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Electrocommunication

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Electrocommunication

Different weakly electric fishes.

Electrocommunication is the communication method used by weakly electric fishes. Weakly electric fishes are a group of animals that utilize a communicating channel that is "invisible" to most other animals: electric signaling. Electric fishes communicate electrically by one fish generating an electric field and a second individual receiving that electric field with its electroreceptors. The receiving side will interpret the signal frequencies, waveforms, and delay, etc.[1] The best studied species are two freshwater lineages- the African Mormyridae and the South American Gymnotiformes.[2] While weakly electric fish are the only group that have been identified to carry out both generation and reception of electric fields, other species either generate signals or receive them, but not both. Animals that either generate or receive electric fields are found only in aquatic (or at least moist) environments due to large resistance of all other media (e.g. air).[3] So far, communication between electric fish has been identified mainly to serve the purpose of conveying information in

  • species recognition
  • courtship and sex recognition
  • motivational status (attack warning or submission) and
  • environmental conditions.

Contents

  • Overview of weakly electric fish 1
  • Electroreceptor organs 2
    • Classification of the two types of receptive organs 2.1
      • Tuberous organs 2.1.1
    • Classification of tuberous organs 2.2
  • Electric organs 3
    • Mormyrids 3.1
    • Gymnotiforms 3.2
  • Signals 4
    • Types of signals 4.1
    • Physical properties of signals 4.2
      • Electric field 4.2.1
      • Active space 4.2.2
      • Frequency and waveform 4.2.3
    • EOD frequency 4.3
    • EOD waveform 4.4
    • Differences and changes in signals 4.5
      • Signals and sex 4.5.1
      • Signals and development stages 4.5.2
      • Signals and dominance status 4.5.3
    • Special signals 4.6
  • See also 5
  • References 6
  • External links 7

Overview of weakly electric fish

A video example of an electric courtship signal in an African weakly electric fish, Paramormyrops sp. recorded in Gabon. This audio recording was made from a male courting a female. The original recording is of the electric signal but here it is converted into sound by the loud speaker. The upper trace shows an oscillogram of the original waveform, the lower figure is a sound spectrogram of the same recording. A photo of the species is shown in the inset panel. The male produces "rasps", which are bursts of high frequency discharges, as his coursthip calling song. The males electric organ discharge waveform is longer in duration than the female's and thus the sound has a lower pitch. (Data from Hopkins and Bass, 1981)

Electric fish are capable of generating an external electric fields or receive electric fields (electroreception). Electric fish can be further divided into three categories: strongly discharging, weakly discharging, and fish that sense but is unable to generate electric fields.[1] Strongly electric fish generate strong electric field up to 500 volts for predatory purposes;[4] Strongly electric fish include both marine and fresh water fishes (two freshwater taxa- African electric catfish (Malapterurus electricus) and the Neotropical electric eel (Electrophorus electricus) and the marine torpedo rays (Torpedo)). Weakly electric fish generates electric fields mainly for communication and electrolocation purposes; weakly electric fish are found in fresh water only and includes African freshwater Mormyridae and Gymnarchus and Neotropical electric knifefishes. Lastly, fish that are only able to detect electrical signals includes sharks, rays, skates, catfishes, and a number of other groups (see Electroreception).[4]

Electric fishes generate discharge from electric organs located near the tail region. Electric organs are mostly derived from muscle cells (myogenic); except for one gymnotiform family has electric organ derived from neurons (neurogenic organs). To detect the electric signals, electric fish has two types of receptive cells- ampullary and tuberous electroreceptors.

Electroreceptor organs

All organisms respond to sufficiently strong electric shocks, but only some aquatic vertebrates can detect and utilize weak electric fields such as those that occur naturally. These aquatic organisms are therefore called electroreceptive. (For an example, human-beings react to strong electric currents with a sense of pain and sometimes a mixture of other senses; however, we cannot detect weakly electric fields and therefore are not electroreceptive.) The ability to sense and utilize electric fields was found almost solely in lower, aquatic vertebrates (fishes and some amphibians). Terrestrial animals, with very few exceptions, lack this electric sensing channel due to low conductivity of air, soil, or media other than aqueous environment. Exceptions include the Australian monotremes, i.e. the echidna which eats mainly ants and termites, and the semi-aquatic platypus that hunts by utilizing electric fields generated by invertebrate prey.[5]

In order to detect weakly electric fields, animals must possess electroreceptors (receptive organs) that detect electric potential differences. For electric fishes, receptive organs are groups of sensory cells rooted in epidermal pits, which look like small spots on the skin. In each receptive organ, there are sensory cells embedded in the bottom of the opened "pit" that faces outside. Electroreceptors detect electric signals by building up a potential difference between the outside environment and the fish body's internal environment. Current flow due to such potential difference further results in a receptor potential that is presynaptic to the sensory fibers. Finally, this receptor potential leads to action potential fired by sensory cells.[6]

Electric fishes carry a variety of sensitive receptive organs that are tuned to different types and ranges of signals. To classify types of electroreceptors, the first differentiation point should be made between

  • A video of courtship and pawning behavior between a male and female Brienomyrus brachyistius can be seen here.
  • Methods for listening to electric fishes at home.

External links

  1. ^ a b Masashi Kawasaki. The electric fish. [1] Retrieved 12/3/2011
  2. ^ Map of Life, 2011
  3. ^ Czech-Damal NU, Liebschner A, Miersch L, et al. (February 2012). "Electroreception in the Guiana dolphin (Sotalia guianensis)". Proceedings. Biological Sciences 279 (1729): 663–8.  
  4. ^ a b c d e f Moller, P. (1995) Electric Fishes: History and Behavior. Chapman & Hall
  5. ^ Electroreception and Communication in Fishes / Bernd Kramer - Stuttgart; Jena ;Lübeck ; Ulm : G. Fischer, 1996. Progress in Zoology ; Vol. 42.
  6. ^ a b c d e Ladich, Friedrich. 2006. Communication in fishes. Enfield, NH: Science Publishers
  7. ^ Carl Hopkins. Electroreception. [2] Retrieved 12/5/2011
  8. ^ Zakon, H. (1986). The electroreceptive periphery. In Electroreception, eds. T. H. Bullock and W. F. Heiligenberg), pp. 103-156. New York: John Wiley & Sons.
  9. ^ a b c Masashi Kawasaki Chapter 7: Physiology of Tuberous Electrosensory Systems. In: Theodore H. Bullock, Carl D. Hopkins, Arthur N. Popper and Richard R. Fay. 2005 (eds), Electroreception. New York: Springer.
  10. ^ a b Bennett MVL (1971) Electric organs. In: Hoar WS, Randall DJ (eds), Fish Physiology. London: Academic Press
  11. ^ Philip K. Stoddard, Electric Signals and Electric Fishes. 2009
  12. ^ Hopkins, CD, Design features for electric communication Journal of experimental biology Vol. 202, 10, 1999, p. 1217
  13. ^ Stoddard PK. (2002) Electric signals: predation, sex, and environmental constraints. Advances in the Study of Behaviour
  14. ^ Hopkins, CD, Temporal structure of non-propagated electric communication signals brain behavior and evolution Vol. 28, 1986, p. 43 [3]
  15. ^ Bossert, WH, The analysis of olfactory communication among animals. Journal of Theoretical Biology. Vol. 5, 3, 1963, p. 443
  16. ^ Squire, A; Moller, P. Effects of water conductivity on electrocommunication in the weak-electric fish Brienomyrus niger (Mormyriformes)animal behaviourVol. 30, 2, 1982, p. 375
  17. ^ a b Hopkins, C.D., Neuroethology of electric communicationannual review of neuroscienceVol. 11, 1, 1988, p. 497
  18. ^ a b Hopkins, C. D. (1972). Sex differences in electric signaling in anelectric fish. Science 176
  19. ^ a b Hagedorm, M.(1986) The ecology, courtship and mating of gymnotiform electric fish. (eds. Bullock, T. H.Heiligenberg, W.), Wiley, NY
  20. ^ Bass, A. H.; Hopkins, C.D., Shifts in frequency tuning of electroreceptors in androgen-treated mormyrid fish, Journal of comparative physiology, Volume 155, Number 6, 713-724, doi:10.1007/BF00611588
  21. ^ Emergence and development of the electric organ discharge in the mormyrid fish, Pollimyrus isidori, G. W. Max Westby and Frank Kirschbaum, JOURNAL OF COMPARATIVE PHYSIOLOGY A: NEUROETHOLOGY, SENSORY, NEURAL, AND BEHAVIORAL PHYSIOLOGYVolume 122, Number 2, 251-271, doi:10.1007/BF00611894
  22. ^ Franchina,C.R.; Salazar, V.L.; Volmar, C. H, and Stoddard, P.K., Plasticity of the electric organ discharge waveform of male Brachyhypopomus pinnicaudatus. II. Social effects journal of comparative physiology b biochemical systemic and environmental physiology Vol. 187, 1, 2001, p. 45
  23. ^ Carl Hopkins, Behavioral Evidence for Species Recognition [4], Retrieved 12/6/2011

References

See also

In electric communication, there are some distinct types of signals that serve special purposes such as courting or aggression. Examples of these special EODs include: "rasps", "chirps" and "smooth acceleration". Rasp is a burst of pulses at a relatively constant frequency performed by some species during courtship. Chirp is a rapid increase or decrease in frequency. Smooth acceleration is a period of tens to hundreds of milliseconds that EOD rate increases but in a smooth way. Due to law of conservation of energy, the amplitude of the EOD might lower for a few percent, but the overall changes in waveform and amplitude Is small. Male gymnotiforms emit these accelerated signals during aggression and courtship. In the fish studied, if courtship goes well and proceeds to spawning, male electric fish starts to use another special type of EOD- the chirp. Chirp also lasts for tens to hundreds milliseconds; however, the increase in frequency was so high that electrocytes could not recover soon enough, and therefore, chirps has a very small amplitude and a waveform deviated from the original waveform.[17][23]

Special signals

Measurements have shown that typically, male electric fish that’s dominating usually has lower EOD frequency and longer EOD duration. An experiment has shown that, when two males are placed in the same fish tank, both fishes enhance their EOD in the first short period of time. However, after leaving the fishes in a dark period (mimicking night time), the male with higher EOD amplitude, which is also usually the male with bigger body size, will further enhance its EOD; on the contrary, the male with smaller body size/ smaller EOD does not enhance its EOD.[22]

Signals and dominance status

There were two hypotheses proposed for the reason why electric signals were modified during the fishes' development stages. Firstly, as stated above, the fishes' electroreceptors are usually tuned to a specific range of frequencies. Therefore, to make effective communication, it is necessary for the electric fishes to narrow the broad frequency spectrum of the larval EOD. Secondly, it is known that the electroreceptors of catfish, gymnotiforms, and most pre-teleost fish are tuned to lower frequencies. Therefore, keeping the low frequency of larval EOD will increase the risk of being detected by predators.[6]

For the myogoenic fishes, this change in signal waveform occurs with the initial larval electrocytes fusing together forming new electrocytes with different shapes, along with redistribution of ion-gated channels, formation of new extracellular structures on the electrocytes, etc. Some pulse fishes also develop accessory electric organs located on other parts of the body; these extra electric organs further play a role in adding phases to the EODs. For the only neurogenic fish known so far, apteronotids, EOD changes during the developmental process seemed to be more dramatic than that of the myogenic fishes, which might indicate that neurogenic electrocytes are more easily prone to modifications. Similar with the myogenic fishes, apteronotids has its electric organ formed by myocytes. As apteronotids matures, new neurogenic electrocytes derived from spinal motoneurons replace the myogenic electrocytes.[10]

Studies done on both gymnotiforms and mormyrids have shown that there are species in both groups that have significant EOD changes from larvae to adult. Gymnotiform larvae all have EODs that are simple, monophasically similar to a single period cosine function, and formed with a very broad spectrum at lower frequency range. It is observed that, as the larvae mature, the frequency spectrum decreases, the discharge waveform becomes sharper, and more complex waveforms that may consist of multiple phases gradually replace the simple larval EOD.[21]

Signals and development stages

Sexual dimorphism in EOD waveforms and frequencies also imposes an influence on active space size. Using Sternopygus marucus as example, males and emit frequencies almost half those of females (80 Hz cf. 150 Hz). However, since most electroreceptors are tuned to signal frequencies that are closer to the receivers' own frequency, the difference in EOD frequency results in a different ability of electric fishes to sense signals from either sex, which further leads to different active space sizes. As measured in Sternopygus marucus by Hagedorn, male fish can only detect females in a range of 6 cm, while female fish can detect a male fish in a much bigger range of 39 cm. This active space size difference is hypothesized to give female fish a better probability of getting closer to potential mates and select an individual to mate with.[4][18][19]

[20][6]. Experiments shown that by injecting 11-KT into female electric fishes, not only did their EOD waveforms and frequencies become closer to that of the males, but their tuberous electroreceptors were also modified to be able to detect signals according to the newly transformed EOD properties. However, when estrogen was applied, the EODs of male electric fish gradually became closer to the female EODs.estrogen 11-keto-testosterone (11-KT) and androgen One of the physiological factors that have been proven to contribute to the sexual dimorphism of EODs is the level of teleost hormone- [19][18] As electric fish mature, some taxa develop differences in EOD between males and females (i.e.

Signals and sex

Electric fishes normally possess a baseline frequency and waveform of their signals, alteration in both qualities occur all the time- among different species, sex, development stages, and dominance status. While different alterations occur to signal generations based on the fishes' identities, the level and types of alteration is limited by the fishes' own sensory system, which is biased to sense signals that have similar frequency to its own discharge frequency.[6]

Differences and changes in signals

Waveform is the shape and form of a wave. Each species of electric fish has their distinct EOD waveform. Pulse-type fishes conduct species recognition by paying attention to the differences of EOD waveform, which include properties such as: EOD duration, number of phases, and form of the phases. Meanwhile, some indirect properties hidden in waveform are also used by pulse-type fishes: amplitude gradient, duration ratios of phases, and the order and signs of phases.

Different waveform representations

EOD waveform

Frequency is the number of occurrences of a repeating event per unit time. Here, EOD frequency is referred to the firing rate of an electric fish. Wave-type fishes carry out species recognition by mediating their EOD frequencies, which include their baseline firing frequencies and modulation of frequencies that results in rising, falling, warbling, and cessations of EOD frequencies. For an example, some gymnotiform species use "chirps," a sudden frequency increase, during courtship.

EOD frequency

[17] Electric fishes communicate with electric signals that possess two main qualities-

Frequency and waveform

One of the biggest factors that affect active space size will be the [water conductance] mediated by solute concentration in the water. It’s been shown that momyrids have adapted its optimal active range in lower-conductivity habitats. One natural phenomenon that supports such theory is that, many species spawn during the time when rivers/ lakes have the lowest conductivity due to heavy rains. Having bigger active space in water with low conductivity will therefore benefit mating and courting.[16] One other explanation tested by Kim and Moller is that, having smaller active space during dry season when mating is not occurring accommodates crowded social spacing without unnecessary signal transmission between individuals.[4]

When transmitting electric signals in aquatic environment, the physical and chemical nature of the surrounding can make big differences on signal transmission. Environmental factors that might impose influences include solute concentration, temperature, and background electric noise (lightning or artificial facilities), etc. To understand the effectiveness of electric signal transmission, it is necessary to define the term "active space-" the area/ volume within which a signal can elicit responses from other organisms. The active space of an electric fish normally has an ellipsoid shape due to the arrangement of dipoles formed by its electric organs. While both electric communication and electrolocation rely on signals generated by electric organs, electrocommunication has an active space tenfold larger than electrolocation because of the extreme sensitivity of tuberous electrocommunication receptors.[15]

Active space

Electric fish generate an electrostatic field shaped like a dipole, with field lines describing a curved arc from positive pole to negative. The electric differs from other communicating modes of communication such as sound or optical, which use signals that propagate as waves. While sound waves for acoustic communication or light waves (electromagnetic waves) for visual communications all propagate, electric signals do not (it is different from electromagnetic waves). As an electric field, the signal magnitude decreases as the inverse cube of distance (1/r^3), which makes the signal sending and formation an energy-costly process. To solve this problem, electric fish match the reflection, refraction, absorption, interference, and so on. As a result, the temporal features, which are very important for electric fish signals, remain constant during transmission.[14]

Electric field built up by a dipole system of magnet. Electric fish generate an electric field in the same manner.

Electric field

Physical properties of signals

There are two types of signals generated by electric fishes: pulse-type and wave-type. A pulse-type EOD is characterized by discrete EOD pulse separated by relatively long silent intervals much longer than the discharges; contrarily, a wave-type EOD has its firing period and silence period approximately the same in length, and therefore a continuous signal with quasi-sinusoidal waveform is formed. Among the mormyrids and gymnotiforms, both pulse-type and wave-type fishes are consistent within grouped by families.[13]

Types of signals

Signals

Apternotids, a member of the wave-type gymnotiforms, is different from all other electric fishes as being the only family possessing neurogenic electrocytes. Electric organ of Apternotids is derived from neurons; more specifically, they are formed from the axons of spinal electromotor neurons. Such structure eliminates one [synapse|synaptic gap] between spinal electromotor neuron and the myogenic electrocytes, which might contribute to Apternotids' highest EOD frequency (>2000 Hz) among electric fishes.[9]

In gymnotiforms, electrocytes diffe between wave-type and pulse-type electric fishes. In wave-type fishes, electrocytes are in a tubular form. In pulse fish the electrocytes tend to be flattened disks. The electocytes also form columns, but unlike the shorter size of electric organ in mormyrids, gymnotiforms have long electric organs that extend throughout almost the entire longitudinal body length. Also different from the stalk system in mormyrids, stalks in gymnotiforms only make one type of innervation at the posterior side of electrocyte. Pulse-type gymnotifoms generally show a higher complexity than the wave-type fishes. For instance, their electrocytes can be either cylindrical or drum-shaped with stalks innervating from either posterior or anterior side. Another important difference is that, unlike mormyrids or wave-type gymnotiforms, electrocytes of pulse-type gymnotiforms are not homogenous along the long electric organ that traverses the fish body. Different parts of the electric organs of some gymnotoiforms are innervated differently or may have different cellular firing properties.

Gymnotiforms

In mormyrids, the electric organ is fairly small and located only in the [4][9][12]

Mormyrids

Electric organs are quite different between mormyrids and gymnotiforms and therefore will be presented separately:

Flow of EOD generation

[11][4] of EODs. These two characteristics (frequency and rhythm) of EODs are also referred to as SPI- sequence of pulse intervals. The command from medullary pacemaker is then passed on by spinal rhythm and frequency Discharge of an electric organ begins with central command from a medullary pacemaker that determines the

[10] Weakly electric fish generates Electric Organ Discharge (EOD) with specialized compartments called

Electric organ

Electric organs

Type Fire according to Method of encoding Found in
Time Coder Frequency of received EOD Fires action potential in a 1:1 ratio to the received EOD Both types of weakly electric fishes
Amplitude Coder Amplitude of received EOD Wave-type: continuously fire at a rate according to EOD amplitude/ Pulse-type: number of bursts in each spike is depend on the amplitude of EOD Both types of weakly electric fishes

Classification of tuberous organs

Tuberous organs, the type of receptive organ used for electrocommunication, can be divided into two types, depending on the way information is encoded: time coders and amplitude coders. There are multiple forms of tuberous organs in each time and amplitude coders, and all weakly electric fish species possess at least one form of the two coders. Time coder fires phase-locked action potential (meaning, the waveform of the action potential is always the same) at a fixed delay time after each outside transient is formed. Therefore, time coders neglect information about waveform and amplitude but focus on frequency of the signal and fire action potentials on a 1:1 basis to the outside transient. Amplitude coders, on the contrary, fire according to the EOD amplitude. While both wave-type and pulse-type fishes have amplitude coders, they fire in different ways: Receptors of wave-type fishes continuously fire at a rate according to their own EOD amplitude; on the other hand, receptors of pulse-type fishes fire burst of spikes to each EOD detected, and the number of spikes in each burst is correlated to the amplitude of the EOD. Tuberous electroreceptors show a V-shaped threshold tuning curve (similar to auditory system), which means that they are tuned to a particular frequency. This particular tuned-in frequency is usually closely matched to their own EOD frequency.[9]

Tuberous organs

Type Structure Function Sensitivity Where found
Ampullary Open pit/ Filled with mucous Electrolocation/ Locate preys 0.01 μV/cm in marine species, 0.01 mV/cm in freshwater; Sensitive to DC fields/ Low frequencies less than 50 Hz Sharks & Rays; Non-teleost fishes; Certain teleosts (mormyrids, certain notopterus, gymnotiforms, catfish); Amphibians (except frogs and toads)
Tuberous Covered by skin - loosely packed with epithelia cells Electrocommunication 0.1 mV to 10 mV/cm/ Tens of Hz to more than 1 kHz. Mormyrid fish; Gymnotiform fish

[8]

Classification of the two types of receptive organs

[7][6]

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