Embryonic sharks will struggle to survive in a warmer ocean

Young sharks that develop inside of protective egg cases called mermaids’ purses will struggle to survive as ocean waters get warmer, according to a new study from the University of Manchester. According to study lead author Daniel Ripley, rising ocean temperatures are reducing the freeze response time that embryonic sharks use to keep from being eaten by predators. 

When an embryonic shark senses the electrical field of potential predators, it responds with a freeze response and stops moving to avoid detection. The longer an embryo can freeze, the greater its chances of evading predators.

For the investigation, Ripley compared the freeze response time of small spotted catshark embryos at water temperatures of 15 and 20 degrees Celsius. 

The study revealed that embryos exposed to a five-degree jump in temperature had a freeze response that was seven times shorter. This reduced response could have major consequences for embryonic sharks as temperatures continue to rise. 

The researchers noted that if warming means embryonic sharks will not be able to freeze as long, the number of sharks surviving to adulthood will decrease.

About 45 percent of shark and ray species lay their eggs inside of a mermaids purse, where the infants develop for around a year before they hatch.

“This study has shown that many shark and ray species may reduce in number owing to increased predation as the oceans warm,” said Ripley.

“It’s hard to say to say how exactly this will impact on the ocean ecosystem, but it’s fair to assume there will be a knock on effect; it’s a major problem which is likely to get worse.”

Ripley explained that many marine animals are cold blooded, which means that rising sea temperatures have important consequences for them.

“It’s widely accepted that the worlds’ oceans are likely to warm in the next 100 years. And according to the US Environmental Protection Agency, sea surface temperatures have been higher over the three previous decades than at any other time since 1880.”

“So the impact of rising ocean temperatures could be catastrophic on species of egg-laying sharks and rays, such as the Brown banded bamboo shark or the Thornback ray.”

“Shark embryos are already very vulnerable in their mermaids purses and our study suggests that one of their key survival strategies – freezing to hide from predators- may be significantly reduced by ocean warming.”

“Some species are already threatened, and others, we simply don’t know enough about their numbers. But ocean warming may further harm their conservation and survival.”

The study is published in the Journal of Conservation Physiology. 

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By Chrissy Sexton, Earth.com Staff Writer