An electric eel (Electrophorus electricus) has current-producing organs made up of electric plates (modified muscle cells) on both sides of its vertebral column running almost its entire body length. The charge—350 volts on average, but as great as 550 volts—is released by the central nervous system. The shock consists of four to eight separate charges, each of which lasts only two- to three-thousandths of a second. These shocks, used as a defense mechanism, can be repeated up to 150 times per hour without any visible fatigue to the eel. The most powerful electric eel, found in the rivers of Brazil, Colombia, Venezuela, and Peru, produces a shock of 400 to 650 volts.

Chondrichthyes are fish that have a cartilaginous skeleton rather than a bony skeleton and include sharks, skates, and rays. Of those animals, one of the most well known, and often feared, is the shark. The 375 species of sharks currently known about range in length from 6 inches (15 centimeters) to 49 feet (15 meters)—only around a dozen are considered to be dangerous to humans. The relatively rare great white shark (Carcharodan carcharias) is the largest predatory fish, with some specimens reaching 20 feet, 4 inches (6.2 meters) long and weighing 5,000 pounds (2,270 kilograms). The largest shark is the whale shark (Rhincodon typus) that measures around 60 feet (20 meters) long; the smallest is the Caribbean Ocean’s deepwater dogfish shark (Etmopterus perryi) that measures 8 inches (20 centimeters) long. The fastest shark is thought to be the shortfin mako (Isurus oxyrinchus), reported to swim 20 miles (32 kilometers) per hour.