The glowing lure
The cookiecutter shark, Isistius brasiliensis, is a master of deep-ocean deception. This small, cigar-shaped shark, which grows to only 42–56 cm (16.5–22 in) long, employs a sophisticated form of biological camouflage called counter-illumination. Its belly is covered in thousands of tiny light-producing organs called photophores, which emit a steady greenish glow. This light matches the faint sunlight or moonlight filtering down from the surface, effectively erasing the shark's silhouette when viewed by predators from below.
The trick lies in what doesn't glow. A distinct, dark, non-luminescent band of skin wraps around the shark's throat, just behind its jaw. From the depths, this dark patch against the glowing belly creates a convincing illusion: the silhouette of a small fish. This lures larger predators like tuna, swordfish, and even other sharks upward to investigate the potential meal. The cookiecutter shark is the only known animal where the absence of bioluminescence is thought to attract prey, while the light it produces is camouflage. This strategy allows the slow-swimming shark to ambush much larger and faster animals.
A specialized bite
When a larger animal is drawn in by the lure, the cookiecutter shark attacks. Its feeding mechanism is highly specialized for its parasitic lifestyle. The shark has fleshy, suctorial lips that create a strong seal against its victim's body. It anchors itself using its 30 to 37 small, hook-like upper teeth. Then, its 25 to 31 large, razor-sharp lower teeth go to work. The shark rotates its body, using its lower jaw like a can opener to scoop out a plug of flesh. The resulting wound is a remarkably clean, circular crater.
This shark travels a long distance every day, a behavior known as diel vertical migration. It spends its days in the dark depths, sometimes as deep as 3,700 meters (12,139 feet), and ascends at night to feed in surface waters, usually staying below 85 meters. Its victims are numerous and diverse, with characteristic circular scars observed on everything from dolphins and whales to great white sharks.