The Mormyridae, sometimes called "elephantfish" (more properly freshwater elephantfish), are a

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

of weakly electric freshwater fish in the order Osteoglossiformes native to

Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...

. It is by far the largest family in the order with around 200 species. Members of the family can be popular, if challenging,

aquarium An aquarium (plural: ''aquariums'' or ''aquaria'') is a vivarium of any size having at least one transparent side in which aquatic plants or animals are kept and displayed. Fishkeepers use aquaria to keep fish, invertebrates, amphibians, aq ...

species. These fish are also known for having large brain size and unusually high

intelligence Intelligence has been defined in many ways: the capacity for abstraction, logic, understanding, self-awareness, learning, emotional knowledge, reasoning, planning, creativity, critical thinking, and problem-solving. More generally, it can ...

. They are not to be confused with the marine and brackish-water callorhinchid elephantfish (family Callorhinchidae) of Southern Hemisphere oceans.

Description and biology

The elephantfish are a diverse family, with a wide range of different sizes and shapes. The smallest are just in adult length, while the largest reach up to . They do, however, have a number of unique features in common. Firstly, their

cerebellum The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cerebe ...

is greatly enlarged, enabling them to interpret complex bio-electrical signals. and to the large size of the valve, Secondly, an auditory

vesicle Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry) In cell biology, a vesicle is a structure within or outside a cell, consisting of liquid or cytoplasm enclosed by a lipid bilayer. Vesicles form nat ...

(a small bladder) is present inside the

labyrinth In Greek mythology, the Labyrinth (, ) was an elaborate, confusing structure designed and built by the legendary artificer Daedalus for King Minos of Crete at Knossos. Its function was to hold the Minotaur, the monster eventually killed by ...

of the left and right

inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in th ...

s. This vesicle, together with a bag with an otolith (sacculum containing the otolith sagitta), itself communicating to the lagena (containing the otolith asteriscus), is in fact unique among vertebrates, completely independent of the other organs; it is neither connected to the labyrinth to which only one otolith bag (the

utriculus The utricle and saccule are the two otolith organs in the vertebrate inner ear. They are part of the balancing system (membranous labyrinth) in the vestibule of the bony labyrinth (small oval chamber). They use small stones and a viscous fluid to ...

containing the otolith lapillus) is attached, nor is it connected to the

swim bladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their current water depth wit ...

(except in

embryos An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

) of which it has the same histological

structure A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such a ...

, nor is it therefore related to the

pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...

. Some species possess modifications of their mouthparts to facilitate electrolocating and feeding on small

invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chorda ...

s 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. The retina is called a "grouped retina", an eye structure seen in mormyrids and a few other fishes. 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. Only a single

gonad A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gonad, the testicle, produces sp ...

is present, located on the left side of their body. The Mormyridae and the closely related genus '' Gymnarchus'' are also unique in being the only vertebrates where the male sperm cell does not have a

flagellum A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates. A microorganism may have f ...

. Among those members of the family lacking extended mouthparts, the body shape and general morphology of the fishes has led to some being known among aquarists by the name of "baby whale", despite the fact that true

whale Whales are a widely distributed and diverse group of fully aquatic placental marine mammals. As an informal and colloquial grouping, they correspond to large members of the infraorder Cetacea, i.e. all cetaceans apart from dolphins and ...

s are

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...

s. Other "mormyrid mammalian misnomers" include the term "dolphin fishes", in reference to certain members of the genus ''Mormyrops''.

Electric fields

Elephantfish possess

electric organs Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by ...

that generate weak

electric field An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field ...

s, and electroreceptors (

ampullae of Lorenzini Ampullae of Lorenzini (singular ''Ampulla'') are electroreceptors, sense organs able to detect electric fields. They form a network of mucus-filled pores in the skin of cartilaginous fish (sharks, rays, and chimaeras) and of basal actinopterygi ...

, knollenorgans, and Mormyromasts) that detect small variations in these electric fields caused by the presence of prey or other objects of different conductivities. This allows them to sense their environment in turbid waters where vision is impaired by suspended matter.The generation of these electric fields and their use in providing the fish with additional sensory input from the environment is the subject of considerable scientific research, as is research into communication between and within species 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. ''

Gymnarchus niloticus ''Gymnarchus niloticus'' – commonly known as the ''aba'', ''aba aba'', frankfish, freshwater rat-tail, ''poisson-cheval'', or African knifefish – is an electric fish, and the only species in the genus ''Gymnarchus'' and the family Gymnarchida ...

'' is a wave fish, producing a near-

sinusoidal A sine wave, sinusoidal wave, or just sinusoid is a mathematical curve defined in terms of the '' sine'' trigonometric function, of which it is the graph. It is a type of continuous wave and also a smooth periodic function. It occurs often i ...

discharge of around 500 Hz. The electric discharge is produced from an electric organ that evolved from

muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of mus ...

, as can also be seen in gymnotiform electric fish, electric rays, and skates. The

convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...

between the South American gymnotiforms and the African Mormyridae is remarkable, with the electric organ being produced by the substitution of the same

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...

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. 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.

Classification

The roughly 221 species of elephantfish are grouped into two subfamilies, the Mormyrinae and Petrocephalinae. The latter has only a single genus: Family Mormyridae * Subfamily Mormyrinae ** '' Boulengeromyrus''

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& Géry, 1968 ** '' Brevimyrus'' Taverne 1971 ** '' Brienomyrus''

, 1971 ** '' Campylomormyrus'' Bleeker, 1874 ** '' Cryptomyrus'' J. P. Sullivan, Lavoué & C. D. Hopkins, 2016 ** '' Cyphomyrus''

Pappenheim Pappenheim is a town in the Weißenburg-Gunzenhausen district, in Bavaria, Germany. It is situated on the river Altmühl, 11 km south of Weißenburg in Bayern. History Historically, Pappenheim was a statelet within Holy Roman Empire. I ...

, 1906 ** '' Genyomyrus''

Boulenger Boulenger is a surname. Notable people with the surname include: * Benjamin Boulenger (born 1990), French footballer * Edward George Boulenger (1888–1946), British zoologist, director of aquarium at London Zoo * George Albert Boulenger (1858– ...

, 1898 ** ''

Gnathonemus ''Gnathonemus'' is a genus of elephantfish in the family Mormyridae. Species There are currently four recognized species in this genus: * '' Gnathonemus barbatus'' Poll 1967 (Angolan mormyrid) * '' Gnathonemus echidnorhynchus'' Pellegrin 192 ...

Gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they ar ...

, 1863 ** '' Heteromormyrus''

Steindachner Franz Steindachner (11 November 1834 in Vienna – 10 December 1919 in Vienna) was an Austrian zoologist, ichthyologist, and herpetologist. He published over 200 papers on fishes and over 50 papers on reptiles and amphibians. Steindachner describ ...

, 1866 ** '' Hippopotamyrus''

, 1906 ** '' Hyperopisus''

, 1862 ** '' Isichthys''

, 1863 ** '' Ivindomyrus''

& Géry, 1975 ** '' Marcusenius''

, 1862 ** ''

Mormyrops ''Mormyrops'' is a genus of weakly electric fish in the family Mormyridae from freshwater in Africa. They are characterized by an elongate head measuring twice as long as high, and no teeth on the palate or the tongue. The genus includes the larg ...

'' J. P. Müller, 1843 ** ''

Mormyrus ''Mormyrus'' is a genus of ray-finned fish in the family Mormyridae. Species There are currently 22 recognized species in this genus: * '' Mormyrus bernhardi'' Pellegrin 1926 (Bernhard's elephant-snout fish) * '' Mormyrus caballus'' Boul ...

'' Linnaeus, 1758 ** '' Myomyrus''

, 1898 ** ''

Oxymormyrus ''Oxymormyrus'' is a small genus of elephantfish in the family Mormyridae. Its members reach about in length and are restricted to the Congo, Campo, Kouilou-Niari, Nyanga and Ogowe river basins in Middle Africa. Taxonomy and species Th ...

'' Bleeker, 1874 ** '' Paramormyrops''

, Thys van den Audenaerde & Heymer, 1977 ** '' Pollimyrus''

, 1971 ** '' Stomatorhinus''

, 1898 * Subfamily Petrocephalinae ** '' Petrocephalus'' Marcusen, 1854

References

External links

* *
Detailed research paper on the sensory and central nervous systems in ''Gnathonemus petersi''

{{Taxonbar , from=Q876569 Mormyridae Fish of Africa Ray-finned fish families

Description and biology

Electric fields

Classification

See also

References

External links