High CO2 levels

Global warming is also affecting sharks and rays

High CO2 levels have a negative impact on biodiversity.

(Bild: APA-PictureDesk/VALERIE MACON / AFP / picturedesk.com)

In the 450 million years that sharks and rays have inhabited the oceans, they have repeatedly benefited from higher temperatures. However, global warming is currently affecting these animals, which are already threatened by overfishing and habitat loss: Because the change is happening too quickly, it is unlikely that the new environmental conditions will benefit them, according to Viennese paleobiologists and colleagues in the journal "Biology". They analyzed which environmental factors are decisive. 

More than 1,200 shark and ray species are currently alive, over a third of which are acutely threatened by overfishing and the destruction of their habitat. The diversity of species of these cartilaginous fish was influenced by a series of events during the Mesozoic era (252-66 million years ago).

Fossil shark and ray teeth document species diversity
An international team of researchers led by Manuel A. Staggl and Jürgen Kriwet from the Institute of Palaeontology at the University of Vienna has investigated which driving forces played a role and how global warming affects their diversity. They used data on climate fluctuations during the Jurassic period (200-143 million years ago) and the Cretaceous period (143-66 million years ago) and estimated species diversity based on fossil shark and ray teeth.

(Bild: Carol Buchanan - adobe.stock.com)

The researchers identified three decisive environmental factors: in the past, higher temperatures, a rise in sea level and more shallow coastal waters due to a strong splitting of the land masses had a positive effect on species diversity. This led to a larger habitat and year-round stable conditions in tropical and subtropical regions that extended far to the north and south. Thanks to their adaptability, cartilaginous fish were able to colonize these stable and complex ecosystems with great biodiversity quickly and efficiently.

Negative effects of high CO2 levels shown for the first time
"For the first time, we showed that higher atmospheric CO2 levels have a negative impact on the diversity of cartilaginous fish," Kriwet told APA. This is because high levels of CO2 in the atmosphere also increase the CO2 content in the oceans, which leads to acidification of the seawater.

"Until now, we have always assumed that this acidification does not affect vertebrates that much," said the palaeobiologist. This may indeed apply to bony fish, "but it appears that cartilaginous fish, which do not have a bony skeleton, may experience changes in embryonic development, among other things". Laboratory tests have provided corresponding evidence of the direct physiological effects of higher CO2 concentrations in the water.

Mechanisms not yet fully understood
However, the exact mechanisms behind the negative effect of CO2 on the biodiversity of sharks and rays have not yet been fully clarified, Staggl said in a press release. The extremely warm climate 80 to 65 million years ago, combined with extremely warm seas and high water levels, led to an enormous increase in the diversity of sharks and rays. Why high CO2 levels apparently did not play a role back then still needs to be clarified.

Due to the negative effects of higher CO2 levels in the oceans coupled with the rapid change in ecosystems caused by the rapid rise in temperature, the researchers believe it is "unlikely" that sharks and rays will benefit from the current global warming. For this reason, "urgent measures are needed to protect them. This is not just about the cartilaginous fish, but about the preservation of entire ecosystems. "Because without the top predators, ecosystems would collapse," emphasized Kriwet in a press release.