| Freshwater elephantfish | |
|---|---|
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| Gnathonemus petersii | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Actinopterygii |
| Order: | Osteoglossiformes |
| Family: | Mormyridae |
| Genera | |
see text | |
TheMormyridae, sometimes called"elephantfish" (more properlyfreshwater elephantfish), are a superfamily ofweakly electric fish in the orderOsteoglossiformes native toAfrica.[1] It is by far the largest family in the order, with around 200 species. Members of the family can be popular, if challenging,aquarium species. These fish have a large brain size and unusually highintelligence.
They are not to be confused with the marine and brackish-watercallorhinchid elephantfish (family Callorhinchidae) of Southern Hemisphere oceans.
The elephantfish are a diverse family, with a wide range of different sizes and shapes. The smallest are just 5 cm (2.0 in) in adult length, while the largest reach up to 1.5 m (4.9 ft). They do, however, have a number of unique features in common. Firstly, theircerebellum is greatly enlarged, enabling them to interpret complex bio-electrical signals,[2] and to the large size of the valve.[3][4] Secondly, an auditoryvesicle (a small bladder) is present inside thelabyrinth of the left and rightinner ears. This vesicle, together with a bag with anotolith (sacculum containing the otolith sagitta), itself communicating to thelagena (containing the otolith asteriscus), is in fact unique among vertebrates, completely independent of the otherorgans; it is neither connected to the labyrinth to which only one otolith bag (theutriculus containing the otolithlapillus) is attached, nor is it connected to theswim bladder (except inembryos) of which it has the same histologicalstructure, nor is it therefore related to thepharynx.[5]
Some species possess modifications of their mouthparts to facilitate electrolocating and feeding on smallinvertebrates buried in muddy substrates. The shape and structure of these leads to the popular name "elephant-nosed fish" for those species with particularly prominent mouth extensions. The extensions to the mouthparts usually consist of a fleshy elongation attached to the lower jaw. They are flexible, and equipped with touch, and possibly taste, sensors. The mouth is not protrusible, and the head (including the eyes), the dorsum, and belly are covered by a thin layer of skin that is perforated with small pores leading to electroreceptors.[citation needed]
The retina is called a "grouped retina", an eye structure seen in mormyrids and a few other fishes.[6] Instead of being smooth, their retina is composed of tiny cups, acting like parabolic mirrors. Because of the murky waters they inhabit, the cones in their eyes have adapted to see only red light. The cups are made of four layers of light-reflecting proteins, funneling red light to areas of cones, intensifying its brightness 10-fold, while the rods are hit by light from other wavelengths.[7] Only a singlegonad is present, located on the left side of their body.[8] The Mormyridae and the closely related genusGymnarchus are also unique in being the only vertebrates where the male sperm cell does not have aflagellum.[9]
Elephantfish possesselectric organs that generate weakelectric fields, and electroreceptors (ampullae of Lorenzini,knollenorgans, and Mormyromasts) that detect small variations in these electric fields caused by the presence of prey or other objects of differentconductivities. This allows them to sense their environment in turbid waters where vision is impaired by suspended matter.[10][11]
Electric fish can be classified into two types: pulse fish or wave fish. Pulse-type discharges are characterized by long intervals between electric discharges, whereas wave-type discharges occur when the interval between consecutive pulses is so brief that the discharges fuse together to form a wave.[12] Theelectric discharge is produced from an electric organ that evolved frommuscle, as can also be seen ingymnotiform electric fish, electric rays, and skates. Theconvergent evolution between the South American gymnotiforms and the African Mormyridae is remarkable, with the electric organ being produced by the substitution of the sameamino acid in the same voltage-gated sodium channel despite the two groups of fish being on different continents and the evolution of the electric sense organ being separated in time by around 60 million years.[13] Convergent changes to other key transcription factors and regulatory pathways in both Gymnotiforms and Mormyridae also contributed to the evolution of the electric sense organ.[14]
The roughly 221 species of elephantfish which are sometimes grouped into two subfamilies, theMormyrinae andPetrocephalinae. The latter has only a single genus:
| Phylogeny[15][16] |
Eschmeyer's Catalog of Fishes classifies the family as follows[17]FamilyMormyridae
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TheMedjed was a sacred fish in Ancient Egypt. At the city of Per-Medjed, better known asOxyrhynchus, whose name means "sharp-nosed" after the fish, archaeologists have found fishes depicted as bronze figurines, mural paintings, or wooden coffins in the shape of fishes with downturned snouts, with horned sun-disc crowns like those of the goddessHathor. The depictions have been described as resembling members of the genusMormyrus.[19]
Media related toMormyridae at Wikimedia Commons
Data related toMormyridae at Wikispecies
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