Shark of the month: Cookiecutter shark

The cookiecutter shark (Isistius brasiliensis), also commonly known as the cigar shark, is a small dogfish shark that prefers warm oceanic waters, usually near islands. This shark prefers deep waters and has been recorded in waters as deep as 3.7 km. It is a diurnal shark, meaning that it makes a nightly migration to the surface and then descends back down during the day. The cookie cutter shark owes its name to the cookie shaped wounds it leaves in its prey. Funny fact that relative to the size of its body, cookie cutter sharks have the largest teeth of all sharks. Moreover, cookiecutter sharks glow greenish due to light-emitting organs in its skin. This green bioluminescence is the strongest known of all sharks and the glow continues even after the shark has been taken out of the water.

Anatomy and appearance

The cookie cutter shark is a small shark that has a long, cigar-shaped body with a rounded snout. At full maturity, it reaches a maximum length of 42 cm for males and 56 cm for females. It has large, green, oval eyes that are placed forward on its head. Behind its eyes are large spiracles, which lead to its respiratory systems to enable them to breath. The mouth is short and filled with 30 to 37 rows of teeth in the upper jaw and 25 to 31 rows of teeth in the lower jaw. The teeth in the upper jaw are narrow, small, straight, and have a smooth-edged cusp. The lower teeth are larger, wider, and serrated, while the bases of the teeth are locked together to create a saw-like edge. The pectoral fins are short and are four-sided. The two dorsal fins are placed near the end of the body. The second dorsal fin is a little bit larger than the first one. The pelvic fin is the largest of the fins on the body. There is no anal fin. The caudal fin is wide with no visible ventral notch. The body is a dark brown color with lighter counter shading on the underside.

Habitat

The cookie cutter shark inhabits all tropic and sub-tropical oceans and is most commonly found between the latitudes of 20°N and 20°S. It prefers warm water temperatures between 18-26°C. In the Atlantic, it has been documented to be in the Bahamas, southern Brazil, Sierra Leone, southern Angola, Cape Verde, Guinea, and South Africa. In the Indo-Pacific, it has been documented in Mauritius, New Guinea, Australia, Japan, and New Zealand. In the Pacific, it has been documented in Fiji, Hawaiian Islands, Galapagos, and Guadalupe Islands.

The cookie cutter shark is known to migrate diurnally, meaning that it migrates from the deep ocean to the upper water column at night. It spends the day at a depth of 1 to 3.5 km. The cookie cutter shark migrates up to around 85 meters at night, and occasionally all the way up to the surface. It is most commonly found near islands, probably for reproductive and predatory reasons.

Diet

Cookie cutter sharks prey on virtually all medium to large sized animals as they take round, cookie cutter like bites out of the sides of animals. Bit scars have been found on cetaceans, pinnipeds, dugongs, sharks, deep-water stingrays, and bony fish. They also hunt and eat entire squids that are 15-30 cm in length, which is around the same size as the shark itself. An attack from a cookie cutter shark will leave a round, crater-like wound that is an average of 5 cm across and 7 cm deep. Diseased or weakened animals seem to be more susceptible to bites, but in some places, healthy animals readily bear the scars from cookie cutter sharks.

The shark feeds by first securing itself to the body surface of its intended prey by closing it spiracles, retracting its tongue, and suctioning its lips in order to create negative pressure to secure a seal. Then, it bits using it narrow upper row of teeth to anchor itself and its bottom teeth to “saw” into the prey. Then, The shark rotates its body to cut a complete circle out of the animal.

Population

Cookie cutter males reach maturity at about 36 cm and females at about 40 cm. Cookie cutter females are ovoviviparous, this means that they give birth to live pups that have developed inside egg cases within the mother. Females give birth to about 6 to 12 live young after a gestation of 12 to 22 months. The pups are fully developed when they are born and instantly able to hunt. It is not known how many cookiecutter sharks habitat the earth’s waters.

Conservation

The International Union for Conservation of Nature (IUCN) has listed the cookiecutter shark under the category of Least Concern. This is due to the fast that these sharks are widely distributed, they have no commercial value, and they are not very susceptible to fisheries.

Relationship to humans

Due to their habitat, cookiecutter sharks are rarely encountered and only a few attacks on humans have ever been documented, and therefore, is not considered to be highly dangerous. However there have been a couple of incidents were these sharks fed on humans in the water.

 

Originally written by Hidden Depths and edited by Isabelle Walter

 

 

 

 

 

 

 

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