Cookiecutter sharks terrorise animals of all sizes
Cookiecutter sharks are known for ripping small, cookie-shaped chunks out of sharks and whales much larger than themselves, but a new study has found they actually terrorise animals of all sizes. The green-eyed, alienesque sharks look like sinister sock puppets made of pastry dough and can grow up to 50 centimetres long. These odd creatures use their pointed teeth to feed off great white sharks ten times their size and are even known to nibble chunks out of human flesh.
Scientists frequently observed cookiecutter markings on larger animals and thus assumed that’s what the sharks primarily ate. But it turns out these sharks munch on animals at the bottom of the food chain as well, giving them a unique role in the ocean ecosystem, a new analysis of shark specimens has discovered.
“They feed on everything, from the biggest, toughest apex predators – like white sharks, orcas, everything you can imagine – down to the smallest little critters,” said Aaron Carlisle, an assistant professor at the School of Marine Science and Policy at the University of Delaware. “There’s not very many animals that do something quite like this.”
Cookiecutter sharks live in tropical and subtropical waters and can inhabit depths of more than 1,500 metres. If humans see cookiecutter sharks, it’s usually near the surface at night, when they come up to hunt larger prey in the upper ocean. The researchers tested the assumption that these sharks mainly eat larger animals in the upper ocean by studying 14 cookiecutter sharks caught around Hawaii by the Monterey Bay Aquarium. The sharks’ stomachs were mostly empty of food, but the team figured out what the animals had been eating by looking at the chemical composition of their tissues. The team also checked for environmental DNA (EDNA), or the presence of DNA left behind even when there is no tissue to study.
“Environmental DNA is an increasingly popular and powerful tool that works under the idea that if an animal swims by in the ocean, it’s going to be shedding DNA in the water,” Carlisle said. “If you take a water sample and filter it out, you can extract the DNA of everything that’s been in that water mass and identify what species were there. So we tried that on their stomach contents.”
The researchers found that the cookiecutter sharks fed mostly on smaller species at lower depths, including crustaceans, squid and small fish. Some of these prey may be small enough for the sharks to swallow whole. In contrast, large animals from the upper ocean made up less than ten per cent of the sharks’ diet. These findings shed light on the behaviour of this cryptic ocean creature, but the sample of sharks was small and from a limited geographic range, so it’s unclear whether this feeding trend is the same throughout cookiecutter sharks’ global range.