Batoids: Order Torpediniformes:
Electric Rays: 69 species
- rounded snout and pectoral disc
- large, kidney-shaped electrogenic organs (derived from branchial musculature) at base of pectoral fins
- body thick and flabby, with soft, loose skin
- eyes small to obsolete (four species are blind)
- 0 to 2 dorsal fins, depending upon species
- caudal fin well developed
- exclusively marine, inhabiting temperate to tropical zones of the Atlantic, Pacific, and Indian oceans
- 11 genera in 4 families
The torpediniform rays have fascinated naturalists since antiquity, being best known for their highly specialized electrogenic organs. These organs are generally kidney-shaped, composed of stacks of 500 to over 1,000 striated muscle plaques (modified from the branchial musculature). These plaques are all enervated on the same side, so that the electricity generated via muscular contraction is summed to produce an external shock. Voltage potentials recorded from different electric rays vary tremendously, having been measured at as little as 8 to 37 volts (narcinids) up to 220 volts (in the torpedinid Torpedo nobiliana). The result is a jolt of electricity ranging from moderately tingly to stunningly powerful. In some forms, the shock is directed upward — where it may serve to deter would-be predators — and in others downward — where it may be used to incapacitate prey.
Recent field research carried out by Chris Lowe, Dick Bray, and Don Nelson off southern California has revealed that the Pacific Torpedo Ray (Torpedo californica) generates two distinct types of electrical pulse (regular 'warning pulses' when pursued, and sharp, powerful blasts to stun prey) and uses several strategies to capture prey (including using bottom topography to sneak up on prey, cupping its pectoral fins and executing a neat barrel roll to manipulate incapacitated prey into the mouth). This may explain how these normally sluggish rays manage to capture surprisingly fast-swimming prey — there is a record of a four-foot (1.2-metre) Pacific Torpedo with a two-foot coho salmon (Oncorhynchus kisutch) in its stomach.
Electric rays were used by the ancient Greeks as a kind of anesthetic, the electricity supposedly numbing the pain of operations and childbirth — in fact, the Greek work for these rays is narke, from which we get our word 'narcotic'.
The torpedinoids are thought by some workers to be among the most primitive of extant batoids, but their highly specialized electrogenic organs — combined with the fact that the oldest known batoid fossils are very guitarfish-like — would seem to argue against this interpretation. The electric rays appear to fall into two suborders, the first composed of the torpedo rays (Torpedinidae) and the Short-tailed Electric Ray (Hypnidae — represented by a single species, Hypnos subnigrum, endemic to Australia), the other composed of the electric rays (Narcinidae) and the shortnose electric rays (Narkidae).