electric ray

Electric rays Fossil range: Eocene–Recent PreЄ Є O S D C P T J K Pg N [1]
Blackspotted torpedo, Torpedo fuscomaculata
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Chondrichthyes
Subclass:	Elasmobranchii
Superorder:	Batoidea
Order:	Torpediniformes
Families
Narcinidae Narkidae Torpedinidae Hypnidae

The electric rays are a group of rays, flattened cartilaginous fish with enlarged pectoral fins, that comprise the order Torpediniformes. They are known for being capable of producing an electric discharge, ranging from as little as 8 volts up to 220 volts depending on species, used to stun prey and for defense.^[2] There are 69 species in four families.

Perhaps the most known members are those of the genus Torpedo, also called crampfish and numbfish, after which the device called a torpedo is named. The name comes from the Latin "torpere", to be stiffened or paralyzed, referring to the effect on someone who handles or steps on a living electric ray.

Torpedo rays are excellent swimmers. Their round disk shaped bodies allow them to remain suspended in the water or roam for food with minimal swimming effort.

Description

Electric rays have a rounded pectoral disc with two moderately large rounded-angular (not pointed or hooked) dorsal fins (reduced in some narkids), and a stout, muscular tail with a well-developed caudal fin. The body is thick and flabby, with soft, loose skin devoid of dermal denticles and thorns. A pair of kidney-shaped electric organs are found at the base of the pectoral fins. The snout is broad, large in the Narcinidae but reduced in all other families. The mouth, nostrils, and five pairs of gill slits are located underneath the disc.^[2][3]

They are bottom dwelling fish, found from shallow coastal waters down to at least 1,000 metres (3,300 ft) depth. They are sluggish and slow moving, propelling themselves along with their tails, rather than using their disc-shaped bodies, as other rays do. They feed on invertebrates and small fish. They lie in wait for prey below the sand or other substrate, using their electricity to stun and capture it.^[4]

Relationship to humans

The electrogenic properties of electric rays have been known since antiquity. The ancient Greeks used electric rays to numb the pain of childbirth and operations.^[2] Scribonius Largus, a Roman physician, recorded the use of torpedo fish for treatment of headaches and gout in his Compositiones Medicae of 46 AD.^[5]

Bioelectricity

The electric ray is known to be the most electro-sensitive of all zoological organisms and their eyes are situated on the top of their head, providing for poor vision which must be balanced by use of other senses, including the detection of electricity. Many species of rays and skates outside the family of the electric ray have electric organs located in the tail, however, the electric ray possesses two large electric organs on each side of its head, where current passes from the lower to the upper surface of the body. The organs are governed by four central nerves from each side of the electric lobe, or specialized brain lobe, which is of a different colour than the rest of the brain. The main nerves branch continuously then attach to the lower side of each plate in the batteries, which are composed of hexagonal columns, in honeycomb formation: each column consists of one hundred and forty to half a million gelatinous plates. In marine fish, these batteries are connected as a parallel circuit where freshwater batteries are found in series, transmitting discharges of higher voltage, as fresh water cannot conduct electricity as well as salt water. It is with such a battery that an average electric ray may electrocute a large fish with a current until 30 amps with a voltage from 50 up to 200 Volts, which gives the effect of a hair dryer being dropped into a tub of water.

Systematics

The four families of the Torpediniformes are typically grouped into two superfamilies: Hypnidae and Torpedinidae in the Torpedinoidea, and Narkidae and Narcinidae in the Narcinoidea. The Torpedinoidea specialize on large prey, which are stunned using their electric organs and swallowed whole, while the Narcinoidea specialize on small prey on or in the bottom substrate. Both groups use electricity for defense, but it is unclear whether the Narcinoidea use electricity in feeding.^[6]

Classification

The order contains over 60 species, grouped into twelve genera and two to four families:^[1]

Order Torpediniformes

• Family Hypnidae (coffin rays) - sometimes placed in Torpedinidae
  • Genus Hypnos
• Family Narcinidae (numbfishes)
  • Genus Benthobatis
  • Genus Crassinarke
  • Genus Diplobatis
  • Genus Discopyge
  • Genus Narcine
• Family Narkidae (sleeper rays) - sometimes placed in Narcinidae
  • Genus Electrolux
  • Genus Heteronarce
  • Genus Narke
  • Genus Temera
  • Genus Typhlonarke
• Family Torpedinidae (electric rays)
  • Genus Torpedo

References

See also

• Electric fish