separate variety of electroreceptor termed knollenorgans are present that function exclusively for encoding the characteristics of the EODs of other individuals, i.e, for communication. Mormyrids possess a third kind of electroreceptor sensitive only the very low frequency biolectric fields of prey organisms. These are called ampullary receptors and are similar to those found on some African knife fishes (notopterids) and catfishes. Related to the neural processing demands of electroreception, mormyrids have evolved massively hypertrophied cerebellums that envelop the rest of the brain, giving them one of the largest brain mass to body mass ratios among vertebrates, roughly equal to that of Homo sapiens.
Electric organ discharges, or EODs are also used for communication by mormyrids. Mormyrid EODs are pulses between one-tenth of a millesecond to 20 milleseconds in duration. While the time interval between the pulses is variable, the pulse waveform characteristics are fixed and species-specific. EOD waveforms can differ radically among co-occuring mormyrid species and reproductive males will often develop distinctive waveforms that function in courtship of conspecific females. In this way, EODs serve a function analagous to visual or acoustic signals in many other groups of organisms.
Impressive examples of EOD variation among co-occuring species can be found within the genera Paramormyrops of Lower Guinea and Campylomormyrus of the Congo River. The hypothesis that EODs may in fact accelerate speciation in these "riverine species flocks" and within mormyrids generally is another active research area. EODs are relatively easy to record from living mormyrids and because of their species-specificity and stereotypy, are often useful aides in recognizing species boundaries and working out the taxonomy of this group.
Adult length of mormyrid species ranges from about 4 to about 150 centimeters.
Recent literature on mormyrid systematics includes Taverne’s taxonomic revision of the family based on osteology (Taverne, 1969; 1971 a; 1971 b; 1972), Bigorne's (1990a) review of the mormyrids of West Africa and revision of Brienomyrus, Pollimyrus, Isichthys and Mormyrops of that region (Bigorne, 1987; 1989; 1990 b), Boden et al.’s (1997) revision of the Marcusenius of Central Africa with eight circumpeduncular scales, Jégu and Lévêque’s (1984) study of Marcusenius of West Africa, and Harder's (2000) published CD-ROM with descriptions and photos of all existing types of specimens of Mormyridae. Despite this recent work, the monophyly of several genera remains poorly supported.
Most of the foregoing work is not explicitly phylogenetic, and only recent molecular studies have provided a well supported tree for the major mormyrid lineages (Alves-Gomes & Hopkins, 1997, Lavoué et al. 2000, Sullivan et al. 2000, Lavoué et al. 2003). Points of agreement between the morphological work of Taverne and the molecular studies are 1) the monophyly of Mormyridae, 2) the sistergroup relationship between Mormyridae and Gymnarchus niloticus, and 3) the basal division of the family into two subfamilies: Mormyrinae and Petrocephalinae, with the latter containing only Petrocephalus.
Another recent development is the use of electric organ discharge (EOD) recordings for species discovery and diagnosis within certain genera (Sullivan et al., 2002; Lavoué et al., 2004). Certain aspects of the EOD appear to be phylogenetically conserved, while others are more variable (Alves-Gomes, 1999; Sullivan & Hopkins, 2001; Sullivan et al., 2000).
Mormyrids have a broader distribution than their Nilo-Sudanic sistergroup,Gymnarchus niloticus, including most of the African continent with the exception of the Sahara, northernmost Mahgreb and southernmost Cape provinces (Roberts, 1975) and are most diverse in the river systems of Central and West Africa.
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).