Anelectric ray (Torpediniformes) showing location of paired electric organs in the head, and electrocytes stacked within it
Inbiology, theelectric organ is anorgan that anelectric fish uses to create anelectric field. Electric organs are derived from modifiedmuscle or in some casesnerve tissue, called electrocytes, and have evolved at least six times among theelasmobranchs andteleosts. These fish use their electric discharges fornavigation, communication, mating,defence, and in strongly electric fish also for the incapacitation ofprey.
In the 19th century,Charles Darwin discussed the electric organs of the electric eel and the torpedo ray in his 1859 bookOn the Origin of Species as a likely example ofconvergent evolution: "But if the electric organs had been inherited from one ancient progenitor thus provided, we might have expected that all electric fishes would have been specially related to each other…I am inclined to believe that in nearly the same way as two men have sometimes independently hit on the very same invention, sonatural selection, working for the good of each being and taking advantage of analogous variations, has sometimes modified in very nearly the same manner two parts in two organic beings".[8] In 1877,Carl Sachs studied the fish, discovering what is now called Sachs' organ.[9][10]
Since the 20th century, electric organs have received extensive study, for example, inHans Lissmann's pioneering 1951 paper onGymnarchus[11] and his review of their function and evolution in 1958.[12] More recently,Torpedo californica electrocytes were used in the first sequencing of theacetylcholine receptor by Noda and colleagues in 1982, whileElectrophorus electrocytes served in the first sequencing of thevoltage-gated sodium channel by Noda and colleagues in 1984.[13]
In mostelectric fish, the electric organs are oriented to fire along the length of the body, usually lying along the length of the tail and within the fish's musculature, as in theelephantnose fish and otherMormyridae.[14] However, in twomarine groups, thestargazers and thetorpedo rays, the electric organs are oriented along the dorso-ventral (up-down) axis. In the torpedo ray, the organ is near the pectoral muscles and gills.[15] The stargazer's electric organs lie behind the eyes.[16] In the electric catfish, the organs are located just below the skin and encase most of the body like a sheath.[1]
Electric organs are composed of stacks of specialisedcells that generate electricity.[13] These are variously called electrocytes, electroplaques or electroplaxes. In some species they are cigar-shaped; in others, they are flat disk-like cells. Electric eels have stacks of several thousands of these cells, each cell producing 0.15 V. The cells function bypumping sodium and potassium ions across theircell membranes via transport proteins, consumingadenosine triphosphate (ATP) in the process.Postsynaptically, electrocytes work much likemuscle cells, depolarising with an inflow of sodium ions, and repolarising afterwards with an outflow of potassium ions; but electrocytes are much larger and do not contract. They havenicotinic acetylcholine receptors.[13]
The stack of electrocytes has long been compared to avoltaic pile, and may even have inspired the 1800 invention of thebattery, since the analogy was already noted by Alessandro Volta.[6][17]
Electric eel anatomy: first detail shows electric organs, made of stacks of electrocytes. Second detail shows an individual cell withion channels andpumps through thecell membrane; Anerve cell's terminal buttons are releasingneurotransmitters to trigger electrical activity. Final detail shows coiledprotein chains of an ion channel.
Electric organs have evolved at least six times in variousteleost andelasmobranch fish.[18][19][20][21] Notably, they haveconvergently evolved in the AfricanMormyridae and South AmericanGymnotidae groups of electric fish. The two groups are distantly related, as they shared a common ancestor before the supercontinentGondwana split into the American and African continents, leading to the divergence of the two groups. A whole-genome duplication event in theteleost lineage allowed for the neofunctionalization of thevoltage-gated sodium channel gene Scn4aa which produces electric discharges.[22][23] Early research pointed to convergence between lineages, but more recent genomic research is more nuanced.[24] Comparative transcriptomics of the Mormyroidea, Siluriformes, and Gymnotiformes lineages conducted by Liu (2019) concluded that although there is no parallel evolution of entire transcriptomes of electric organs, there are a significant number of genes that exhibit parallel gene expression changes from muscle function to electric organ function at the level of pathways.[25]
The electric organs of all electric fish are derived fromskeletal muscle, anelectrically excitable tissue, except inApteronotus (Latin America), where the cells are derived fromneural tissue.[13] The original function of the electric organ has not been fully established in most cases; the organ of the African freshwater catfish genusSynodontis is however known to have evolved from sound-producing muscles.[26]
Electrocytes evolved from an existing excitable tissue,skeletal muscle.[13] Electrocytes are assembled into stacks to create largervoltages (and into multiple stacks to create largercurrents, not shown). Electric fish may have diphasic discharges (as shown), or discharges of other kinds.
Electric organ discharges (EODs) need to vary with time forelectrolocation, whether with pulses, as in the Mormyridae, or with waves, as in the Torpediniformes andGymnarchus, the African knifefish.[27][28][29] Many electric fishes also use EODs for communication, while strongly electric species use them for hunting or defence.[28] Their electric signals are often simple and stereotyped, and the same on every occasion.[27]
The ability to produce electricity is central toNaomi Alderman's 2016science fiction novelThe Power.[34] In the book, women develop the ability to release electrical jolts from their fingers, powerful enough to stun or kill.[35] The novel references the ability of fish such as the electric eel to give powerful shocks, the electricity being generated in a specially modified strip or skein of striated muscle across the girls' collarbones.[36]
The poet and author Anna Keeler's short story "In the Arms of an Electric Eel" imagines a girl who, unlike an electric eel, does feel the electric shocks she generates. Agitated and depressed, she unintentionally burns herself to death with her own electricity.[37]
^Walsh, John (1773). "On the Electric Property of the Torpedo: in a Letter to Benjamin Franklin".Philosophical Transactions of the Royal Society of London (64):461–480.
^Williamson, Hugh (1775). "Experiments and observations on theGymnotus electricus, or electric eel".Philosophical Transactions of the Royal Society of London (65):94–101.
^Hunter, John (1773). "Anatomical Observations on the Torpedo".Philosophical Transactions of the Royal Society of London (63):481–489.
^Hunter, John (1775). "An account of theGymnotus electricus".Philosophical Transactions of the Royal Society of London (65):395–407.
^abAlexander, Mauro (1969). "The role of the voltaic pile in the Galvani-Volta controversy concerning animal vs. metallic electricity".Journal of the History of Medicine and Allied Sciences.XXIV (2):140–150.doi:10.1093/jhmas/xxiv.2.140.PMID4895861.
^Sachs, Carl (1877). "Beobachtungen und versuche am südamerikanischen zitteraale (Gymnotus electricus)" [Observations and research on the South American electric eel (Gymnotus electricus)].Archives of Anatomy and Physiology (in German):66–95.
^Lavoué, S. (2000). "Phylogenetic relationships of mormyrid electric fishes (Mormyridae; Teleostei) inferred from cytochrome b sequences".Molecular Phylogenetics and Evolution.14 (1). R. Bigorne, G. Lecointre, and J. F. Agnese:1–10.Bibcode:2000MolPE..14....1L.doi:10.1006/mpev.1999.0687.PMID10631038.
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