Itt írjon a(z) ElectricOrgansFish-ról/ről
Function
Electric fish are divided into two major groups based on the strength of their EOD. Strong electric fish’s EOD falls between 10 to 600V and is typically used for predatory and defensive purposes (James S. Albert and William G.R. Crampton, 2005). Weak electric fish discharge is less than 1V, these include fish from Gymnotiformes order (South African knifefish), Mormyridae & Gymnarchus species from the Osteoglossiformes order (Stoddard, P. K., 2010).
Weak Electric Fish:
Active Electrolocation:
Active electrolocation is the ability of weak electric fish to recognize distortions in there own EOD for navigational purposes by differentiating between the electrical properties of objects (which cause characteristic distortions of the EOD) from those of the neighbouring water. The recepted signal is translated to an electrical image on it is skin depicting information on the size, location and material of the object (A. A. Caputi, Budelli, Grant, & Bell, 1998) ; D.Babineau 2006). This ability is of great importance for nocturnal electric fish species (G. Von Der Emde 2007)
An object’s distance from the fish is determined by the area of skin stimulated during electroreception. Several criteria modify the amplitude of the EOD, as such most of these rely on more than one criteria to depict, distance is exceptionally calculated with soley amplitude(Gottwald, Bott, & von der Emde, 2017). The electroreceptors closest to the object translating the EOD, register an increase amplitude, these electroreceptors would be surrounded with other electroreceptors of decreased amplitude (A. A. Caputi et al., 1998). The mexican hat profile (fig. 3) characterizes this mechanism.. T G. Von Der Emde showed that object distance is independent of size, shape and material of the object (Gerhard von der Emde, 2004).There is no focusing mechanism for electric images, as such objects can be effectively recognized at maximum of 4 cm distance from the fish (von der Emde & Emde, 2010).
The EOD’s peak-to-peak amplitude drop in an electroreceptor compared to that of the electroreceptors around it also gives an indication of the object’s conductivity. The electrical resistance of the object is characterized by it’s impedance, a high impedance object (non-conductor) will translate into a lower EOD peak-to-peak amplitude and vice versa. recording at local electroreceptors. Since non-conductors will have a lower change in rmsLEOD their mexican hat profile character is horizontally inverted compared to a conductor’s profile as seen in Fig 3 (G.Von.Der Emde 1999) .
Capacitance detection is used to discriminate between animate and inanimate objects, both Mormyridae and Gymnotiformes are capable of this however the mechanism is not yet fully understood in Gymnotiformes. In Mormyridae, capacitance of an object (the ability to store electrical charge) is measured via the EOD amplitude & the EOD waveform and timing. The presence of membrane electrical gradients in living animals can be detected as capacitance values (G. Von Der Emde 1999).
Jamming Avoidance Response:
Two weakly electric fish in each other’s vicinity with similar EOD frequencies will alter their EOD frequency, the fish with the lower EOD will lower it is EOD and vice-versa (Akira Watanabe And, 1963). The resultant alteration is termed J.A.R. (Jamming Avoidance Response), it is due to the interaction of the 2 electric fields producing ‘beat’ patterns with a frequency called ‘beat rate’. The beat rate frequency is identical to the difference between the frequency of the 2 electrical fields, depending on the range of this frequency the electrosensory ability of the fish may weaken. Eigenmannia viscerens (ghost knifefish) will exhibit J.A.R. behaviour if the beat rate frequency is in the range of 3-8Hz(Tan, Nizar, Carrera-G, & Fortune, 2005). A beat rate frequency of 4Hz elicits J.A.R. behaviour in Gymnarchus niloticus (African knifefish) (Heiligenberg, 1975).
Electrocommunication:
Weak electric fish are known to communicate via discharging a modulated EOD targeted for reception by another fish. Communication related to EOD modulation is deciphered into courtship and agnostic information (Ho, Fernandes, Alves-Gomes, & Smith, 2010). The EOD of a particular species is typically uniform and characteristic when not being used for communication. Modulations of the EOD are categorized into chirps and gradual frequency rises (GFRs) on the basis of change in frequency over a period of time. Chirps are large frequency changes (50-600Hz) over a short duration (10-1000ms) while GFRs are characterized by a frequency change of <100Hz over a longer duration (10ms-60s)(Turner, Derylo, de Santana, Alves-Gomes, & Smith, 2007).
Fish belonging to the Mormyridae family send information pertaining to their species’s sex and size with their characteristic, invariable EOD. They are able to modify their inter-discharge-interval (IDI) to vary information sent regarding agnostic and courtship behaviour, this has been shown in recordings of Mormyrus rume discharge during different behavioural periods shown in Table 2. It is apparent that IDI dynamics can be used to determine behavioural state governed by the current activity the fish is partaking in. Determination of these behavioural states are not solely done by researchers but as is shown by synchronization behaviour, certain IDI patterns instigated an echo response between one another(Gebhardt, von der Emde, & Alt, 2012). IDIs may also be relevant in species discrimination, this has been shown in Petrocephalus bovei and Campylomormyrus rhynchophorus (Baier & Kramer, 2007).
A separate study carried out on Mormyrus rume proboscirostris found that there was a direct correlation between the hierarchy level and the peak-to-peak EOD amplitude, weight and size of the fish (Worm, Kirschbaum, & von der Emde, 2017). In Eigenmannia virescens and Apteronotus leptorhynchus (Gymnotiformes) hierarchy level has also been linked to EOD amplitude, frequency and waveform (Hagedorn & Heiligenberg, 1985).
Sexual dimorphism also plays a role in the modulation of EOD, in this regard a great degree of species difference can be noticed. Androgens play a role in the development of sexual dimorphism which are accredited to morphological changes such as thickening of the skin, which in turn alters the electrocyte function (Hopkins, 1988). Male Sternopygus macrurus (Hopkins, 1988) and Apteronotus albifrons discharge at a lower frequency and longer waveform (retaining the species characteristic wave) . Male Apteronotus leptorhynchus and Apteronotus rostratus discharge at higher frequencies than their female counterparts (Ho et al., 2010) . Eigenmannia virescens and Apteronotus leptorhynchus courtship involves the male continuously discharging marginal higher frequency chirps, in an attempt to stimulate spawning (release of unfertilized eggs) by the female (Hagedorn & Heiligenberg, 1985). Sternopygus macrurus EOD’s modulation related to courtship has been observed to follow a pattern of a rise in frequency followed by a decrease to resting discharge frequency. The resting discharge frequency is different in males and females, the oscillations in frequency play a role in attracting and recognising potential mates (Hopkins, 1974).
Strong Electric Fish
Fish in this section are capable of 10-600V (James S. Albert and William G.R. Crampton, 2005), this high voltage EOD is used to stun during predatory or defensive behaviour. EODs of these fish last 0.03s, they are fired in rapid successions of pulses termed trains. A shock is a succession of trains. The electromotive force of the EODs is species specific and varies with the condition of the fish (Tee-Van, 1948). Strong electric fish include species belonging to Torpediniformes, Paradoxoglans, Malapretus, Electrophorus and stargazer family. The order of Torpediniformes (Electric rays) is further subdivided into the families Torpedinidae, Hypnidae and Narcidinae(Nacinidae and Narkidae subfamilies).
Electric rays:
Hypnos monopterygius (Coffin ray) is the only member of the family Hypnidae, commonly thought of as a subfamily for Torpedinidae. Torpedo marmorata (Marbled torpedo ray) can produce an EOD of up to 200V with a frequency of up to 200Hz (Fishbase.org 5132). P.Belbenoit (Belbenoit, 1985) described 2 predatory behaviours in this species, jumping and creeping responses. Along with this 2 defensive behaviours were also described. All of these behaviours use EOD sets to stun and immobilize their target. The sequence of events during predatory jumping behaviour are the jumping response followed by an engulfing response and suction responses. During the jump response an immediate tail stroke lifts the ray, subsequent gliding. The first EOD pulses are discharged during the jump response. The aim of the fish is to entrap it is prey beneath it is disc, when accomplished the movement of the prey underneath the discs stimulates the engulfing response. The engulfing response can be split into 2 phases, the first phase involves stunning the prey with EODs while phase 2 involves bringing the prey closer to the mouth with a tailstroke. Torpedo nobiliana (Atlantic torpedo ray) can produce EODs within the range of 50-60V (Grundfest, 1960), however up to 220V ad 600HZ have been recorded (Sharktrust, n.d.). Hypnos monopterygius (Coffin ray) has been observed to deliver up to 50 successive shocks in 10 mins. Voltage and power of EOD progressively decline with increasing fatigue. Close relation to torpedos, Hypnidae & Torpedos have no labial cartilage connecting two jaws, other electric rays do. (Tee-Van, 1948).
Electric eel:
The Electrophorus electricus (electric eel) belongs to the order of Gymnotiformes, it is the only strong electric fish in this group. It possesses 3 electric organs (the main organ, Hunter’s organ and Sach’s organ) which are capable of producing 2 types of EOD, a high voltage one and a low voltage one. The main organ and ⅔ of Hunters organ are responsible for the high voltage discharge which can reach up to 600V (+100V per 30 cm body length). Sach’s organ along with ⅓ of Hunters organ produces low voltage discharge of 1-10V (reference 1 & 2). Low Voltage EOD is used for active electrolocation and electrocommunication. A continuous low voltage discharge is produced with varying frequency during rest (1Hz) and during swimming (10Hz). Low voltage EOD activity is higher during the night. Electric eels use their strong electric discharge to hunt by immobilizing prey, this is done by stimulating efferent neurons, subsequently eliciting muscle contraction in turn evokes tetanic contractions of the prey’s muscle(K. C. Catania, 2015a). K. Catania (K. Catania, 2014) recorded high frequency (400 Hz) high voltage EOD pulses just 10-15ms before the eel strikes, he noted it took 3-4ms for the prey to be immobilized.
Curling behaviour further strengthens the shock by intensifying the electric field, this is done by placing the tail (negative pole) behind the prey, since the head is the positive pole the electrical field is greatly intensified between these 2 extremities. Curling behaviour is used when dealing with large prey (which are more resistant) and when repositioning of captured fish (in mouth) for swallowing is necessary. Repositioning starts with the electric eel capturing its prey with a high voltage discharge and an initial strike, the prey is captured lengthwise, as such it cannot be swallowed (This can be seen in Fig. 4 A). The eel orients its head and tail into curling position and the fish is released simultaneous with high voltage discharges as can be seen in Fig. 4 B&C. A new opportunity for the eel to capture the fish presents itself (Fig. 4D). It can be commonly observed in young eels and in medium sized eels but rarely in large eels, since the high voltage discharge increases by 100V for 30cm body length it seems logical that juvenile and medium sized eels would struggle more when hunting hence profiting greatly from this mechanism (K. C. Catania, 2015a). It has also been shown that the high voltage discharge is also used to locate prey during striking, modification of strike direction (K. C. Catania, 2015b).
