Current Understanding of Shark Electroreception
The sensory environment of the marine world is vastly different from that above water, and as such, many marine animals have sensory capabilities not possessed by terrestrial animals. One of these senses is electroreception, the ability to detect and locate the source of electric currents. Because animal nervous systems send signals to muscles using electric currents, a moving animal creates small amounts of electricity. Since air is a poor conductor, terrestrial animals do not produce detectable electric fields; however, Kalmijn showed that in saltwater, which is a decent conductor, the electricity produced by animal muscle contractions creates small electrical currents in the water (Kalmijn 1974). This same study also showed that additional electric currents are created by the difference in voltage between the internal environment of an animal and the surrounding saltwater. Some marine animals, including sharks, have evolved the ability to detect these bioelectric fields.
The first evidence of electroreception in elasmobranchs (the group of marine animals including sharks, skates, and rays) came from a study by Dijkgraaf and Kalmijn in which it was shown that a blindfolded shark would flee from the approach of a steel wire, but would not react to the approach of a glass rod (Dijkgraaf and Kalmijn 1962). Since then, elasmobranch electroreception has become a well-established fact of shark biophysics, and it is well known that sharks use electroreception to locate prey. Recent research has focused on the exact techniques that sharks use to extrapolate the position of the source of an electric field from the field itself. Additionally, although the purpose of electroreception in sharks was long thought to be limited to locating prey, there is growing interest in the idea that sharks could use electric currents induced by the Earth’s magnetic field in order to sense this magnetic field. This capability would allow them to use electroreception not only to locate prey, but also for long-distance navigation.
It has become accepted fact in shark biology that electroreception’s primary purpose is prey detection and location. This purpose was demonstrated by Heyer in a study in which sharks were shown to attack an activated electrode in the proximity of a dead fish, rather than going for the dead fish itself (Heyer et al 1981). This behavior strongly suggests that to zero in on the precise location of prey, sharks use electroreception, and not odor of the fish or other stimuli.
However, how sharks pinpoint the source of an electric field is not entirely understood. Electric field lines emanate from places of positive charge and terminate in places of negative charge, following a curved path. Because the strength of the electric field at any given location depends on the magnitude of the charges producing it as well as the distance from the charges, the intensity of the field does not give definitive information about the location of its source. Kim showed, through the correlation of behavioral analysis and theoretical calculations, that the typical swaying motion of a shark’s head allows it to extrapolate the shape of the field and to align its forward direction with the flow of the current, which allows it to follow the current to the vicinity of the source (Kim 2006). At close range, the shark can use difference in intensity of field experienced across its snout (where its electroreceptors are located) to pinpoint the source. However, a study by Kaijura and Holland comparing the electroreception precision of scalloped hammerhead sharks and sandbar sharks found no difference between the two species (Kaijura and Holland 2002). This finding loosely suggests that wider spacing of electroreceptors does not significantly affect a shark’s electroreception precision, as otherwise the hammerheads would have had an advantage over their narrow-headed cousins. Further research is needed to determine the relationship between swaying motion and detection of intensity differences in sharks’ ability to pinpoint the source of an electric field.
More recently, it has been suggested that sharks, in addition to using electroreception for short-range prey location, could also use this sense in long-range navigation by detecting electric currents induced in a shark swimming through the Earth’s geomagnetic field. The sensory organs of electroreception are the ampullae of Lorenzini, gel-filled canals on the heads of elasmobranchs (Kalmijn 1971). The gel is conductive, with resistance approximately equivalent to that of seawater (Kalmijn 1974). Since a conductor moving through a magnetic field (as long as a component of its motion is perpendicular to the field) will experience an induced current, movement of the shark relative to the Earth’s magnetic field will induce a voltage difference across the ampullary canal. Paulin showed through calculations that, if a shark is swimming in such a way that its heading direction varies slightly around the average direction of its path, then the voltage difference induced across the ampullary canals will be strong enough for the shark to detect and theoretically use to navigate (Paulin 1995). This finding ties in to Kim’s 2006 study, which demonstrated that sharks do move with swaying motions; Kim suggested that this movement aided in electroreception, but it could serve a dual purpose by enabling magnetoreception as well.
In addition to the theoretical plausibility of shark magentoreception, further suggestion that magnetoreception might exist in sharks comes from the mounting evidence that many shark species complete long, directed migrations. A study of twenty white sharks tracked from 1995 to 2005 found that the sharks completed long distance seasonal migrations from the California coast to the Hawaiian Islands and another offshore feeding ground 2,500 kilometers west of the Baja Peninsula (Weng et al. 2007). How the sharks successfully navigate between such distant locations is currently unknown but researchers speculate that it could be explained by magnetoreception.
A study by Edrén and Gruber on young lemon sharks in the Bahamas involved displacing the sharks up to 16 kilometers from their observed home range; nearly all of the displaced sharks quickly returned to their home ranges (Edrén and Gruber 2005). Edrén and Gruber briefly speculate that the sharks could be employing infrasound detection to find their way back; however, magnetoreception seems to be another possible explanation. Most of the lemon sharks, once released, immediately began heading east before adjusting course to head back to their native waters. This behavior seems to suggest a compass-based sense as opposed to a sound-based one. Although not mentioned by the authors, this is also a potentially intriguing finding because the shark would induce a stronger voltage difference across its ampullary canals by swimming perpendicular to the geomagnetic field lines (which run north-south). This effect occurs because the magnitude of the induced voltage difference (in this case, the magnitude of the detectable current across the shark’s ampullary canals) is proportional to the component of the shark’s speed that is perpendicular to the magnetic field. In other words, if a shark is swimming due west or due east (perpendicular to the geomagnetic field lines), the induced electric field it could detect would be strong, whereas if it were swimming northeast, northwest, southwest, or southeast, the induced electric field would be weaker, and if it were swimming due north or due south (parallel to the geomagnetic field lines), there would be no induced electric field.
Theoretical modeling and correlative behavioral evidence strongly hint at the existence of shark magnetoreception, but there has yet to be a well-designed, controlled experiment to prove that sharks possess this sense. There has been at least one poorly designed experiment, though. Shark magnetoreception was mistakenly “confirmed” in a 2005 study in which it was shown that sharks could be conditioned to expect food when a magnetic field was generated in their tank (Meyer and Holland 2005). However, the authors of the study produced an artificial magnetic field through the use of a magnetic coil system. This device is basically a cube with wires coiled around it; the wires are arranged in such a way that when current is switched on and sent through them, a magnetic field is generated in the enclosed space. Since the charged particles in wire currents also produce electric fields, it is impossible to tell if the sharks were reacting to the sudden appearance of a magnetic field, or if they were reacting to the changing electric field created by live wires. This study is indicative of the lack of solid proof of elasmobranch magnetoreception, as well as the perils of biologists without adequate physics experience.
Electroreception in sharks is a fascinating and still-mysterious sensory system. Its role in prey detection has been well-estabished, but the exact mechanism by which sharks use electroreception to pinpoint prey location is less well understood. This ability to locate the source of a magnetic field may have to do with the shark’s ability to detect differences in intensity in the electric field across its snout, in addition to the swaying motion of the shark’s head as it swims. Models indicate that this same swaying motion could also play a role in sharks’ hypothesized ability to detect the geomagnetic field using magnetically induced electric currents across their electroreceptors. Navigational magnetoreception as a result of electroreception is theoretically plausible and matches behavioral evidence, but has yet to be proven. Future study in shark electroreception could focus on determining the exact mechanism by which a shark locates the source of an electric field; this knowledge could potentially be used to build precise electro-detectors, which could have uses as far-fetched as marine search-and-rescue or enemy boat detection. An understanding of shark magnetoreception is unlikely to yield many useful human inventions – we can already detect the geomagnetic field with a handheld compass – but could prove extremely important in shark conservation, by proving, for example, that large metal industrial structures underwater could disrupt shark migrations. Our understanding of shark electroreception has come a long way in the fifty years since its discovery, but there is still a copious amount of useful information to be gained from further research.
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