Understanding cookiecutter sharks
University of Delaware examines the diet and habitat of the elusive Cookiecutter shark

Date:
August 23, 2021
Source:
University of Delaware
Summary:
A little understood species of shark, known for taking cookie
cutter- shaped bites out of everything from white sharks and whales
to the rubber coated sonar sensors on submarines and even underwater
electrical cables, is the subject of a new study. While the deep
sea dwelling Cookiecutter shark will take a chomp out of anything
it encounters in the upper reaches of the ocean, it really feasts
on crustaceans, squid and small fish found in their habitat.



FULL STORY ==========================================================================
For years, researchers studying marine life in the wild would occasionally
come across animals -- such as dolphins, swordfish, leatherback sea
turtles, whales, white sharks and even humans -- with oddly shaped plugs
of tissue taken out of their bodies. Those fresh bites and scars were
almost like someone took a cookie cutter and surgically removed a hunk of tissue. These bites were not only restricted to animals, as submarines
in the 1970s and 1980s were having their rubber coated sonar sensors
bitten in this same fashion and underwater electrical cables were also
found to have the odd-shaped bites.


========================================================================== Eventually, it was discovered that the culprit was a small shark that
is distributed throughout the world's tropical and subtropical oceanic
waters named the Cookiecutter shark (Isistius brasiliensis). While these
sharks are widely distributed throughout the world, and may be one of
the more common sharks in the ocean, because they live in the deep sea,
and are never held in captivity there is little known about them --
especially when it comes to their eating habits.

A new study led by the University of Delaware's Aaron Carlisle has
uncovered the potential diet and habitat of these Cookiecutter sharks,
showing that while they might chomp on everything they can get their
jaws on in the upper reaches of the ocean to supplement their diets,
they primarily feed on the little critters they share a habitat with
such as crustaceans, squid and small fish.

Carlisle, assistant professor in the School of Marine Science and Policy
in UD's College of Earth, Ocean and Environment, said that for years, researchers assumed that Cookiecutter sharks were coming up at night,
feeding on whales and bigger animals, and then heading back to the deep
ocean and hanging out during the day. But it was an assumption. The
feeding habits of Cookiecutters have been little studied -- Carlisle
said there have been maybe 150 Cookiecutter stomachs studied around the
world over the last 50 years.

When researchers did study the sharks, they could only look at what was physically in the sharks' stomachs -- which sometimes, was nothing at
all - - and make inferences from the bites they had seen on their larger species, but they could not employ the more advanced scientific methods
now available to researchers.

"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." Collaborative research For this study,
the researchers used 14 Cookiecutter sharks that were collected by
the Monterey Bay Aquarium from the Central Pacific around Hawaii. They
utilized a variety of biochemical tracing techniques -- including stable isotope analysis, fatty acid analysis, and environmental DNA -- to help
better understand the feeding habits and habitats of the species.

John O'Sullivan, a coauthor on the study and the director of collections
at the Monterey Bay Aquarium which is planning to open a new deep-sea
exhibit featuring live deep water animals next year, said that by
utilizing a number of different biochemical tests in the study, the
researchers were able to get well-developed answers to their questions
about the Cookiecutter's diet and possible behavior.

"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."
The researchers examined the sharks' stomachs and found that they were
mostly empty. While that would stump researchers in the past, by utilizing these modern techniques, they could extrapolate the potential diets of
the sharks.



==========================================================================
For instance, using the environmental DNA of their stomach contents
could give them an idea of what the sharks had eaten.

"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.

Habitat patterns The researchers were also able to make inferences about
their habitat.

The Cookiecutter sharks have generally only been observed near surface
waters during the night, leading researchers to believe that they exhibit vertical migration, where they ascend to shallow waters at dusk and
return to deeper waters at dawn. However, this might only be the case
for the larger Cookiecutter sharks, whereas the smaller, baby sharks
might not vertically migrate at all.

"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 to advance the knowledge of these deep-water 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." O'Sullivan said that he is hopeful that through efforts like this paper,
as well as the new deep sea exhibit at the Monterey Bay Aquarium, they
can help to spread awareness of the importance of these creatures to
the general public.

"It's about doing good science and about making wise choices," said
O'Sullivan.

"It's about good animal welfare, and it's about engaging
the public. The best science, no matter how exciting
it is, if the public can't get engaged, it falls short." ========================================================================== Story Source: Materials provided by University_of_Delaware. Original
written by Adam Thomas.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Aaron B. Carlisle, Elizabeth Andruszkiewicz Allan, Sora L. Kim,
Lauren
Meyer, Jesse Port, Stephen Scherrer, John O'Sullivan. Integrating
multiple chemical tracers to elucidate the diet and habitat
of Cookiecutter Sharks. Scientific Reports, 2021; 11 (1) DOI:
10.1038/ s41598-021-89903-z ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210823125805.htm

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