question: How do sharks detect electrical signals fish emmit when they move?
comment: They have special organs called the ampullae of Lorenzini. They're basically clusters of pores with electroreceptors. These organ cells are specialized to have high concentrations of voltage dependent ion channels. The cells are also connected very tightly to one another by gap junctions. There are some other anatomic features that also help direct the current inwards and prevent current leakage. What the sharks actually sense is the difference in electric potential between the skin and the base of the pore. Fish swim around using their muscles, which produce electric fields by contracting. Salt water is a good conductor, and the movement of the conductor in the presence of Earth's magnetic field produces an electric field. Sharks sense this and their ampullae of Lorenzini are excited.
passages: The electric field sensors of sharks are called the ampullae of Lorenzini. They consist of electroreceptor cells connected to the seawater by pores on their snouts and other zones of the head. A problem with the early submarine telegraph cables was the damage caused by sharks who sensed the electric fields produced by these cables. It is possible that sharks may use Earth's magnetic field to navigate the oceans using this sense.
  
  Section::::Examples.:Bony fish.
   Electroreception, or electroception, is the ability to detect electric fields or currents. Some fish, such as catfish and sharks, have organs that detect weak electric potentials on the order of millivolts. Other fish, like the South American electric fishes Gymnotiformes, can produce weak electric currents, which they use in navigation and social communication. In sharks, the ampullae of Lorenzini are electroreceptor organs. They number in the hundreds to thousands. Sharks use the ampullae of Lorenzini to detect the electromagnetic fields that all living things produce. This helps sharks (particularly the hammerhead shark) find prey. The shark has the greatest electrical sensitivity of any animal. Sharks find prey hidden in sand by detecting the electric fields they produce. Ocean currents moving in the magnetic field of the Earth also generate electric fields that sharks can use for orientation and possibly navigation.
   Some fish, such as catfish and sharks, have organs that detect weak electric currents on the order of millivolt. Other fish, like the South American electric fishes Gymnotiformes, can produce weak electric currents, which they use in navigation and social communication. In sharks, the ampullae of Lorenzini are electroreceptor organs. They number in the hundreds to thousands. Sharks use the ampullae of Lorenzini to detect the electromagnetic fields that all living things produce. This helps sharks (particularly the hammerhead shark) find prey. The shark has the greatest electrical sensitivity of any animal. Sharks find prey hidden in sand by detecting the electric fields they produce. Ocean currents moving in the magnetic field of the Earth also generate electric fields that sharks can use for orientation and possibly navigation.
   Sharks and rays (members of the subclass "Elasmobranchii"), such as the lemon shark, rely heavily on electrolocation in the final stages of their attacks, as can be demonstrated by the robust feeding response elicited by electric fields similar to those of their prey. Sharks are the most electrically sensitive animals known, responding to DC fields as low as 5 nV/cm.
   Section::::Senses.:Electroreception.
  
  The ampullae of Lorenzini are the electroreceptor organs. They number in the hundreds to thousands. Sharks use the ampullae of Lorenzini to detect the electromagnetic fields that all living things produce. This helps sharks (particularly the hammerhead shark) find prey. The shark has the greatest electrical sensitivity of any animal. Sharks find prey hidden in sand by detecting the electric fields they produce. Ocean currents moving in the magnetic field of the Earth also generate electric fields that sharks can use for orientation and possibly navigation.
  
  Section::::Senses.:Lateral line.
   All chondrichthyans have highly sensitive electrical receptors called the "ampullae of Lorenzini" located in their snouts. These tiny gel-filled sacs sense electric current from prey at very close distances, typically less than one meter. They use these short-range sensors when feeding or searching for food. They do not use electrical receptors to track animate objects over long distances; other senses such as audition and olfaction are the primary drivers.
   One additional behavior observed in sharks is their orientation to the Earth's magnetic field. The Earth's magnetic field acts as a uniform field, B. When the shark swim through the Earth's magnetic field with its own velocity v, the shark induces its own electric current. Sharks swimming though the Earth's magnetic field create a voltage gradient that could act as a physical basis for compass orientation.
  
  Section::::Behavior.:Weakly electric fish.
   Behavioral studies have also provided evidence that sharks can detect changes in the geomagnetic field. In one experiment, sandbar sharks and scalloped hammerhead sharks were conditioned to associate a food reward with an artificial magnetic field. When the food reward was removed, the sharks continued to show a marked difference in behavior when the magnetic field was turned on as compared to when it was off.
  
  Section::::Temperature sensing ability.
   Section::::Examples.:Dolphins.
  
  Dolphins have evolved electroreception in structures different from those of fish, amphibians and monotremes. The hairless vibrissal crypts on the rostrum of the Guiana dolphin ("Sotalia guianensis"), originally associated with mammalian whiskers, are capable of electroreception as low as 4.8 μV/cm, sufficient to detect small fish. This is comparable to the sensitivity of electroreceptors in the platypus. To date (June 2013), these cells have been described from only a single dolphin specimen.
  
  Section::::Examples.:Bees.
   Electroreception (or electroception) is the ability to detect electric fields. Several species of fish, sharks, and rays have the capacity to sense changes in electric fields in their immediate vicinity. For cartilaginous fish this occurs through a specialized organ called the Ampullae of Lorenzini. Some fish passively sense changing nearby electric fields; some generate their own weak electric fields, and sense the pattern of field potentials over their body surface; and some use these electric field generating and sensing capacities for social communication. The mechanisms by which electroceptive fish construct a spatial representation from very small differences in field potentials involve comparisons of spike latencies from different parts of the fish's body.
   The paddlefish ("Polyodon spathula") hunts plankton using thousands of tiny passive electroreceptors located on its extended snout, or rostrum. The paddlefish is able to detect electric fields that oscillate at 0.5–20 Hz, and large groups of plankton generate this type of signal. "See: Electroreceptors in paddlefish"
  
  Electric fishes use an active sensory system to probe the environment and create active electrodynamic imaging.
   When a shark swims through the Earth's magnetic field, electromagnetic induction – a phenomenon which generates voltage in an electrical conductor moving through a magnetic field – creates an electric field around the shark. Minute differences in the Earth's magnetic field at different locations result in minute differences in the induced electric field which may be detected by the shark's sensitive electroreceptors, especially as the head region moves back and forth during swimming (Lohmann and Johnsen 2000).
  
  Section::::Recent findings.
   Craig O’Connell, previously a partner with Sharkdefense, completed his Master’s Thesis at Coastal Carolina University with regards to magnetic shark repellents as selective shark bycatch reduction devices. His studies involved Southern Stingrays ("D. americana"), adults nurse sharks ("G. cirratum"), blacktip sharks ("C. limbatus"), adult lemon sharks ("N. brevirostris"), and Atlantic Sharpnose sharks ("R. terranovae") and showed a reduction of shark catch on demersal longlines and rod-and-reel studies. Craig is currently a PhD student at U Mass Dartmouth, continuing shark magnetoreception studies.
   Section::::Surface wave detection in "Aplocheilus lineatus".:Directional orientation.
  
  When stimuli are presented within a detectable range, orientations toward the stimulus by "A lineatus" are extremely precise: the response is almost perfect with angles up to 150° in either direction from forwards, and precision falls off but not much when waves come from the rear of the fish.
   The only orders of mammals that are known to demonstrate electroception are the dolphin and monotreme orders. Among these mammals, the platypus has the most acute sense of electroception.
  
  A dolphin can detect electric fields in water using electroreceptors in vibrissal crypts arrayed in pairs on its snout and which evolved from whisker motion sensors. These electroreceptors can detect electric fields as weak as 4.6 microvolts per centimeter, such as those generated by contracting muscles and pumping gills of potential prey. This permits the dolphin to locate prey from the seafloor where sediment limits visibility and echolocation.
   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. In vertebrates, electroreception is an ancestral trait, that is to say that it was present in the last common ancestor of all vertebrates.. This form of ancestral electroreception is called ampullary electroreception, with the receptive organs themselves called Ampullae of Lorenzini. Not all vertebrates that possess electroreception have Ampullae of Lorenzini. Ampullae of Lorenzini exist in cartilaginous fishes (sharks, rays, chimaeras), lungfishes, bichirs, coelacanths, sturgeons, paddlefish, aquatic salamanders, and caecilians. Other vertebrates that have electroreception such as catfish, gymnotiformes, Mormyridiformes, monotremes, and at least one species of cetacean all have different secondarily derived forms of electroreception. Ampullary electroreception is "passive", and is used predominantly in predation. Two groups of teleost fishes are weakly electric and engage in "active" electroreception: the Neotropical knifefishes (Gymnotiformes) and the African elephantfishes (Notopteroidei). A rare terrestrial exception is the Western long-beaked echidna which has about 2,000 electroreceptors on its bill, compared to 40,000 for its semi-aquatic monotreme relative, the duck-billed platypus.
   Weakly electric fishes provide an example of electrocommunication, together with electrolocation. These fish use an electric organ to generate an electric field, which is detected by electroreceptors. Differences in the waveform and frequency of changes in the field convey information on species, sex, and identity. These electric signals can be generated in response to hormones, circadian rhythms, and interactions with other fish. Some predators, such as sharks and rays, are able to eavesdrop on these electrogenic fish through passive electroreception.
  
  Section::::Modes.:Touch.
  
  Touch is a key factor in many social interactions. Here are some examples:
   Section::::Behavior.
  
  Section::::Behavior.:Sharks.
   Spiny dogfish sharks have been remotely controlled by implanting electrodes deep in the shark's brain to a remote control device outside the tank. When an electric current is passed through the wire, it stimulates the shark's sense of smell and the animal turns, just as it would move toward blood in the ocean. Stronger electrical signals—mimicking stronger smells—cause the shark to turn more sharply. One study is funded by a $600,000 grant from Defense Advanced Research Projects Agency (DARPA). It has been suggested that such sharks could search hostile waters with sensors that detect explosives, or cameras that record intelligence photographs. Outside the military, similar sensors could detect oil spills or gather data on the behaviour of sharks in their natural habitat. Scientists working with remote control sharks admit they are not sure exactly which neurons they are stimulating, and therefore, they can't always control the shark's direction reliably. The sharks only respond after some training, and some sharks don't respond at all. The research has prompted protests from bloggers who allude to remote controlled humans or horror films featuring maniacal cyborg sharks on a feeding frenzy.
   Passive electroreception is carried out solely by ampullary electroreceptors in fish. It is tuned to low frequency signals (below one up to tens of Hertz).
  
  Fish use passive electroreception to supplement or replace their other senses when detecting prey and predators. In sharks, sensing an electric dipole alone is sufficient to cause them to try to eat it.
  
  Section::::Electrocommunication.
  
  Weakly electric fish can also communicate by modulating the electrical waveform they generate, an ability known as electrocommunication.
  
  They may use this for mate attraction and territorial displays. Some species of catfish use their electric discharges only in agonistic displays.
   Section::::Sharks.
   BULLET::::- The ampullae of Lorenzini allow sharks to sense electrical discharges.
  
  BULLET::::- Electric fish are able to produce electric fields by modified muscles in their body.
  
  Section::::Sensory systems.:Pain.
  
  Experiments done by William Tavolga provide evidence that fish have pain and fear responses. For instance, in Tavolga's experiments, toadfish grunted when electrically shocked and over time they came to grunt at the mere sight of an electrode.
   Electrolocating fish use this ability to detect prey, locate other fish, avoid predators, and navigate by the Earth's magnetic field. Electroreceptors probably evolved once or twice early in vertebrate evolution, but the sense was apparently lost in amniotes, and in a large number of the Actinopterygii (ray finned fishes) only to reappear independently in two teleost clades. In fish, the ampullary receptor is a specialized receptor that it uses to sense these electric fields and allows the fish to follow electric field lines to their source. Sharks primarily use specialized receptors, called Ampullae of Lorenzini, to detect their prey's low frequency DC fields and may also use their receptors in navigation by the Earth's magnetic field. Weakly electric fish use their ampullary receptors and tuberous receptors to detect the weakly electric fields produced by other fish, as well as for possible predator avoidance. Passive electrolocation contrasts with active electrolocation, in which the animal emits its own weak self generated electric field and detects nearby objects by detecting the distortion of its produced electric field. In active electrolocation the animal senses its own electromotor discharge or reafference instead of some externally generated electric field or discharge
   Section::::Signals.:EOD frequency.
  
  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.
  
  Section::::Signals.:EOD waveform.
   Any moving conductor, such as sea water, induces an electric field when a magnetic field such as the Earth's is present. The electric fields induced in oceanic currents by the Earth's magnetic field are of the same order of magnitude as the electric fields that sharks and rays are capable of sensing. This could mean that sharks and rays can orient to the electric fields of oceanic currents, and use other sources of electric fields in the ocean for local orientation. Additionally, the electric field they induce in their bodies when swimming in the magnetic field of the Earth may enable them to sense their magnetic heading.
  