The giant cookiecutter shark (Isistius sp.) is a hypothetical giant species of cryptid cookiecutter shark posited to exist in the Arctic Ocean off Alaska and possibly Greenland. Although no sightings of such an animal have ever been reported, a colleague of shark specialist Eugenie Clark (1922 – 2015) working near Alaska once discovered a dead narwhal (Monodon monoceros) which, when brought alongside the vessel, was found to have been injured with round bites, strongly resembling the distinctive bites of a cookiecutter shark, but a great deal larger. As the narwhal had been discovered at night and only examined come morning, it is unclear if it had been killed by the perpetrator of the bites, or scavenged during the night.
Ordinary cookiecutter sharks generally do not exceed 2 ft (60 cm) in length, and Clark later suggested that the bites could have been made by a hypothetical giant species. However, marine biologist Richard Martin has suggested that the perpetrator was more likely a Pacific sleeper shark (Somniosus pacificus), which is known to scavenge the carcasses of marine mammals. According to Martin, similar "giant cookiecutter" injuries have been discovered on narwhals off Greenland, where the related Greenland shark (Somniosus microcephalus) has allegedly been shown to be responsible. It is not known how sleeper sharks might biomechanically produce the distinctive circular bite pattern.