Electric Rays: a Shocking Use of

Muscle Power

The electric rays (order Torpediniformes) 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 that have been modified from the gill 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. These rays produce regular 'warning pulses' when pursued and sharp, powerful blasts to stun prey. In addition, the Pacific Torpedo employs 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. This shocking ability may also explain why although the non-electrogenic Bat Ray (Myliobatis californica) often turns up among White Shark (Carcharodon carcharias) stomach contents the Pacific Torpedo has yet to meet such a toothy end.

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 word for these rays is narke, from which we get our word 'narcotic'.

 

ReefQuest Centre for Shark Research
Text and illustrations R. Aidan Martin
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