1050 Electric Organ return to the opposite pole of the source. This is 9. Zakon HH, Unguez GA (1999) Development and important in freshwater fish with water conductivity far regeneration of the electric organ. J exp Biol – below the conductivity of body fluids (usually below 202:1427 1434 μ μ 10. Westby GWM, Kirschbaum F (1978) Emergence and 100 S/cm for tropical freshwaters vs. 5,000 S/cm for development of the electric organ discharge in the body fluids, or, in resistivity terms, 10 kOhm × cm vs. mormyrid fish, Pollimyrus isidori. II. Replacement of 200 Ohm × cm, respectively) [4]. the larval by the adult discharge. J Comp Physiol A In strongly electric fish, impedance matching to the 127:45–59 surrounding water is especially obvious, both on a gross morphological level and also regarding membrane physiology. In freshwater fish, such as the South American strongly electric eel, there are only about 70 columns arranged in parallel, consisting of about 6,000 electrocytes each. Therefore, in this fish, it is the Electric Organ voltage that is maximized (500 V or more). In a marine environment, this would not be possible; here, it is the current that should be maximized. Accordingly, in Definition the strong electric rays, such as the Torpedo species, So far only electric fishes are known to possess electric there are many relatively short columns arranged in organs. In most cases myogenic organs generate electric parallel, yielding a low-voltage strong-current output. fields. Some fishes, like the electric eel, use strong – The number of columns is 500 1,000, the number fields for prey catching or to ward off predators, while of electrocytes per column about 1,000. The discharge others use weak fields for electrolocation and commu- amplitude is only 50 V in air, corresponding to a massive nication. power output of greater than 1 kWat the peak of the pulse. Specialized organs in electrosensitive fishes – mostly For an unknown reason, marine electric fish generate derived from muscle tissue – that give off electrical (unusually large) postsynaptic potentials (PSPs) rather discharges, both pulse-like and sinusoidal, under the than muscle action potentials. control of the nervous system. References ▶Electric Senses in Monotremes: Electroreception and Electrolocation in the Platypus and the Echidna 1. Moller P (1995) Electric fishes: history and behavior. ▶Electrolocation Chapman & Hall, London ▶ 2. Kramer B (1996) Electroreception and communication Electroreceptor Organs in fishes. Gustav Fischer Verlag, Stuttgart ▶Reafferent Control in Electric Communication 3. Kramer B (1990) Electrocommunication in teleost fishes: ▶Temporal Coding in Electroreception behavior and experiments. Springer, Berlin Heidelberg New York 4. Bennett MVL (1971) Electric organs. In: Hoar WS, Randall DJ (eds) Fish physiology. Academic Press, London New York, pp 347–491 5. Zakon HH (1988) The electroreceptors: diversity in structure and function. In: Atema J, Fay RR, Popper AN, Electric Organ Discharge Tavolga WN (eds) Sensory biology of aquatic animals. Springer, Berlin Heidelberg New York Tokyo, pp 813–850 6. Gosse JP (1984) Mormyriformes. In: Daget J, Gosse JP, BERND KRAMER Thys van den Audenaerde DFE (eds) Check-list of University of Regensburg, Institute of Zoology, the freshwater fishes of Africa (CLOFFA). Office Rech Regensburg, Germany Sci Tech Outre-Mer (ORSTOM), Bondy (France)/Mus R Afr Cent (MRAC), Tervuren (Belgium), pp 63–124 Synonyms 7. Albert JS, Campos-da-Paz R (1998) Phylogenetic systematics of Gymnotiformes with diagnoses of 58 EOD; organ discharge clades: a review of available data. In: Malabarba LR, Reis RE, Vari RP, Lucena ZM, Lucena CAS (eds) Definition Phylogeny and classification of neotropical fishes. Certain fish possess an electric organ that, on brain – Edipucrs, Porto Alegre, pp 419 446 command, generates a three-dimensional electric dipole 8. Kramer B, van der Bank FH, Wink M (2004) The Hippopotamyrus ansorgii species complex in the Upper field around their bodies. Compared to incidental stray Zambezi River System with a description of a new fields as measured close to any organism [1], an electric species, H. szaboi (Mormyridae). Zoologica Scripta organ discharge (EOD) is characterized by a stronger 33:1–18 amplitude, higher temporal and spatial stability, and a Electric Organ Discharge 1051 species-specific orientation that is adapted to its discrete frequency lines only where all the energy is function. concentrated. These frequencies are the fundamental frequency (which is the repetition frequency of the Characteristics discharge “pulse,” or of a single signal period), and Quantitative Description the higher harmonics which are integer multiples of Strongly electric fish all generate monopolar (D.C.) the fundamental. pulses. They are head-positive or head-negative in Pulse EODs are single-cycle clicks repeated at rates horizontally attacking fish, and dorsal-negative or from below 1 to about 65 Hz at rest. Pulse discharges dorsal-positive in vertically attacking fish. The time are separated by pauses that are long (and often course of an individual EOD pulse is that of a muscle variable) compared to the duration of an EOD. The E action potential, or of a postsynaptic potential (PSP) amplitude spectrum of a single pulse shows energy in the marine skates, rays, and stargazers (skates possess over a broad and continuous frequency range with a only weak organs). For the human touching a fish, flat peak region; that is, the signal is broadband. perceived amplitudes range from mild discomfort Frequencies of peak amplitude are usually below associated with the EODs of the weakest strong-electric 10 kHz (but may be as high as 25 kHz). For fish, the marine stargazers (up to 5 V with its dorsal intraspecific and interspecific waveform or frequency surface in air), to intense pain caused by, for example, differences see the entry “electric communication and the electric catfish’s(▶Malapterurus electricus) electrolocation.” or the electric eel’s EODs (▶Electrophorus electricus; Wave EODs represent a continuous drain of energy for several hundred Volts). the sender; there are no strong-electric wave fish. Com- There are two phenotypes of weakly electric fresh- pared with most wave EODs, pulse EODs are of lower water fish (Mormyriformes, Gymnotiformes), pulse repetition rate and stronger amplitude. Pulse EODs may and wave species (Fig. 1). be detected over a greater distance because of their usu- Played through a loudspeaker, wave EODs sound ally stronger amplitude. This would be an advantage tonal and are termed “hummers,” whereas pulse species for both communication and active electrolocation. are sometimes termed “buzzers.” In wave EODs, pulse However, wave EODs compensate for being weak by duration and the inter-pulse interval are of about strongly contrasting from background noise by their the same length, and merge into a constant-frequency harmonic structure. There is no or little D.C. component wave. Frequencies range from about 50 to about to the EOD of wave fishes (only a few studied), unlike 1,800 Hz. The amplitude spectrum of a wave EOD, that of many pulse species, making them less prone to such as that of ▶Eigenmannia virescens, shows a few detection by certain predators [2,3,4]. Electric Organ Discharge. Figure 1 Pulse and wave discharges. Left Oscillograms of EODs (head-positivity is upwards); right amplitude spectra with the amplitudes expressed as dB attenuation relative to the strongest spectral component. Same time and frequency axes. Pulse EODs, such as that of the African snoutfish Gnathonemus petersii, are short and broad-band; they are repeated at highly variable rates. The wave EOD of the South American knifefish Eigenmannia virescens is of constant frequency and harmonically structured. 1052 Electric Organ Discharge Higher Level Structures spikes(about1msapart)evokesonlyasingle Astwospike-generatingmembranesarrangedinseries discharge.Thetripletcanberecordedexternally. witheachothertendtodesynchronizeeachother’s Thepacemakernucleusisamidlinestructureof activity,eachelectrocytemustbeinnervatedseparately 16–20relativelysmallneuronslocatedjustventrallyto toreceivethecentralcommandsynchronously[2]. themedullaryrelaynucleus.Thecellsarefunctionally Theelectriccatfishprobablyhasthesimplestcommand coupledbygapjunctions.Incontrasttothecellsof systemforcontrollingitselectricorgan.Itconsists therelaynucleus,theirdendritesextendfarbeyond ofonlytwogiantelectromotoneurons(>100μmdiam- theconfinesofthenucleus,intothesurrounding eter)inthefirstspinalsegment,oneoneitherside.Both reticularformationandlongitudinallyrunningfibre cellsarecloselycoupledelectrotonicallybypresynaptic tracts,wheretheyarepresumablycontactedbythe fibres,andbehavefunctionallyasaunit.Eachgiant mostdiversesources.Itisprobablytheseafferentinputs cellinnervatesthemillionsofelectrocytesonitsside thatmediatetheeffectofvirtuallyanykindofsensory ofthebody. inputonamormyrid’sdischargerate. Ingymnotiforms,thecommandsystemhasfour Command-associatedcorollarydischarges“inform” levelsfromperipheraltocentral:spinalelectromotoneur- afferentbrainareas,suchastheELL(electrosensory ons,medullaryrelaycells,medullarypacemakercells, laterallinelobe),ofareafferencetobeexpected andmesencephalicprepacemakercells.Pacemakerand fromthefish’sownelectroreceptororgansthatis relaycellseitherformtwoseparate,butcloselyadjacent, evokedbythefish’sownEOD[7].Thecorollary midlinenuclei(forexample,ina▶Hy po po pmulus e s dischargesgreatlyfacilitatethetaskofsepara- fishspecies),orareintermingledinasinglenucleus,