Isistius is the genus that includes the cookiecutter shark.
The cookiecutter shark is usually identified as Isistius brasiliensis, a small deep-sea shark named for the round plugs of tissue it removes from much larger animals. Its bite leaves a distinctive crater-like wound, giving the shark its common name.
Cookiecutter sharks use specialized lower teeth and powerful suction to clamp onto prey. Their targets include tuna, billfish, dolphins, seals, whales and occasionally submarines or underwater cables. The shark can also produce light from photophores on its underside, a form of counterillumination that may help disguise its silhouette in the dim ocean.
The genus Isistius contains more than one recognized species, so “cookiecutter shark” is not itself the genus name. Isistius is a taxonomic label, while Isistius brasiliensis is the species most commonly meant by the common name.