The cookiecutter shark, a small inhabitant of the deep ocean, presents a fascinating subject. Its name hints at a distinctive characteristic that sets it apart. This creature has developed remarkable adaptations for survival in its challenging environment.
Physical Description and Habitat
This shark has a slender, cigar-shaped body, typically measuring between 42 to 56 centimeters (16.5 to 22 inches) in length. Its large, green eyes are adapted for low-light conditions in its deep-water home. The skin is generally dark brown to gray, with a darker band, or “collar,” around its gill area.
A striking feature is its bioluminescence, produced by tiny light-emitting organs called photophores covering its entire underside, excluding the dark collar. This greenish glow serves as a form of camouflage known as counter-illumination, helping the shark blend with the faint sunlight filtering down from the surface. From below, its glowing body might appear as general ambient light, while the dark collar could mimic the silhouette of a smaller, unsuspecting fish, effectively luring larger predators closer.
The cookiecutter shark lives in the warm, deep waters of tropical and subtropical oceans worldwide, often found near islands. During daylight hours, it resides in the mesopelagic zone, or twilight zone, at depths ranging from 1,000 to 4,000 meters (3,300 to 13,100 feet). As night falls, these sharks undertake a daily vertical migration, ascending closer to the surface, sometimes as shallow as 85 meters, to forage for food before descending again at dawn.
The Unique Feeding Method
The cookiecutter shark’s feeding mechanism allows it to extract a perfectly round plug of flesh from its prey, which gives the shark its common name. It initiates an attack by using its thick, fleshy lips to create suction and attach itself to the target. This suction is achieved by retracting its basihyal, similar to a tongue, and closing its spiracles, creating a low-pressure seal.
The shark’s dentition is arranged for this parasitic feeding style. Its upper jaw contains a row of small, hook-like teeth, which serve to anchor the shark to the victim’s body. The lower jaw features a single, fused row of larger, triangular, sharp teeth. These lower teeth, numbering between 25 and 31, are the primary cutting instruments.
Once attached, the cookiecutter shark employs a twisting or rotating motion of its body. This action, combined with the slicing power of its lower teeth, allows the shark to carve out a circular plug of flesh. The shark then detaches, leaving a cookie-shaped crater on its victim.
Impact on Marine Life and Man-Made Objects
The cookiecutter shark parasitizes marine animals, leaving circular wounds on creatures significantly larger than itself. Its targets include cetaceans like whales and dolphins, pinnipeds such as seals, and large fish like tuna and swordfish. Even other sharks, including great white sharks, have been observed bearing these distinctive scars.
While these bites rarely prove fatal to such large animals, they do leave behind crater wounds, typically averaging 5 centimeters (2 inches) across and 7 centimeters (2.8 inches) deep. The presence of these marks is widespread, with some populations of spinner dolphins off Hawaii showing nearly every adult with scars from cookiecutter shark encounters.
Beyond marine life, these sharks have also interacted with man-made objects, causing damage. During the 1970s, United States Navy nuclear submarines experienced operational issues due to cookiecutter shark bites on their sonar domes. These attacks caused sound-transmitting oil to leak, impairing navigation, and were initially mistaken for an unknown enemy weapon before the shark was identified as the culprit.
The sharks also have been known to bite underwater communication cables. In response to these incidents, fiberglass covers were installed around submarine sonar domes to prevent further damage. While rare, there have been a handful of documented interactions with humans, but due to their small size and deep-water habitat, cookiecutter sharks are not generally considered a threat to people.