Why Noah's ark does not work

The strategy of a Noah's Ark will fail. In the most brutal sense, this is the conclusion of a first study of its kind that shows which marine species have the ability to survive in a world of increasing temperatures and oceans becoming acidic.

Remnants two out of two, even of medium size, may have little chance of persisting on a planet with a modified climate. For many species, "we will need large populations," says Melissa Pespeni, a biologist at the University of Vermont, who led the new study on the reaction of hundreds of thousands of sea urchin larvae to experiences where their seawater was moderately or extremely acidic.

The study was published on June 11, 2019 in the journal Acts of the Royal Society B.

Rare relief

Pespeni and his team were surprised to discover that the rare variations in the DNA of a small minority of sea urchins were very useful for survival. These rare genetic variants are "a bit like having a winter coat among fifty light jackets when the weather is under twenty in Vermont," says Pespeni. "It's this coat that allows you to survive." When water conditions were made extremely acidic, the frequency of these rare variants increased in the larvae. It is these genes that allow the next generation of sea urchins to alter the functioning of various proteins, such as those used for making their hard but easy-to-dissolve shells and managing the acidity of their cells.

However, maintaining these rare variants in the population, plus another necessary genetic variation, more common and allowing to react to a range of levels of acid in the water, requires many individuals .

"The larger the population, the more variation will be rare," said Reid Brennan, a postdoctoral researcher at UVM's Pespeni Laboratory and lead author of the new study. "If we reduce the population size, then we will have less fodder for evolution – and less likely to have the rare genetic variation that could be beneficial."

In other words, some organisms may persist in a climate-change world because they are able to change their physiology – think about sweating more; some may migrate, perhaps further north or upstream. But for many others, their only hope is to evolve, saved by the potential for change that is waiting for rare segments of DNA.

Quick adaptation

The purple sea urchins that the UVM team studied in their Vermont lab are part of natural populations that extend from Baja, California to Alaska. Located in the rocky reefs and kelp forests, these pungent creatures are a favorite snack for sea otters – and a key species to shape life in the intertidal and subtidal zones. Because of their large numbers, geographic extent and diversity of living conditions, sea urchins exhibit "permanent genetic variation," scientists noted. This makes purple sea urchins likely to survive in the rough future of an acidified ocean – and good candidates to understand how sea creatures can adapt to rapidly changing conditions.

It is well understood that rising global average temperatures is a fundamental factor in the imminent extinction that is facing a million species or more, as indicated by a recent report by the US government. UN on biodiversity. But it's not just the averages that are increasing. These are perhaps the hottest moments – or the most acidic ones – that test the limits of an organism and control its survival. And, as the UVM team writes, "the genetic mechanisms allowing rapid adaptation to extreme conditions have rarely been explored".

Currency in the current sea

The new study used an innovative "one-generation breeding" experiment that started with 25 adult sea urchins caught in the wild. Each female produced approximately 200,000 eggs from which scientists were able to extract pool DNA from approximately 20,000 surviving larvae living in different water conditions. This huge number of individuals has clearly shown scientists that purple sea urchins have a genetic heritage that allows them to adapt to extremely acidic seawater. "This species of sea urchin will perform well in the short term, they can respond to these low pH conditions and have the genetic variation needed to evolve," says Reid Brennan of UVM. "As long as we do our part to protect their habitats and maintain their important populations."

But facing the fierce challenge of rapid climate change can be very expensive. "We hope that the evolution will occur and it is surprising and exciting that these rare variants play such a powerful role," said Melissa Pespeni, assistant professor in the UVM biology department and expert on ocean ecosystems . "This discovery has important implications for the long-term persistence of species, and these rare variants are a kind of currency that children have to spend," she says. "But they can only spend it once."