The cookiecutter shark ( Isistius brasiliensis ) is a small, yet fascinating shark species that inhabits tropical and subtropical oceanic waters worldwide. Reaching perhaps 22 inches at full size, this shark has a thin, cigar-shaped body and a short snout. Its unique feeding habits have earned it the name "cookiecutter" shark. Let's delve into the intriguing world of the cookiecutter shark and explore its diet and feeding habits.
Physical Characteristics
The cookiecutter shark has a distinctive appearance. Its body is generally dark brown to black on the dorsal side and lighter brown on the ventral side. They have a dark collar marking its throat. The entire ventral surface, except for the dark collar, is covered in a dense network of tiny photophores, organs that emit light. These sharks possess small dorsal fins set far back on the body and lack an anal fin. Their eyes are located closer to the tip of the snout than those of other sharks. One of the most notable features of the cookiecutter shark is its large, triangular cusped lower teeth, arranged in 25 to 32 rows.
Distribution and Habitat
The cookiecutter shark has a global distribution, occurring in tropical and temperate oceans. They have been found in the Atlantic, Pacific, and Indian oceans. These sharks are mesopelagic, meaning they inhabit the mid-depths of the ocean. During the day, they stay at least 300 feet below the surface, with their depth thought to exceed two miles. At night, they migrate towards shallower waters to feed.
Unique Feeding Strategy
The cookiecutter shark is known for its unique feeding strategy, which involves taking round, cookie-cutter-shaped bites out of larger marine animals. While they do eat smaller animals like squid whole, they also target larger prey such as tuna, whales, dolphins, and seals.
The cookiecutter shark employs a clever hunting technique. It is bioluminescent, able to emit a greenish light from its belly. This light attracts larger fish. The dark patch of the throat, against the glow of the underside, is thought to appear like a small fish when viewed from deeper waters. When a larger animal approaches, the shark uses its large, fleshy lips to create suction, attaching itself to the prey. Then, it rotates its body and uses its sharp lower teeth to carve out a section of flesh. The scar that remains is reminiscent of one that could be caused by a cookie cutter. After taking its bite, the cookiecutter shark detaches and swims away to enjoy its meal.
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Diet Composition
The diet of the cookiecutter shark is more diverse than previously thought. While they are known to feed on large marine animals, recent studies have revealed that smaller micronektonic and forage species also play a significant role in their diet.
Traditional View
Traditionally, it was believed that cookiecutter sharks primarily fed on large animals such as whales and tuna. This assumption was based on the observation of bite marks on these animals. However, empirical data on the diet of cookiecutter sharks was limited.
Recent Findings
Recent research using biochemical tracers, including stable isotope analysis (SIA), fatty acid analysis (FAA), and environmental DNA (eDNA), has provided new insights into the trophic ecology of cookiecutter sharks. These studies have shown that small micronektonic and forage species are the most important prey group for cookiecutter sharks across their size range. Larger mesopelagic species or species that exhibit diel vertical migration are also important prey items.
Environmental DNA metabarcoding has revealed new prey items for cookiecutter sharks and demonstrated that eDNA can be used to identify recent prey in stomachs frozen for extended periods.
Dietary Shift
There is evidence of a potential shift in diet and/or habitat with size and season. Smaller sharks may stay in deeper waters and not ascend to the surface until they reach a certain size.
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Ecological Role
The cookiecutter shark plays a unique role in pelagic food webs. It feeds on a wide range of prey, from the largest apex predators to small, low trophic level species. This allows them to overlap with the depth distribution of the sharks throughout the diel cycle.
Reproduction
Little is known about the reproduction of cookiecutter sharks. They are believed to be mostly solitary, interacting with other members of the species mainly to mate. Male cookiecutters reach maturity at about 14 inches, and females mature at roughly 16 inches.
This species is ovoviviparous, meaning that the pups develop inside egg cases within the mother. Each litter contains 6-12 live young that are born after 12-22 months. After birth, the young sharks are capable of hunting for themselves.
Interaction with Humans
Cookiecutter sharks are not considered dangerous to people because they reside in deep-water habitats. However, there have been a few documented cases of cookiecutter shark attacks on humans. Most of these incidents involved long-distance ocean swimmers at night.
During the 1970s, cookiecutter sharks caused damage to United States Navy submarines. The sharks bit the neoprene boots of sonar domes, causing the sound-transmitting oil inside to leak. This impaired the submarines' ability to navigate.
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Conservation Status
The International Union for Conservation of Nature and Natural Resources (IUCN) has classified both types of cookiecutter sharks as species of least concern since the early 2000s. This is because their small size limits their appeal to commercial fishing operations. However, they are occasionally caught on accident by fisheries targeting other species. Protecting and restoring our oceans benefits wildlife around the world, including cookiecutter sharks.