The Cookiecutter Shark (Isistius brasiliensis) is a widely distributed deep-sea creature known for its small size and unique feeding method. It gains its common name from the distinctive, perfectly circular wounds it leaves on much larger animals, suggesting a specialized predatory style. Understanding this oceanic shark requires a close look at its morphology, particularly its body shape and specialized mouth.
General Body Profile and Dimensions
The Cookiecutter Shark possesses a slender, cylindrical shape, often described as cigar-like, earning it the nickname “cigar shark.” It is one of the smallest known shark species; adult females reach a maximum length of about 22 inches (56 cm), with males being slightly smaller. The body is generally dark chocolate brown or grayish-brown on the upper surface, becoming lighter underneath.
The snout is short and bulbous, and the eyes are notably large and oval, reflecting its life in the dimly lit ocean depths. The fins are relatively small and positioned far back on the body. The two dorsal fins lack spines and are set toward the rear, with the first originating just ahead of the pelvic fins. The caudal fin is broad and nearly symmetrical, featuring a large lower lobe that aids in rapid, short bursts of movement.
The Striking Dark Collar and Bioluminescence
A prominent feature is the dark brown or black band encircling the gill slits and throat area, resembling a collar. This band is unique because it is the only part of the shark’s underside that does not produce light. The rest of the ventral surface is covered densely with light-producing organs called photophores.
These photophores emit a greenish glow, a process known as bioluminescence. This light serves a dual purpose, primarily functioning as camouflage through counter-illumination, matching the faint downwelling sunlight to hide its silhouette from predators below. The dark collar breaks this camouflage and is hypothesized to mimic the silhouette of a smaller fish. This dark patch acts as a lure, attracting larger predators who mistake the collar for potential prey.
The Specialized Jaws and Teeth
The mouth is the most defining feature, being short and bordered by enlarged, fleshy lips that create a suction mechanism. The jaw structure shows a significant difference between the upper and lower teeth. The upper jaw contains 30 to 37 rows of small, narrow, sharp teeth, which are used to firmly anchor the shark to its prey.
The lower jaw holds the specialized cutting tools: 25 to 31 rows of much larger, broader, triangular teeth. These lower teeth are interlocked at their bases, forming a continuous, serrated, saw-like cutting blade. This unique arrangement means the shark must shed and swallow an entire row of lower teeth at once, unlike most sharks that replace individual teeth. The powerful bite is supported by heavily calcified cartilage, enabling the shark to latch on before excising a plug of flesh with the lower blade.
The Signature Bite
The specialized dental anatomy results in a distinctive, perfectly circular, crater-like wound left on its prey. This feeding strategy is considered ectoparasitic, as the shark removes a non-lethal plug of tissue from animals much larger than itself, including whales, dolphins, seals, and large fish like tuna and marlin. The wounds average about 2 inches (5 cm) across and can be as deep as 2.8 inches (7 cm), though wounds up to 5 inches (13 cm) in diameter have been recorded.
To achieve this clean cut, the shark attaches itself to the victim, then uses its lower teeth to slice out the plug of flesh while rotating its entire body. Evidence of the Cookiecutter Sharkâs reach extends beyond marine life; bite marks have been found on the rubber sonar domes of submarines and deep-sea cables. The prevalence of these scars is high in some areas, with almost every adult spinner dolphin off Hawaii bearing marks from these tiny predators.