Cookiecutter Sharks Use Glow In The Dark Ability To Attract Prey 10x Its Own Size

By McKenna Hardie, B.S. Graduate Student, Florida Institute of Technology & Shark Angels Intern
Scientific Study: A predatory use of counterillumination by the squaloid shark, Isistius brasiliensis by Dr. Edith Widder, 1998

Background

The Cookiecutter shark, Isistius brasiliensis, gets its common name from the way the shark attacks its prey. The Cookiecutter shark takes bite sized, circular, chunks of flesh from its prey, leaving behind perfectly circular wounds on prey. The prey of Cookiecutter sharks typically consists of much larger and faster fish and cetaceans. Cetaceans are marine mammals that are whales, porpoises, and dolphins. Large, whole squid has also been found in the stomachs of Cookiecutter sharks (Widder, 1998). Teeth on the upper row are small and thorn-like whereas the teeth on the bottom row are large, triangular, and flat (Widder, 1998). In order to conserve energy, the teeth will get replaced all at once; this is unique to the Cookiecutter shark.

Also unique to the Cookiecutter shark is its enlarged liver which offsets the heavy weight of its head and jaw region from the hard, calcified cartilage (Widder, 1998). This liver is so large, it almost takes up the entire body cavity (Widder, 1998).

Cookiecutter sharks can be found in the open ocean or nearshore in tropical waters. These sharks are extremely small, around 20 inches, and will swim together in schools (Widder, 1998). Cookiecutter sharks are slow swimmers. Using vertical migration, these sharks will migrate to the surface at night in order to hunt prey. During the day the sharks will return back to the deep ocean in order to escape predators.

The Cookiecutter shark has many unique morphologies that make it a successful predator. What the researchers are determining here is how a small, slow shark is able to successfully attack prey much larger and faster than it. It has been noted that Cookiecutter sharks have another characteristic unique to them, bioluminescence. Bioluminescence is the ability for an organism to produce light. This article goes into great detail to determine if bioluminescence plays a role in the predatory success of Cookiecutter sharks.

Bioluminescence  

Cookiecutter sharks are able to produce bioluminescence on their ventral side, the underside of an organism, due to tiny light producing organs called photophores (Widder, 1998). In this article, Widder used prior observations from the research of F.D. Bennett regarding the bioluminescence of Cookiecutter sharks. In Cookiecutter sharks, the photophores surround the scales of the body except for one dark, banded, area around the mouth that doesn’t produce light (Bennett, 1840). The light produced by Cookiecutter sharks is constant and doesn’t increase or decrease with agitation or friction (Bennett, 1840). Bioluminescence lasted longest in the areas near the jaws and pectoral fins (Bennett, 1840). No luminescence occurred on the black collar (named the “dog collar”) around the throat of the Cookiecutter shark (Bennett, 1840).

Organisms that inhabit the deep ocean often use bioluminescence to defend against predators or camouflage their silhouettes. Many predators find prey by searching for an organism’s silhouette above that is visible against the light provided by the sun and moon (Widder, 1998). Organisms use counterillumination in order to hide their silhouettes from predators (Widder, 1998). The ventral side of the organism is lightened up using bioluminescence in order to blend into the lighter background above.

Findings & Results

The “dog collar” is darkly pigmented and does not contain light producing photophores. Researchers determined that this dark band around the Cookiecutter shark’s neck is actually used as a lure. The dark band resembles a silhouette of a small fish when the rest of the shark’s silhouette is broken up due to bioluminescence (Widder, 1998). Furthermore, when Cookiecutter sharks travel in schools they create the deception of a school of fish (Widder, 1998). This is a remarkable strategy for creating a temptation to attract larger prey. The schooling also prevents the prey from counterattacking because they are outnumbered by Cookiecutter sharks (Widder, 1998). In actuality, it is not the luminescence produced that attracts prey but rather the dark collar that is shaped by the light to appear as a silhouette of a small fish. By the time the large prey realizes the shark is not a small fish, the Cookiecutter shark is already in close enough range to attack. Cookiecutter sharks have larger, oval, well-developed eyes which aid in the ability for the shark to see its prey before the prey notices their deception (Widder, 1998).

Momentum from the prey swimming upward towards the shark helps the Cookiecutter shark latch on and move in a circular fashion around the prey in order to quickly remove a perfectly circular chunk of flesh (Widder, 1998). Cookiecutter sharks must rotate while latched onto the prey because of the two different sizes/shapes of teeth that it has (Widder, 1998).  If the prey fights back and creates a lot of movement, the shark is able to use an oral vacuum in order to stay attached to the prey until it is able to take out a round of flesh (Widder, 1998).

The oral vacuum can be used as an explanation for how a Cookiecutter shark can devour a squid whole. When the squid goes in for the attack of what it perceives to be a small fish, the Cookiecutter shark opens its jaws and creates an oral vacuum that sucks in the squid whole (Widder, 1998).

Conclusions

The researchers concluded that Cookiecutter sharks use bioluminescence to aid in their ambush/deception predatory style (Widder, 1998). The dark band around the neck is the lure, not the part of the fish that is producing light. The “dog collar” is effective because it mimics the silhouette of a small fish, attracting larger, hungry predators who will soon become prey. Cookiecutter sharks have developed differential blending, an adaptive trait that allows certain areas of an organism to blend and other areas to be highly contrasted to the background (Widder, 1998). The migration of sharks from the deep to the surface is key because that determines the effectiveness of using bioluminescence to hide the shark (Widder, 1998). If the background is not light enough, the shark will fail to blend in and won’t be able to use the collar as a lure.

The energy used during bioluminescence and vertical migration is expensive. By using the collar as a lure to attract prey to them, Cookiecutter sharks are saving a large amount of energy that would be used to search for prey (Widder, 1998). Typically, the ability for deep sea organisms to produce bioluminescence is used for defending against predators and avoiding detection from predators (Widder, 1998). However, in this case, Cookiecutter sharks use their bioluminescence to find food, adding on to how unique these small sharks really are.

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References
Bennett, F. D. (1840). Narrative of a whaling voyage round the globe, from the year 1833 to 1836 (Vol. 2). Рипол Классик.
Widder, E. A. (1998). A predatory use of counterillumination by the squaloid shark, Isistius brasiliensis. Environmental Biology of Fishes, 53(3), 267-273.

Article Citation
Widder, E. A. (1998). A predatory use of counterillumination by the squaloid shark, Isistius brasiliensis. Environmental Biology of Fishes, 53(3), 267-273.