A new study led by the University of Delaware’s Aaron Carlisle has uncovered that while Cookiecutter sharks might chomp on everything they can get their jaws on in the upper reaches of ocean to supplement their diets, they primarily feed on crustaceans, squid and small fish.
For years, researchers assumed Cookiecutter sharks were coming up at night to feed on whales and bigger animals before returning to the deep ocean during day time. However, this was just an assumption as no one really knew how they fed in detail. To study their feeding habits further, Carlisle led a research team that studied 150 stomachs of these creatures over 50 years around the world which revealed some pretty interesting findings about them.
With the use of more advanced scientific methods, researchers are now able to study sharks in ways that were not possible before. For example, when scientists studied what was inside a shark’s stomachs–which sometimes included nothing at all—they could only make inferences from the bites they had seen on their larger species and couldn’t employ these new techniques.
“At the end of the day, the paradigm was that the sharks would primarily feed on these larger animals, but we just didn’t have any empirical data. So our question was, ‘Are we biased by what we’re seeing?’ ” said Carlisle. “It turns out that all these bites we see on marine mammals and larger sport and commercial fishes and things really make up a relatively smaller amount of their diet.”
Carlisle noted that it is unique to have an animal that will feed on creatures from the top and the bottom of the food chain.
“These animals occupy a unique ecological role in the world’s oceans,” said Carlisle. “They feed on everything from the biggest, toughest apex predators—like white sharks, orcas, everything you can imagine—down to the smallest little critters. There’s not very many animals that do something quite like this.”
For this study, the researchers studied 14 Cookiecutter sharks that were collected by the Monterey Bay Aquarium from around Hawaii. They used a variety of techniques to better understand their feeding habits and habitats.
Since the Monterey Bay Aquarium is planning to open a new deep-sea exhibit featuring live animals next year, researchers needed answers for their questions about Cookiecutter’s diet and behavior. Using biochemical tests in coauthor John O’Sullivan’s study, they were able to do so successfully.
“It’s important to do more than one test and the reason for that is the same reason that people want to get more than one opinion for any medical situation,” said O’Sullivan. “The broader the test range is, with their techniques and methods, the more sound you can feel, collectively, about your results, and that just helps improve the scientific methods.”
Researchers examined the sharks’ stomachs and found that they were mostly empty. However, using modern techniques like environmental DNA of their contents allowed them to extrapolate potential diets for these sharks.
“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,” said Carlisle. “So 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.”
Using this approach, they identified several prey species from seemingly empty stomachs, including previously unknown prey.
The researchers were also able to make inferences about their habitat. They have generally only been observed near surface waters during the night, leading them to believe that they exhibit vertical migration where they ascend shallow waters at dusk and return deeper water at dawn. However, this might not be true for larger Cookiecutter sharks whereas smaller baby ones don’t vertically migrate all together.
“The little guys, we think that they may stay down deep. It appears that they don’t start going up to the surface until they get to be a certain size,” said Carlisle. “But again, nobody’s ever really seen a baby Cookiecutter shark so we don’t really know. We’re trying to make these inferences based on indirect metrics of what they’re eating and what their ecology is.”
Carlisle stressed that with the deep ocean increasingly being exploited by fisheries and other extractive industries, it is important to continue advancing our knowledge of these species.
“Most of the animals that live in the deep sea, we call it life in the slow lane. They live a long time, and they don’t make very many babies,” said Carlisle. “Most of these deep sea animals are the poster children of things you don’t want to fish for because they just don’t have the ability to reproduce very quickly and rebound after being exploited.”
Read the full study here: https://www.nature.com/articles/s41598-021-89903-z