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Active Electrolocation
Active electrolocation in electric fish is similar to echolocation in bats.
In active electrolocation, fish discharge electric signals (EODs) into the
surrounding
water. Any object within the fish’s electrical field will distort the signal.
Depiction of a fish using electrolocation. The fish creats an electric field by discharging an EOD pulse. The object (shown here as a red circle) distorts the field slightly. THe fish detects the distortion in the elcetric field and determines the location, shape, size, and electrical properties of the object. Image used with permission of Masashi Kawasaki |
The fish detects these disruptions using electroreceptors on its skin surface.
Areas on the fish’s body where electroreceptors sense a distortion in
the EOD form an “electric image” (von der Emde 1999) that the fish
can interpret to gain information about the object.
Depiction of an object creating an "electric image" on the body surface of a weakly electric fish. Image adapted from Gerhard von der Emde (von der Emde 1999) |
Electric fish can gather a variety of information using active electrolocation. They can determine the distance of an object from themselves, its size shape, and its electrical conductivity. They do this by interpreting various aspects of the electric image that the object projects on them: size, amplitude, location on the body, and intensity. The Elephant Nose Fish Gnathonemus petersii can even distinguish between living and non-living material (von der Emde 1999).
Examples of electrolocation:
How the weakly electric Elephant Nose Fish (Gnathonemus petersii) sense objects in their environment using electricity.
Electrical Properties: An object may conduct electricity
better than the surrounding water, or not as well, or not at all. If an object
conducts better than the water, the
electric signals a fish discharges will be attracted to the conductor, and
bend toward it. When the fish detects the EOD, it will have an area of concentrated
current usually surrounded by an area of low current density: the "Mexican
hat profile" (von der Emde 1999). Less conductive or nonconductive objects
produce the opposite effect (von der Emde 1999).
| Depiction of electric field disruption by conducting or non-conducting objects. Notice how the conductor bends Electric signals toward it, causing a concentration of signal to be received by the receptors on the fish's body. In contrast, the non-conductor causes an area of less signal reception on the body by blocking teh electric signal. Image adapted from Gerhard von der Emde (von der Emde 1999) |
Object Location and Shape: A fish using electrolocation deduces
an objects shape and location relative to its own body simply by the shape
and location of the electric image the
object makes. Where the electric image falls depends on where the impeding
object lies, so analysis of the electric image location on the body will tell
where the object is in relation to the body. Similarly, the shape of the electric
image reflects the shape of the object. (von der Emde 1999)
Distance: The distance of an object from the fish is not as simple to determine as its
location or shape, because the electric image does not directly reflect distance
the same way is does shape and location. A large circular electric image could
be caused by a large sphere close to the body, or by a small sphere further
away.
| Depiction of the two different electric images produced by similar sized objects at varying distances. Notice how the farther away object creats a lighter, fuzzier, and larger image than the closer object. Image adapted from Gerhard von der Emde (von der Emde 1999) |
Gerhard von der Emde offers a comparison explaining how he believes Elephant Nose fish determine distance electrically: “Each object casts and image like an electrical shadow” on the body surface of the fish, which becomes larger when the object is further away. In addition, the edges of the shadow become “fuzzier” and the maximum “darkness” of the shadow decreases” (von der Emde 1999). The fish determines distance by comparing the images fuzziness to its darkness. Other researchers have different hypotheses (Budelli and Caputi 2000), and there is some evidence that distance determination varies between species (von der Emde 1999).
NOTE: In passive electrolocation, the electric fish simple detects electric fields of other animals. In active electrolocation, the fish is both creating and detecting the electric field, and located objects by analyzing disruptions in the field.
This website was created as a part of a class project in the Animal Physiology Class at Davidson College.