22 Jun.
2021

Ocean warming could hit shark survival

Infant sharks that live in the familiar mermaids’ purses found on most beaches in the UK and throughout the world are more vulnerable to predation because of ocean warming, new research suggests.

According to Daniel Ripley from The University of Manchester, higher temperatures reduce freeze response times which the animals employ to avoid being eaten by predators.

The study by the ecophysiologist is funded by the Biotechnology and Biological Sciences Research Council and The University of Manchester’s Knowledge and Innovation hub for Environmental stability.

It is published in the Journal of Conservation Physiology today (17 June).

If an embryo employs a freeze response, it stops moving so that predators - including large fish and other sharks - won’t detect them.

That explains why being able to elicit a freeze response is key to surviving predation during embryonic development – and the longer an embryo can freeze, the better chance it has of not being detected by predators.

In the lab Ripley compared the freeze response time of small spotted catshark embryos - which are 7 to 8cm long - at a water temperature of 15C and a water temperature of 20C.

The 5C temperature rise resulted in a 7-fold decrease in the time the animals froze following a predator simuli, mimicked by gently flicking the egg case.

And that could have major consequences for embryonic sharks in a warming world. Being able to freeze is key to avoiding predators and if warming means infant sharks will not be able to freeze as long, it could reduce the number of sharks surviving to adulthood.

Around 45% of shark and ray species lay eggs which grow inside a mermaids purse, which can last for around a year before they hatch.

The purses come in various colours, shapes and textures, depending on the species of shark.

Beachcombers often spot the empty shell cases on the beach, though the live egg cases often lie tangled up with sea weed in shallow waters and rockpools.

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

“It’s hard 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.

“Many marine animals are cold blooded so rising sea temperatures have important consequences for them.”

He added: “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.”

“Ocean Warming Impairs the Predator Avoidance Behaviour of Elasmobranch Embryos” is published in the Journal of Conservation Physiology and an embargoed copy is available.

 

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