Fish
Cookiecutter Shark
Isistius brasiliensis · also called cookiecutter shark, cigar shark, luminous shark
The cookiecutter shark is barely half a metre long, and it feeds by biting round, ice-cream-scoop craters of flesh out of tuna, dolphins, seals, great white sharks, and whales — animals that outweigh it by thousands of times. It lures them in with a belly that glows in the dark.
Overview
The cookiecutter is a small, cigar-shaped shark of the warm open ocean worldwide. It spends the day deep — 1,000 metres or more — and rises toward the surface at night, a vertical migration of over a kilometre, to feed. It is an ectoparasite: it does not kill its prey, it takes a bite and lets it swim off.
The bite
The shark seals its sucker-like lips onto the prey’s skin, anchors its small upper teeth, drives in its huge band-saw lower teeth, and spins its body, cutting out a neat plug about 5 cm across and 7 cm deep. Most big tropical fish and mammals carry the round scars. It sheds its lower teeth in one connected row and swallows them to recycle the calcium.
Glow and lure
The cookiecutter’s underside is covered in photophores that produce a green glow, hiding its silhouette from below (counter-illumination). But a dark collar band does not glow — and from beneath, that dark shape in a field of light may look like a small fish, drawing large predators close enough to bite.
Biting machines
Cookiecutter sharks have bitten submarine sonar domes, undersea cables, and research instruments, and — very rarely — open-water swimmers at night in warm seas.
Status
The cookiecutter shark is Least Concern — wide-ranging, common, and not fished.
Cookiecutter Shark: common questions
How does the cookiecutter shark actually feed?
It is a parasite that takes bites out of animals far bigger than itself and lets them swim away alive. The shark approaches a large fish or marine mammal, presses its fleshy, ridged lips against the skin to form a seal, and holds on by suction. It anchors the narrow, hooked upper teeth, sinks its enormous interlocking lower teeth into the flesh, and then rotates its whole body, sometimes assisted by the prey's own forward motion. The lower teeth work like a rotary blade and carve out a clean, round or oval plug of skin, blubber, and muscle, leaving a distinctive crater-shaped wound. Almost every adult swordfish, tuna, dolphin, and large whale in tropical seas carries these scars or the pale round scars they heal into.
Why does it glow?
The cookiecutter shark's entire underside is packed with photophores that emit a soft green light, and this serves as counter-illumination camouflage — the glow matches the dim light filtering down from the surface so that, seen from below, the shark's silhouette disappears against the background. But there is a twist — a band of skin around the throat and "collar" does not glow and stays dark. The leading idea is that this dark collar, hanging in a field of glow, looks from below like the small silhouette of a little fish or squid. A tuna or seal coming up to grab an easy meal instead finds a shark attached to its flank. The shark's light organs can keep glowing for hours after death.
Does the cookiecutter shark bite submarines and people?
Yes to submarines, rarely to people. During the Cold War, US Navy submarines suffered mysterious damage to their rubber sonar dome coverings and the neoprene boots around electrical cabling, and the culprit turned out to be cookiecutter sharks biting the equipment; the domes had to be re-covered in fibreglass. The sharks have also bitten undersea telecommunications cables (causing outages) and oceanographic instruments. Bites on living humans are very rare and involve people in warm open ocean at night — a small number of long-distance swimmers and survivors of vessel sinkings have documented cookiecutter wounds, including one well-known 2009 case of a swimmer crossing the Alenuihaha Channel in Hawaii.
How big is a cookiecutter shark and where does it live?
It is tiny for a shark that attacks whales — females reach about 56 centimetres and males about 42, roughly the size of a large cucumber, which is why it was once called the "cigar shark." It lives in warm oceanic water around the world, usually far from land but often concentrated near oceanic islands. During the day it stays in deep water, commonly between 1,000 and 3,700 metres, and each night it rises through the water column, sometimes almost to the surface, to feed, descending again before dawn. This daily vertical migration of over a kilometre brings it into contact with the surface-dwelling and mid-water animals it parasitises.