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, and has been recorded from as deep as 3.7 km (2.3 mi). It migrates vertically up to 3 km (1.9 mi) every day, approaching the surface at dusk and descending with the dawn. Reaching only 42–56 cm (17–22 in) 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 dark brown in color, with light-emitting photophores covering its underside except for a dark "collar" around its throat and gill slits.
The name "cookiecutter shark" refers to its feeding habit of gouging round plugs, like a cookie cutter, out of larger animals.