The cookiecutter shark is either of two sharks in the genus Isistius, which belongs to the dogfish shark family, Squalidae. The dogfish sharks are part of the order Squaliformes, which also includes the bramble sharks and rough sharks. I. brasiliensis, commonly called the cookiecutter shark and sometimes referred to as the cigar shark, has been studied comprehensively. The biology of I. plutodus, known as the largetooth cookiecutter shark, is not well known.
The bodies of both sharks are small and cigar-shaped. Body coloration is gray or grayish brown, though the fins have lighter edges. The cookiecutter shark has a broad, dark band around its neck, but the largetooth cookiecutter lacks this band. Both sharks have a short, bulbous snout: that of the largetooth cookiecutter shark is somewhat shorter. Both sharks have two dorsal, or top, fins, set far back on the body, but no anal fin. Roughly the same size, the dorsal fins lack the rigid frontal spine found on certain other sharks. The tail fin of the cookiecutter shark is much larger than that of the largetooth cookiecutter.
The bladelike lower teeth of both sharks are notably large, with a single cusp, or point. The lower teeth of the largetooth cookiecutter are the largest of all sharks in proportion to body size. The upper teeth are small and shaped like narrow, slightly curved spikes. On the body surfaces of both sharks are flat, block-shaped teethlike structures called dermal denticles.
The cookiecutter shark has organs called photophores on its lower body that glow brightly in the dark. Scientists believe that the greenish glow, which resembles that of a firefly, might be used to locate other sharks of the same species or to attract prey. Both cookiecutter sharks have large, fleshy lips, which they use to attach themselves to large prey before cutting out chunks of flesh with their powerful jaws and large teeth. The cookiecutters feed on marlin, albacore, tuna, wahoo, dolphinfish, dolphins, and other sharks, along with large invertebrates, including crustaceans and squid.
Both cookiecutter sharks can grow to a maximum length of about 16 to 20 inches (41 to 51 centimeters) and presumably give birth to live young. A schooling shark with a wide geographical range, the cookiecutter shark inhabits the Atlantic Ocean off the coast of the Bahamas and southern Brazil to Cape Verde, Guinea to Sierra Leone, and southern Angola and South Africa, including the island of Ascension; the southern Indian Ocean and southwestern Pacific Ocean from Mauritius to New Guinea and western Australia; and the Pacific from Japan and Lord Howe Island to Hawaii and the Galápagos Islands. Its vertical range extends from just below the ocean surface, where it may come to feed at night, to about 11,500 feet (3,500 meters) deep. The largetooth cookiecutter shark lives in deeper waters of the western North Atlantic, off the coast of Alabama in the United States and in the Gulf of Mexico; and in the western North Pacific, off the coast of Okinawa.
Neither of the cookiecutter sharks has attacked humans, and neither is fished commercially. They affect the fishing industry, however, with the damage they cause to important commercial species, such as tuna, with their unique feeding technique. Presumably in search of a meal, the cookiecutter shark has attacked the rubber domes over the radar apparatuses of nuclear submarines, prompting the United States Navy to add protective plastic tops. (See also dogfish sharks.)
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