Mormyroidea

The Mormyroidea are a superfamily of fresh water fishes endemic to Africa that, together with the families Hiodontidae, Osteoglossidae, Pantodontidae and Notopteridae, represents one of the main groups of living Osteoglossiformes. They stand out for their use of weak electric fields, which they use to orient themselves, reproduce, feed, and communicate.
There is no consensus regarding its superior biological classification as some experts state that it belongs to the suborder Osteoglossoidei, while others to the Notopteroidei. In either case, the mormyriformes include the gymnarchids and mormyrids and represent the largest superfamily within the order Osteoglossiformes with about two hundred and thirty-three subordinate taxa that are distributed across various watersheds existing throughout tropical Africa south of the Sahara, including the Nile, Turkana, Gambia, and northern South Africa.
These fish have a large brain and an unusual intelligence, they feed on benthic and allochthonous invertebrates, as well as some crustaceans found in marshy and sandy areas of rivers and lakes. Most of its species are sociable, and although their reproductive form is little known, they generally reproduce during the rainy season and their electrical organs transmit signals with the capacity to influence their reproductive and hormonal behavior.
According to the International Union for Conservation of Nature, the conservation status of 66.7% of the species is Least Concern and 10.8% is Threatened species. Furthermore, according to the same institution, the extinction rate of the taxon – at least in the northern region of the African continent – reaches 44.4%, while 55.6% of the individuals are threatened.

Etymology

The term Mormyriformes derives from Greek mormyros, μορμύρος, μόρμυρος, a species of fish that would probably be Lithognathus mormyrus, and from Latin forma, with the same connotation as the English form.
Its synonymy Mormyridae is considered the valid taxonomic status according to the Integrated Taxonomic Information System or ITIS of North America. Although several authors included it within the taxonomic category "order" until after the second half of the 20th century, it is considered as a "superfamily" since the mid 1990s.

Distribution and ecology

Ecology

This superfamily has a wide adaptability and can be found in freshwater river systems "with high concentrations of suspended solids and reduced transparency", a water hardness of up to 20 °dH and a salinity level of less than 1%. The habitats of these fishes are dominated by mud and/or sandy substrates, plant debris, marginal grasses, filamentous algae or clumps of aquatic plants, while the watercourse can be variable: there are species that inhabit barely torrential waters and others in basins where the water flow is high although with presence of pools and rocks that provide areas with lower streamflow.

Distribution

These fishes are distributed in various rivers, lakes, and swamps in Africa, the Nile, and the higher temperature sectors of South Africa, with an area of approximately.
1n 1909, George Albert Boulenger visited the Congo and described forty-seven endemic species, fourteen of them in northern West Africa in the Congo region, eight in West Africa on the Congo and other rivers, seven in the Nile, six in both the Nile and Lake Chad and the Niger and Senegal, and two in Lake Victoria.
In 2003, Didier Paugy, Christian Lévêque, and Guy G. Teugels published a regional synthesis of West African fishes and identified a total of fifteen genera with 41 species in Lower Guinea and fourteen genera with 44 species in West Africa. In 2006, Christian Lévêque and Didier Paugy analyzed the composition of the fish fauna in the most representative rivers and lakes of the main ichthyological provinces of Africa and determined the presence of fifteen species in the Nile, fourteen in Chad, twenty-seven in Niger, sixteen in the Volta River, ten in the Konkouré River, thirteen in the Jong, eight in the Sassandra, ten in the Bandama, fifteen on the Sanaga, twenty-two on the Ogôoué, six on the Ruaha, 109 on the Congo, and ten on the Zambèze. Additionally, within the freshwater or epicontinental species of the richest African aquatic systems, 19 of them are found in the Niger River, 75 in the Congo River and 16 others in the Zambezi River.
In 2008, Melanie L. J. Stiassny, Guy G. Teugels and Carl Hopkins evaluated the geographic distribution of genera in the subfamily Mormyrinae and indicated that at least fourteen are found in Lower Guinea; the remainder can be found in Congo as in the case of Genomyrus, Angola as in Heteromormyrus, Nilo-Sudan as in Hyperopisus and Cyphomyrus, and South Africa as in Cyphomyrus. In addition, the same authors indicated that at least six species in the subfamily Petrocephalinae are in Lower Guinea, while in 2012, several researchers from the universities of Regensburg and Heidelberg, in conjunction with the South African Institute for Aquatic Biodiversity, indicated the presence of several new species in the rivers Luongo, Lufubu, Zambezi, Boro, Cunene, Thoage, and the Okavango Delta.

Morphology

Sizes and shapes

The superfamily Mormyridae has a high diversity within its more than 200 species and subspecies, with a range of sizes and shapes that varies according to the family of membership and their respective genus. The smallest can measure around in their adult stage, while the largest can reach, although a specimen belonging to Gymnarchus niloticus which reached a size of is known to exist in the Loumbila reserve, near Ouagadougou. Its body has cycloid scales, small eyes – which in the case of mormyrids are covered with skin – and a mouth that is not protractile which may vary depending on the genus:.

The genera Campylomormyrus, Gnathonemus and Mormyrus possess a particularly prominent extending mouth that usually consists of a flexible fleshy elongation attached to the lower jaw and is equipped with touch and probably taste sensors, which is why they are popularly called "elephant-nose fishes".
The genera Mormyrops, Brienomyrus, Hippopotamyrus, Marcusenius, Petrocephalus, and Pollimyrus possess small barbs and usually lack the extended mouthparts of elephantfishes, hence they are called "Nile river pikes".
The genus Gymnarchus has a prominent snout with "strong, pointed or notched teeth that line up in a single row on both jaws."
Brain and cerebellum

The brain of this superfamily is one of the largest among fishes and has a body-proportional size comparable to that of humans, with a brain-to-body mass ratio ranging from 1/52 to 1/82, and possibly associated with the ability to interpret bioelectrical signals. Since the pioneering work of Michael Pius Erdl in 1846, several researchers have made efforts toward describing the development of this organ and its functionality.
Thus, based on the analysis of larvae and embryos of Pollimyrus Isidori, it is known that "the brain develops very rapidly: the corpus cerebelli and cerebellar structures, i.e. eminentia granularis, lobus caudalis and transitorius, lobi lineae lateralis, are formed in 40 days, whereas valve development needs 180." They possess a hypertrophy in the cerebellum, which the literature refers to as mormyrocerebellum or gigantocerebellum, "probably related to his unique electrogenic and electroreceptive abilities" and to the large size of the valve, which in turn relates to the electrosensory system present in these fish.
It has been found that for species living in oxygen-deficient aquatic environments, they protect their brains from damage caused by hypoxia through efficient use of existing oxygen. Furthermore, among its species, Gnathonemus petersii was found to hold the record among vertebrates – including humans – as the one whose brain consumes at least 60% of all body oxygen.

Electric organs

Mormyriformes can produce weak electric signals with a specialized organ discovered in 1951 by the British-Ukrainian researcher Hans Lissmann after observing a live specimen of Gymnarchus niloticus. Such an organ is evolutionarily derived from muscle cells, and there is a degree of convergent evolution in form and function with the Gymnotiformes of South America, especially in the sensory apparatus for detecting and processing electrical signals involving electrocommunication and electrolocation processes.

Ampullary organ

are electroreceptors "extremely sensitive to low-frequency fields of biotic or abiotic origin and are generally used in the context of passive electrolocation", with a high sensitivity of 0.01 mV/cm and sensitive to DC fields or frequencies lower than 50 Hz.

Tuberous organs

These fish have two types of tuberous electroreceptor: the Knollenorgan and the Mormyromast. Both organs are found in adult individuals, where they are lightly covered by epithelial cells and skin, while their sensitivity ranges from 0.1 mV and 10 mV/cm/ Tens of Hz up to more than a kHz.
The knollenorgan was first described in 1921 as an epidermal organ by the German anatomist Victor Franz for a Marcusenius species, although without discovering its function. It is composed of a set of receptor cells that can reach between 40 and 60 microns in diameter; these are located under the skin and have a sensitivity of approximately 0.1 mV/cm.
The mormyromast appeared under the name Schnauzenorgan in a paper by Walter Stendell in 1914, where he described it as a combination of the sensory and glandular apparatus for a species of Mormyrus. The larval form of the receptor, the promormyromast, differs from the adult's in cellular composition, distribution in the epidermis and innervation. This organ is one of the most abundant in mormyriforms, with a high concentration of electroreceptors in the epidermis per cm²: for example, for Gnathonemus petersii there are about 2000 per cm², versus a maximum of 50 receptors per cm² for ampullary organs and knollenorgans. In 2009, Engelmann and colleagues showed that G. petersii actively moves its elongated "elephantnose" chin to localise prey accurately, i.e. this species has an active sensory-motor loop that links electroreception to "active motor exploration of the environment".