Mormyridae

The family Mormyridae, sometimes called "elephantfish" (more properly freshwater elephantfish), are freshwater fish in the order Osteoglossiformes native to Africa.[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 are also known for having large brain size and unusually high intelligence.

Freshwater elephantfish
Gnathonemus petersii
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Class:
Actinopterygii
Order:
Osteoglossiformes
Family:
Mormyridae
Subfamilies

Mormyrinae
Petrocephalinae

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

Description and biology

The elephantfishes are a diverse family, with a wide range of different sizes and shapes. The smallest are just 5 centimetres (2.0 in) in adult length, while the largest reach up to 1.5 metres (4.9 ft). They do, however, have a number of unique features in common. Firstly, the cerebellum is greatly enlarged, giving them a brain to body size ratio similar to that of humans (though other sources give the brain/body proportion as 'similar to that of birds and marsupials'; Helfman, Collette & Facey 1997, p. 191). This is likely to be related to the interpretation of bio-electrical signals. Secondly, is the presence of an auditory vesicle (a small bladder), included inside the labyrinth of the left and right inner ear. 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 containing the otolith lapillus) is attached, nor is it connected to the swim bladder (except in embryos) of which it has the same histological structure, nor is it therefore related to the pharynx.[2]

Some species possess modifications of the mouthparts to facilitate feeding upon small invertebrates buried in muddy substrates. The shape and structure of these leads to the popular name of "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. Their mouths are non-protrusible, and their 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 of their eyes is called a "grouped retina", a type of eye structure seen in mormyrids and a few other fish.[3] Instead of being smooth, their retina is composed of tiny cups, acting like parabolic mirrors. Because of the murky waters, 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.[4] Only a single gonad is present, located on the left side of their body.[5] Mormyridae and their close relative Gymnarchus are also unique in being the only vertebrates where the male sperm cell does not have a flagellum.[6]

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 whales are mammals. Other "mormyrid mammalian misnomers" include the term "dolphin fishes", in reference to certain members of the Genus Mormyrops.

Electric fields

Elephantfishes possess electric organs and are thus notable for their ability to generate weak electric fields that allow the fishes to sense their environment in turbid waters where vision is impaired by suspended matter.[7][8]

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.[9] Gymnarchus niloticus is a wave fish, producing a near-sinusoidal discharge of around 500 Hz. The electric discharge is produced from an electric organ that evolved from muscle, as can also be seen in Gymnotiform electric fish, electric rays and skates. The convergent evolution between the South American Gymnotiforms and the African Mormyridae is remarkable, with the electric organ being produced via the substitution of the same amino 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.[10] 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.[11]

Classification

There are about 221 species of elephantfish, grouped into two subfamilies, Mormyrinae and Petrocephalinae. The latter has only a single genus:

Phylogeny based on the following works:[12][13]
Mormyridae
Petrocephalinae

Petrocephalus

 
Mormyrinae
 

Myomyrus

 
 
 

Mormyrops

 
 
 
 

Brienomyrus

 
 

Isichthys

 
 
 
 

Mormyrus

 
 
 
 
 

Pollimyrus

 
 

Stomatorhinus

 
 
 
 

Paramormyrops

 
 
 

Cryptomyrus

 
 
 

Boulengeromyrus

 
 

Ivindomyrus

 
 
 
 
 
 
 
 

Hyperopisus

 
 
 

Brevimyrus

 
 

Hippopotamyrus

 
 
 
 
 
 

Campylomormyrus

 
 

Gnathonemus

 
 
 
 

Genyomyrus

 
 

Marcusenius

 
 
 
 
 
 
 
 
 
 

Family Mormyridae

References
  1. Froese, Rainer, and Daniel Pauly, eds. (2017). "Mormyridae" in FishBase. April 2017 version.
  2. Orts, S. (1967): Contribution to the comparative anatomy and systematics of Mormyroïdes(RAOS Prize 1966) - Ann. Acad. Roy. Sc. d'Outre Mer, Classe des Sc. Nat. et Méd., Bruxelles, XVII, 3, 1-89, 30 figs, 3 tabs, 8 pls, 1967 (in French)
  3. Francke M., Kreysing M., Mack A., Engelmann J., Karl A., Makarov F., Guck J., Kolle M., Wolburg H., Pusch R., von der Emde G., Schuster S., Wagner HJ., Reichenbach A. (2014). "Grouped retinae and tapetal cups in some Teleostian fish: Occurrence, structure, and function" (PDF). Prog Retin Eye Res. 38: 43–69. doi:10.1016/j.preteyeres.2013.10.001. PMID 24157316.CS1 maint: uses authors parameter (link)
  4. Elephant-Nosed Fish Has Funky Eyes, Too
  5. Communication Behavior and Sensory Mechanisms in Weakly Electric Fishes
  6. "Mormyridae | Mormyridae - African weakly electric fishes". Archived from the original on 2013-06-18. Retrieved 2013-09-15.
  7. Bustami, H.P. (2007). Smart elephant fish navigates in darkness with electric fields. life-of-science.net
  8. name=EoF>The generation of these electric fields and their use in providing the fishes with additional sensory input from the environment is the subject of considerable scientific research, as is research into communication between and within species
  9. Caputi, A. A. (1999). "The electric organ discharge of pulse gymnotiforms: the transformation of a simple impulse into a complex spatiotemporal electromotor pattern". Journal of Experimental Biology. 202 (# (Pt 10)): 1229–1241. PMID 10210664.CS1 maint: uses authors parameter (link)
  10. Arnegard, M. E.; Zwickl, D. J.; Lu, Y.; Zakon, H. H. (2010-12-02). "Old gene duplication facilitates origin and diversification of an innovative communication system--twice". Proceedings of the National Academy of Sciences. 107 (51): 22172–22177. doi:10.1073/pnas.1011803107. ISSN 0027-8424.
  11. Gallant, Jason R.; Traeger, Lindsay L.; Volkening, Jeremy D.; Moffett, Howell; Chen, Po-Hao; Novina, Carl D.; Phillips, George N.; Anand, Rene; Wells, Gregg B.; Pinch, Matthew; Güth, Robert (2014-06-27). "Genomic basis for the convergent evolution of electric organs". Science. 344 (6191): 1522–1525. doi:10.1126/science.1254432. ISSN 0036-8075. PMC 5541775. PMID 24970089.
  12. Lavoué, S., Sullivan, J.P. & Hopkins, C.D. (2003). "Phylogenetic utility of the first two introns of the S7 ribosomal protein gene in African electric fishes (Mormyroidea: Teleostei) and congruence with other molecular markers". Biological Journal of the Linnean Society. 78 (2): 273–292. doi:10.1046/j.1095-8312.2003.00170.x.CS1 maint: uses authors parameter (link)
  13. Sullivan, J.P., Lavoué, S. & Hopkins, C.D. (2000). "Molecular systematics of the African electric fishes (Mormyroidea: Teleostei) and a model for the evolution of their electric organs". Journal of Experimental Biology. 203 (Pt 4): 665–683. PMID 10648209.CS1 maint: uses authors parameter (link)
  14. Sullivan, J.P., Lavoué, S. & Hopkins, C.D. (2016). "Cryptomyrus: a new genus of Mormyridae (Teleostei, Osteoglossomorpha) with two new species from Gabon, West-Central Africa". ZooKeys (561): 117–150. doi:10.3897/zookeys.561.7137. PMC 4768369. PMID 27006619.CS1 maint: uses authors parameter (link)
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