The cookie cutter shark, Isistius brasiliensis, is a small species of shark that inhabits the deep oceans. This animal is often nicknamed the “cigar shark” due to its elongated, cylindrical body shape. It has captured the attention of marine biologists and the public for its highly specialized and unusual feeding strategy.
Geographical Distribution and Vertical Migration
The cookie cutter shark inhabits the warm, oceanic waters of all major tropical and warm-temperate basins worldwide. This cosmopolitan distribution includes the Atlantic, Pacific, and Indian Oceans, typically between the latitudes of 20°N and 20°S. They are strictly pelagic, meaning they live in the open water column rather than near the coast or ocean floor, though they are sometimes found near oceanic islands.
The shark undertakes one of the most extensive daily migrations of any known vertebrate, a behavior called diel vertical migration. During the day, they descend to the bathypelagic zone, spending time at extreme depths, often below 1,000 meters and potentially as deep as 3,700 meters. As dusk approaches, the sharks migrate upward, sometimes reaching depths as shallow as 85 to 300 meters, to hunt in the surface waters of the epipelagic zone.
This massive vertical movement, covering up to three kilometers daily, is a strategic adaptation to their deep-sea lifestyle. They spend their days in the dark, cold depths to avoid larger predators and then follow the migrating prey toward the surface at night. This movement pattern means the shark is rarely encountered by humans, which is why much of the information about the species comes from specimens caught as bycatch.
Distinctive Physical Characteristics
The cookie cutter shark is relatively small, with females reaching a maximum length of about 56 centimeters and males slightly smaller. Its body is cigar-shaped with a short, rounded snout, and it possesses two small, spineless dorsal fins set far back on its body. The coloration is dark brown or grayish, with the underside being slightly lighter, which provides some camouflage in the deep ocean.
A distinctive feature is a dark band, often called a “collar,” that wraps around the gill area of the shark. The most unique characteristic is the bioluminescence produced by organs called photophores that densely cover the entire underside of the shark, excluding the dark collar. These photophores emit a vivid green glow, which is the strongest bioluminescence known in any shark species.
The shark’s dentition is highly specialized. The upper jaw holds 30 to 37 rows of small, pointed teeth used to anchor to prey. The lower jaw features 25 to 31 rows of much larger, triangular, and razor-sharp teeth that are interlocked at their bases, forming a continuous, saw-like cutting edge.
Specialized Feeding Behavior
The shark’s name is derived from its unique, parasitic feeding method, which leaves a characteristic circular wound on its victims. This species targets animals far larger than itself, including various whales, seals, tuna, swordfish, and even great white sharks. The dark, non-luminescent collar against the glowing underside is hypothesized to function as a lure, mimicking the silhouette of a smaller, vulnerable fish when viewed from below.
When a larger animal approaches the apparent “small fish” lure, the cookie cutter shark rapidly strikes. It uses its fleshy, suctorial lips to create a vacuum and attach itself firmly to the side of the larger animal. With its small upper teeth securing its grip, the shark then bites down with its large, sharp lower teeth.
To excise the plug of flesh, the shark rotates its entire body, using the lower teeth like a rotating cookie cutter. This action cleanly removes a perfectly round plug of tissue before the shark quickly detaches and swims away. The resulting wounds are distinct and have even been found on non-living objects, such as the sonar domes of submarines and deep-sea cables.