ADW: Electrophorus electricus: INFORMATION

Geographic Range

Electrophorus electricus, more commonly known as the electric eel, occupies the northeastern portions of South America. This includes the Guyanas and Orinoco Rivers as well as the middle and lower Amazon basin.

(Berra, 2001)

• Biogeographic Regions
• neotropical
  • native

Habitat

E. electricus dwell mainly on the muddy bottoms of rivers and occasionally swamps, prefering deeply shaded areas. However, they must surface rather frequently because they are air breathers, gaining up to 80 percent of their oxygen through this method. This feature allows E. electricus to survive comfortably in water that has a very low concentration of dissolved oxygen.

(Riis-Johannessen, 2001)

• Habitat Regions
• tropical
• freshwater

• Aquatic Biomes

• rivers and streams

• Wetlands
• swamp

Physical Description

Electric eels are not really eels, they are actually ostariophysians, but have a strong physical resemblance to true eels. The body is long and snake-like, lacking caudal, dorsal and pelvic fins. Body length can be as long as 2.5 m. They also have an extremely elongated anal fin, which is used as a means of locomotion. It is cylindrical in shape with a slightly flattened head and large mouth. The vital organs to the fish are all in the anterior portion of the body and only take up about 20 percent of the fish. The posterior portion of the body contains the electrical organs. They do have gills, though it is not their primary source of oxygen intake. Electric eels are obligatory air breathers. They receive almost 80 percent of their oxygen through their highly vascularized mouth. A thick, slimy skin covers the entire body of E. electricus. The skin is used as a protective layer, often from their own electrical current that is produced. Electric eels range from gray to brownish/black in color with some yellowish coloration on the anterior ventral portion of the body.

(Riis- Johannessen, 2001; Berra, 2001; Val and de Almeida-Val, 1995)

• Other Physical Features
• bilateral symmetry

• Range mass

  20 (high) kg

  44.05 (high) lb

• Range length

  2.5 (high) m

  8.20 (high) ft

Development

The development of electric organs in E. electricus happens very soon after birth. There has been evidence that fish as small as 15 mm have begun electrical organ development. This initial growthof a weak electrical organ allows for orientation. Strong electric organs do not develop until the fish is approximately 40 mm. Observations have shown small juveniles surrounding the head of the parent; this is most likely before the young develop their own orientation organs.

(Brown, 1957; Moller, 1995; Berra, 2001)

Reproduction

Electric eels reproduce during the dry season. The eggs are deposited in a well-hidden nest made of saliva, built by the male. In field observations, an average of 1200 embryos were hatched. Fecundity counts have been documented as high as 17,000 eggs. The electric eel is thought to be a fractional spawner.

(Moller, 1995)

• Key Reproductive Features
• iteroparous
• seasonal breeding
• sexual
• fertilization
  • external
• oviparous

• Breeding season

  dry season

• Average number of offspring

  1200

Males will defend their nest and the fry vigorously.

• Parental Investment
• male parental care

Lifespan/Longevity

The lifespan of electric eels in the wild is unknown. In captivity males live between 10 and 15 years, while females usually survive between 12 and 22 years.

(Cormier, 2000)

• Typical lifespan

  Status: captivity

  10 to 22 years

Behavior

Although electric eels have the potential to be fairly aggressive animals, they are not. They really only use their strong electric organ discharges for predation and defensive purposes. Weak electric organ discharges are used for electrolocation as well as identification of foreign objects. This is especially important because of their poor eyesight. They are nocturnal animals that live in muddy dark waters, so they must rely on electricity for sensing. Electric eels tend to stay relatively rigid in order to fully use their electrical capabilities. They have a positive charge near the head, while the tail end is negative. When scanning their environment with electric current, they begin at the tail and finish with the head. In order to do this the fish must be able to swim backward. The polarity of the fish itself helps to create this electric field that dictates much of the animal’s behavior.

The use of electrical organs has been studied in great detail. From many different experimental situations it is clear that the eels are able to detect an electrical circuit in the water and differentiate between a closed and an open system. It has also been determined that electric eels are very sensitive to the changes in water conductivity. It is the electroreception system that allows them to navigate through the muddy dark waters they dwell in.

(Moller, 1995; Berra, 2001; Riis-Johannessen, 2001)

• Key Behaviors
• natatorial
• nocturnal
• motile
• solitary

Communication and Perception

The Sachs organ is the primary source of communication among E. electricus. This organ transmits a weak signal, only about 10V in amplitude. These signals are used in communication as well as orientation, useful not only to find prey but also thought to play an important role in finding and choosing a mate.

Scientists have been able to determine through experimental information that E. electricus has a well developed sense of sound. They have a Weberian apparatus that connects the ear to the swim bladder which greatly enhances their hearing capability.

(Berra, 2001; Brown, 1957; Moller, 1995)

• Communication Channels
• acoustic
• electric

• Perception Channels
• tactile
• chemical

Food Habits

To find prey E. electricus uses its weak electric organ, also known as the Sachs organ. This transmits a weak pulsating signal, thought to be used for locating and directional purposes. Once prey is found the electric eel will use a much larger electrical current to stun the fish. This is done with the two larger electric organs, the Main and Hunters organs. The shock itself does not kill the prey, but it is usually sufficiently stunned. Since eels lack maxilla teeth, it is difficult to eat a fish that is thrashing about. However, since the prey is fairly stationary eels are able to open their mouths to create a suction, which allows them to eat the prey with ease. Most adult electric eels will feed on smaller fish, while juveniles will prey mainly on smaller invertebrates.

(Berra, 2001; Riis-Johannessen, 2001)

• Primary Diet
• carnivore
  • piscivore

  • eats non-insect arthropods

Predation

Predation of electric eels is usually prevented by their electric shocking capabilities. They can produce voltage as high as 650 volts. Although this shock is rarely deadly it is enough to deter most predators. These defensive electrical pulses are created by two organs in E. electricus, the Main and Hunters organs. It is the strength of these two organs and the electric eels pulsating electric current that classifies it as a strongly electric fish.

(Berra, 2001; Brown, 1957)

Economic Importance for Humans: Positive

E. electricus have very little economic value to humans. Occasionally they are eaten by locals of the Amazon area; however they are commonly avoided due to the electrical shocks that can be given out up to eight hours after death. Although there is no commercial value, the electric eel has been a constant source of study for many years. The scientific community is very interested in studying the electrical capabilities of these fish. Of electric fish, E. electricus is the best documented species.

(Moller, 1995)

• Positive Impacts

• research and education

Economic Importance for Humans: Negative

Electric eels can be very dangerous to humans because of their strong electric capabilities. They are able to produce enough voltage to severely injure humans and other animals.

(Cormier, 2000)

Conservation Status

• IUCN Red List

  Least Concern

  More information

Contributors

William Fink (editor), University of Michigan-Ann Arbor.

Traci Valasco (author), University of Michigan-Ann Arbor.

Glossary

Neotropical

living in the southern part of the New World. In other words, Central and South America.

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

electric

uses electric signals to communicate

external fertilization

fertilization takes place outside the female’s body

fertilization

union of egg and spermatozoan

freshwater

mainly lives in water that is not salty.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

male parental care

parental care is carried out by males

motile

having the capacity to move from one place to another.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother’s body.

piscivore

an animal that mainly eats fish

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

swamp

a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.

tactile

uses touch to communicate

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

References

Berra, T. 2001. Freshwater Fish Distribution. San Diego: Academic Press.

Cormier, L. 2000. “More Information about Electric Eels” (On-line).

Accessed
November 10, 2002
at http://whozoo.org/Intro2000/tashcorm/tempagetwo.htm.

Keynes, R. 1957. Electric Organs. Pp. 323-343 in M Brown, ed. The Physiology of Fishes, Volume II. New York: Academic Press, Inc..

Moller, P. 1995. Electric Fishes: History and Behavior. New York: Chapman & Hall.

Riis-Johannessen, T. 2001. “The Electric Eel” (On-line).

Accessed
Oct. 30, 2002
at http://www.chm.bris.ac.uk/webprojects2001/riis/electriceels.htm.

Val, A., M. de Almeida – Val. 1995. Fishes of the Amazon and their Environment: Physiological and Biochemical Aspect. New York: Springer.