The parasite shark
The Cookie Cutter Shark (Isistius brasiliensis) is a deep water shark that earned its name due to the wounds that it inflicts upon many marine animals. It latches onto its prey and takes out large chunks of the organism in the same way a cookie cutter carves through cookie dough. The wounds are non fatal and said to resemble that of an ice cream scoop. The cookie cutter is known as a facultative parasite, meaning that it exhibits parasitic behaviour, but it does not rely on a host organism to be able to complete it’s life cycle.
Cookie cutters are very small shark, with the largest males reaching only 16.5 inches, whereas the females can reach 22 inches. They belong to the dogfish family, Squalidae. They are dark brown in colouration, but the underside is covered by many photophores which are organs that produce light. The genus name, Isistius refers to the Egyptian god of light, Isis, this is a reference to the photophores, an iconic feature of the shark.
The habitat range for cookie cutters is extremely wide, inhabiting the deep waters of tropical and temperate regions meaning that they are a highly abundant species. They typically reside deep in the ocean, at around 3200 feet, the deepest they have been recorded is 3.7km. At night the shark moves up the water column to around 300 feet, this is an action called “diel vertical migration”.
It is theorised that populations of cookie cutters is higher around islands, as there is a higher chance of finding a mate, or prey, however this has not been backed up with conclusive proof.
In order to take advantage of the lack of visibility in deep water, the cookie cutter developed photophores, these work in two ways, the first is increase camouflage to aid predator avoidance, the second is to lure in unsuspecting prey. There is a dark brown band of skin around the gills and neck of the shark, and because of the camouflage provided by the photophores, this dark band looks like a small fish. This makes the ambush predator tactic significantly more effective. The success of this method is greatly increased when the shark travels in schools.
The mouth of cookie cutters is extremely specialised, the top teeth have evolved to be narrow and act as an anchor whereas the bottom teeth are triangular in shape and are used for slicing through the flesh of nearly any large marine organism. The tongue is retracted within the mouth, causing the pressure to drop, this in turn creates a suction seal. Once secured to their prey the shark will then rotate it’s body, slicing through the prey. The carving motion is likely aided by the flailing prey. The wounds that are inflicted by this average 5cm across and 7cm deep and while not fatal, can leave the prey susceptible to infection. While the cookie cutter prefers to feed in this parasitic manner, they also consume squid, likely by using the same strong suction they use to latch onto other organisms.
The teeth are also a valuable source of calcium for the shark, as they can swallow the whole row of teeth and replace them with fresh ones, this is due to the low level of nutrients in the deep water.
Cookie cutter sharks don’t only harm marine organisms, they have been recorded causing damage to U.S. Navy submarines, causing fear of unknown enemy weaponry. Once discovered that it was only sharks, a simple fiberglass installation was implemented to negate the damage.