Electroreception and electrogenesis

discovered electroreception in 1950 through his observations of Gymnarchus niloticus. The electroreceptive function of these organs was established by R. W. Murray in 1960. In 1921, the German anatomist Viktor Franz described the knollenorgans (tuberous organs) in the skin of the elephantfishes, again without knowledge of their function as electroreceptors. In 1949, the Ukrainian-British zoologist Hans Lissmann noticed that the African knife fish (Gymnarchus niloticus) was able to swim backwards at the same speed and with the same dexterity around obstacles as when it swam forwards, avoiding collisions. He demonstrated in 1950 that the fish was producing a variable electric field, and that the fish reacted to any change in the electric field around it. == Electrolocation ==

Electroreceptive animals use the sense to locate objects around them. This is important in ecological niches where the animal cannot depend on vision: for example in caves, in murky water, and at night. Electrolocation can be passive, sensing electric fields such as those generated by the muscle movements of buried prey, or active, the electrogenic predator generating a weak electric field to allow it to distinguish between conducting and non-conducting objects in its vicinity. Passive electrolocation In passive electrolocation, the animal senses the weak bioelectric fields generated by other animals and uses it to locate them. These electric fields are generated by all animals due to the activity of their nerves and muscles. A second source of electric fields in fish is the ion pump associated with osmoregulation at the gill membrane. This field is modulated by the opening and closing of the mouth and gill slits. Passive electroreception usually relies upon ampullary receptors such as ampullae of Lorenzini which are sensitive to low frequency stimuli, below 50 Hz. These receptors have a jelly-filled canal leading from the sensory receptors to the skin surface. the animal senses its surrounding environment by generating weak electric fields (electrogenesis) and detecting distortions in these fields using electroreceptor organs. This electric field is generated by means of a specialised electric organ consisting of modified muscle or nerves. Animals that use active electroreception include the weakly electric fish, which either generate small electrical pulses (termed "pulse-type"), as in the Mormyridae, or produce a quasi-sinusoidal discharge from the electric organ (termed "wave-type"), as in the Gymnotidae. Many of these fish, such as Gymnarchus and Apteronotus, keep their body rather rigid, swimming forwards or backwards with equal facility by undulating fins that extend most of the length of their bodies. Swimming backwards may help them to search for and assess prey using electrosensory cues. Experiments by Lannoo and Lannoo in 1993 support Lissmann's proposal that this style of swimming with a straight back works effectively given the constraints of active electrolocation. Apteronotus can select and catch larger Daphnia water fleas among smaller ones, and they do not discriminate against artificially-darkened water fleas, in both cases with or without light. These fish create a potential usually smaller than one volt (1 V). Weakly electric fish can discriminate between objects with different resistance and capacitance values, which may help in identifying objects. Active electroreception typically has a range of about one body length, though objects with an electrical impedance similar to that of the surrounding water are nearly undetectable. Elephantfish emit short pulses to locate their prey. Capacitative and resistive objects affect the electric field differently, enabling the fish to locate objects of different types within a distance of about a body length. Resistive objects increase the amplitude of the pulse; capacitative objects introduce distortions. The Gymnotiformes, including the glass knifefish (Sternopygidae) and the electric eel (Gymnotidae), differ from the Mormyridae in emitting a continuous wave, approximating a sine wave, from their electric organ. As in the Mormyridae, the generated electric field enables them to discriminate accurately between capacitative and resistive objects. == Electrocommunication ==

s create electric fields powerful enough to stun prey using modified muscles. Some weakly electric knifefishes appear to mimic the electric eel's discharge patterns; this may be Batesian mimicry, to deceive predators that they are too dangerous to attack. Electric catfish frequently use their electric discharges to ward off other species from their shelter sites, whereas with their own species they have ritualized fights with open-mouth displays and sometimes bites, but rarely use electric organ discharges. When two glass knifefishes (Sternopygidae) come close together, both individuals shift their discharge frequencies in a jamming avoidance response. Brachyhypopomus males produce a continuous electric "hum" to attract females; this consumes 11–22% of their total energy budget, whereas female electrocommunication consumes only 3%. Large males produced signals of larger amplitude, and these are preferred by the females. The cost to males is reduced by a circadian rhythm, with more activity coinciding with night-time courtship and spawning, and less at other times. Fish that prey on electrolocating fish may "eavesdrop" on the discharges of their prey to detect them. The electroreceptive African sharptooth catfish (Clarias gariepinus) may hunt the weakly electric mormyrid, Marcusenius macrolepidotus in this way. This has driven the prey, in an evolutionary arms race, to develop more complex or higher frequency signals that are harder to detect. Some shark embryos and pups "freeze" when they detect the characteristic electric signal of their predators. == Evolution and taxonomic distribution ==

In vertebrates, passive electroreception is an ancestral trait, meaning that it was present in their last common ancestor. The ancestral mechanism is called ampullary electroreception, from the name of the receptive organs involved, ampullae of Lorenzini. These evolved from the mechanical sensors of the lateral line, and exist in cartilaginous fishes (sharks, rays, and chimaeras), lungfishes, bichirs, coelacanths, sturgeons, paddlefishes, aquatic salamanders, and caecilians. Ampullae of Lorenzini appear to have been lost early in the evolution of bony fishes and tetrapods, though the evidence for absence in many groups is incomplete and unsatisfactory. Passively-electrolocating groups, including those that move their heads to direct their electroreceptors, are shown without symbols. Non-electrolocating species are not shown. Fish able to deliver electric shocks are marked with a red lightning flash . Bony fish Two groups of teleost fishes are weakly electric and actively electroreceptive: the Neotropical knifefishes (Gymnotiformes) and the African elephantfishes (Notopteroidei), enabling them to navigate and find food in turbid water. Monotremes is a monotreme mammal that has secondarily acquired electroreception. Its receptors are arranged in stripes on its bill, giving it high sensitivity to the sides and below; it makes quick turns of its head as it swims to detect prey. The platypus localises its prey using almost 40,000 electroreceptors arranged in front-to-back stripes along the bill. The arrangement is highly directional, being most sensitive off to the sides and below. By making short quick head movements called saccades, platypuses accurately locate their prey. The platypus appears to use electroreception along with pressure sensors to determine the distance to prey from the delay between the arrival of electrical signals and pressure changes in water. Experiments have shown that echidnas can be trained to respond to weak electric fields in water and moist soil. The electric sense of the echidna is hypothesised to be an evolutionary remnant from a platypus-like ancestor. Bees Until recently, electroreception was known only in vertebrates. Recent research has shown that bees can detect the presence and pattern of a static charge on flowers. == See also ==