Ocean Warming Threatens Baby Sharks in the Great Barrier Reef

Researchers found the hatchlings of the egg-laying epaulette shark are weakened by rising sea temperatures

An epaulette shark (Hemiscyllium ocellatum) photographed in captivity at the Adventure Aquarium in Camden in 2009.
Warming waters cause the sharks to hatch early and underdeveloped, making them vulnerable to predation. bbcjk.king via Flickr under Creative Commons Attribution 2.0 Generic license

In the face of climate change, epaulette sharks (Hemiscyllium ocellatum) are tough. They can withstand several threats caused by global warming. Falling oxygen levels, for example, are no match for the shark because of its ability to survive without oxygen for long periods of time. Combined with their buff pectoral fins, this oxygen-free trick actually gives them the astounding ability to leave the water where they can breathe behind and "walk" on land.

But one symptom of climate change is giving the critter a run for its money: ocean warming. When faced with warmer waters, epaulette shark babies emerged from their eggs earlier and weaker than normal, according to a study published this month in Nature.

Researchers put the sharks to the test under controlled settings designed to simulate increasing ocean temperatures expected by the end of the century in the Great Barrier Reef where the sharks live. The Great Barrier Reef's ocean temperature is usually 80.6 degrees Fahrenheit, but February 2020 was the hottest month on record, with some parts of the region reaching five degrees Fahrenheit above average, reports the New York Times.

When placed in water at 87.8 degrees Fahrenheit, epaulette shark embryos ate their yolk sacs faster and emerged weaker and smaller than average.

"The hotter the conditions, the faster everything happened, which could be a problem for the sharks," lead author Carolyn Wheeler, Ph.D. candidate the University of Massachusetts and James Cook University in Australia, tells Graham Readfearn of the Guardian.

In the wild, female epaulette sharks will lay eggs and leave them unprotected until they hatch after four months, reports Amy Woodyatt for CNN. During this time, the shark eggs are enclosed in an egg case, sometimes called a "mermaids' purse." Egg cases are often vulnerable to being eaten by other sharks or larger fish.

Some sharks face up to 90 percent mortality at the embryonic stage, according to past research. For pups that survive long enough to hatch, they usually emerge fully developed and strong enough to swim. So, in theory, faster development as embryos and an earlier hatch date would mean less time spent in an unprotected egg case. left vulnerable to predators. But for epaulette sharks, hatching early is not beneficial to survival.

When the baby sharks in the study hatched ahead of schedule, Wheeler and her team put their fitness to the test with “chase trials,” which consisted of a researcher nudging the shark’s dorsal fin every three minutes, reports Barbara Moran for WBUR. These trials showed the pups would likely be weakened, tired, and unable to hunt for food successfully in the wild.

As of 2015, the epaulette shark has been listed as "least concern" on the ICUN Red List, but researchers are concerned the shark's plight during development in their experiment signals danger to other shark species.

"If this shark is having trouble coping with ocean warming conditions, that's going to be a huge problem for shark species that are less tolerant and not as robust to changes in their environment." co-author Jodie Rummer, marine biologist at James Cook University, Australia, tells Tobi Loftus and Chloe Chomicki of Australia’s ABC News. The epaulette shark's ability to adapt and survive in other global warming threats have made it the perfect organism to study how other sharks would survive against similar threats.

Other sharks are adapting to warmer waters by migrating to cooler coasts, however. Bull Sharks forgo their migration to Floridian waters and instead stay in North Carolina estuaries to give birth. This move resulted in a spike in the bull shark population. The effects of this migration are still being studied possibly affect ecosystems that initially did not house the bull shark.

"Sharks are important predators that keep ocean ecosystems healthy. Without predators, whole ecosystems can collapse, which is why we need to keep studying and protecting these creatures," Wheeler tells CNN.

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