The cookie cutter shark (Isistius brasiliensis), also known as the cigar shark, has a unique appearance. Its distinctive features contribute to its name. This small deep-sea shark exhibits several unusual characteristics, from its body shape to its specialized mouth and teeth, and its capacity for bioluminescence.
Compact Body and General Size
The cookie cutter shark has a small, slender, cylindrical, cigar-shaped body. Males grow up to 42 centimeters (16.5 inches) in length, while females reach up to 56 centimeters (22 inches). Its coloration is usually dark brown or grayish on its upper side, fading to a lighter shade on its underside. This countershading helps it blend with its environment in the deep ocean.
The shark’s eyes are large and oval, positioned towards the front of its head. It has two small dorsal fins located far back on its body. Its caudal fin is large with a prominent lower lobe. The pectoral fins are roughly trapezoidal. These features contribute to its streamlined, torpedo-like form.
The Distinctive Dark Collar
One of the most recognizable features of the cookie cutter shark is the dark band, or “collar,” that encircles its throat and gill region. This dark coloration stands out against the lighter underside of the shark’s body. The band is non-luminescent, unlike the rest of the shark’s belly.
The dark collar is thought to play a role in the shark’s hunting strategy. It is hypothesized to mimic the silhouette of a smaller fish from below, potentially luring larger prey towards the shark. This unique marking distinguishes Isistius brasiliensis from similar species, such as Isistius plutodus, which lacks this dark collar.
Specialized Teeth and Mouth
The cookie cutter shark’s mouth is small and circular, with enlarged, fleshy, suctorial lips that give it a suction-cup appearance. Its teeth are specialized and arranged differently in the upper and lower jaws. The upper jaw contains 30 to 37 rows of small, pointed, and erect teeth.
In contrast, the lower jaw has fewer teeth, typically 25 to 31, which are much larger, triangular, and serrated. These lower teeth interlock at their bases, forming a continuous, saw-like cutting edge. When feeding, it attaches with its suctioning lips, using its upper teeth to grip prey while its lower teeth carve out a circular plug of flesh, giving the shark its common name. The cookie cutter shark sheds its entire lower row of teeth as a single unit, and is believed to ingest them to recycle calcium.
Bioluminescent Features
The cookie cutter shark produces its own light, a phenomenon known as bioluminescence. Its entire ventral (underside) surface, except for the dark collar, is densely covered in tiny light-producing organs called photophores. These photophores emit a consistent greenish glow.
This greenish luminescence can persist for several hours even after the shark has died. The bioluminescence serves as a form of camouflage called counter-illumination, making the shark’s silhouette less visible to predators or prey looking up from below by matching the faint light filtering down from the surface.