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All about the amazing cookiecutter shark

June 20, 2019
BY DONNELL TATE, Harbor Report , Lahaina News

French naturalists Jean Ren Constant Quoy and Joseph Paul Gaimard originally described the cookiecutter shark during the 1817-20 exploratory voyage of the corvette Uranie, giving it the name Scymnus brasiliensis, because the type specimen was caught off Brazil.

In 1865, American Ichthyologist Theodore Nicholas Gill coined the new genus Isistius for this species, after Isis, the Egyptian goddess of light.

The cookiecutter shark (Isistius brasiliensis), also called the cigar shark, is a species of small dogfish shark in the family Dalatiidae. This shark occurs in warm, oceanic waters worldwide, particularly near islands, perhaps for reproductive purposes or because they hold congregations of large prey animals.

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Male cookiecutter sharks mature at 14 inches and can grow to a size of 16 inches. Females mature at 15 inches and can reach up to 22 inches.

In the central and eastern Pacific, it occurs from Fiji north to the Hawaiian Islands, and east to the Galapagos, Easter and Guadalupe Islands. Fresh wounds observed on marine mammals suggest that this shark may range as far as California in favorable, warm years.

It has been recorded from as deep as 3.7 kilometers (2.3 miles). It migrates vertically up to 3 kilometers (1.9 miles) every day. It spends the day at a depth of 1-3.7 kilometers (0.62-2.3 miles), and at night it rises into the upper water column, usually remaining below 85 meters (279 feet), but on rare occasions venturing to the surface. It descends with the dawn.

Reaching only 1722 inches in length, the cookiecutter shark has a long, cylindrical body with a short, blunt snout, large eyes, two tiny spineless dorsal fins, and a large caudal fin. It is medium brown dorsally, lighter below, and has a distinct dark collar around the gill region, with light-emitting glands called photophores densely covering its underside, producing a vivid green glow.

These glands produce an enzyme called luciferose that converts chemical energy to light energy, causing the glow. Transparent cells surrounding the glands also allow the light to shine through the cookiecutter shark's skin. The intrinsic green luminescence of the cookiecutter shark is the strongest known of any shark, and has been reported to persist for several hours after it has been taken out of the water.

Set apart from the glowing underside, the darker, non-luminescent collar tapers at both sides of the throat, and has been hypothesized to serve as a lure, by mimicking the silhouette of a small fish from below, via its ventral photophores. The appeal of the lure would be multiplied in a school of sharks. If the collar does function in this way, the cookiecutter shark would be the only known case of bioluminescence in which the absence of light attracts prey, while its photophores serve to prevent premature detection by incoming would-be predators.

The ventrally positioned photophores serve to disrupt its silhouette from below by matching the downwelling light, a strategy known as "counter-illumination." It is common among bioluminescent organisms found in the intermediate oceanic depths, between approximately 300 and 3,300 feet. As the shark can only match a limited range of light intensities, its vertical movements likely serve to preserve the effectiveness of its disguise across various times of day and weather conditions

Unlike other sharks, the retina of the cookiecutter shark has ganglion cells concentrated in a concentric area, rather than in a horizontal streak across the visual field. This may help to focus on prey in front of the shark. This shark has been known to travel in schools, which may increase the effectiveness of its lure as well as discourage counter-attacks by much larger predators.

Cookiecutter sharks are ovoviviparous. Females have two functional uteruses. Hatching takes place between 12-22 months with 6-12 pups per litter. Newborn cookiecutter sharks measure 5.5 to 6 inches. When the young emerge, they are fully developed and able to hunt for food by themselves.

 
 

 

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