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As oceans' CO2 level rises, urchins demonstrate evolution in action

SANTA BARBARA, Calif. — The violet bottom-dwelling, prickle-backed spheres wriggling in the tank in Gretchen Hofmann's lab aren't really known for their speed.

But these sea urchins adapt so quickly they're helping answer a question that's key to understanding ocean acidification:

As fossil-fuel emissions disrupt marine life, will evolution come to the rescue?

Like Darwin's finches or Great Britain's peppered moths, these hedgehogs of the sea increasingly embody nature's stunning capacity for resilience.

A number of plants and animals threatened by souring seas, including some mussels, abalone, rock oysters, plankton and even a few fish, appear likely — at least at first — to adjust or evolve. But few seem as wired as these saltwater pincushions to come through the next several decades unscathed.

Yet work with urchins, as well as other species, suggests that acidification sooner or later may still push these and other marine organisms beyond what they can tolerate.

"Evolution can happen, and it can happen quickly," said Hofmann, a marine biologist at the University of California, Santa Barbara, who has studied urchins for years. "But concerns about extinctions are very real and very valid. Biology can bend, but eventually it will break."

The oceans are absorbing a quarter of the carbon dioxide emitted by burning coal, oil and natural gas. That, researchers say, is causing sea chemistry to change faster than it has for tens of millions of years.

Which plants and animals can accommodate these more corrosive seas — and for how long — will depend on many factors, from where they live to their population sizes to the depth of stress they face from other forces, such as warming temperatures and pollution. Survival will vary species by species. Not everything will make it.

"This kind of change is not free; evolution is not a gentle sport," said Stephen Palumbi, an evolutionary ecologist at Stanford University, who also works extensively with urchins. "When evolution happens, it's because the unfit are dying. It's pretty brutal."

And that's when things work well.

• • •

In the late 2000s, commercial urchin fisherman Bruce Steele feared things would not go so well. And for good reason.

Urchins graze on algae, drive out kelp and are eaten by sea otters, sunflower stars and humans. Steele, a scuba diver, had made his living since the 1970s scooping the spiny delicacies off the seafloor to sell to sushi restaurants. But when he read a research paper about acidification, he saw right away what it could mean for his business — and for the ocean he loves.

"When you start knocking out the very bottom of the food chain, it's incredibly terrifying," Steele said. "But that's what the research is showing us."

Increasing CO2 not only makes oceans more corrosive, it reduces carbonate ions, which everything from scallops to crabs, coral and sea urchins need to build shells or skeletons. So Steele dialed up Hofmann, his local university expert on spiky echinoderms.

"She thought it was a crank call or something, because, I don't know," Steele said. "I guess people figure a sea urchin diver's not going to be reading a whole bunch of science."

Hofmann dismissed Steele at first, but quickly called back and started investigating his concerns. She exposed sea urchin larvae to high-CO2 water and made a troubling discovery: Their bodies often got smaller.

"Overall, their body size really matters in how well they swim and how much food they get," Hofmann said. "So if you're smaller, it's really bad news if you're a baby sea urchin in the water."

But Hofmann noticed something else, too: Some larvae didn't change at all.

"When we started raising our babies in the lab, we saw that some of them shrank," Hofmann said. "But, in fact, some of them didn't. There were some in there that didn't respond the same way."

Hofmann knew enough about genetics to know that distinction might prove important.

• • •

In the century after Charles Darwin returned from the Galapagos carrying birds with different-shaped beaks, these finches came to represent the power of natural selection. As the birds expanded to areas with new foods, variations in their genetic code allowed new traits to emerge.

Such selection can be simple, elegant and fast. In Manchester, England, a common tree moth evolved from mostly speckled ivory to black in just decades. Soot from the Industrial Revolution had killed lichen and darkened local trees, which scientists believe allowed birds to more readily pick off the lighter insects. Once pollution was controlled, the tree trunks grew light again, and ivory-colored moths returned to dominance.

Hofmann suspected variations in urchin DNA left some predisposed to handle acidified seas. Nature, quite by accident, had been preparing a long time for this very moment.

It was all about the water. Water chemistry close to shore is rarely static. Ocean CO2 can vary with the time of day or the tides, when plants suck up CO2 to grow, or when animals die and decay.

The change is more pronounced in Northern California and the Pacific Northwest. When heavy winds blow along shore, deep, cold water that naturally holds more CO2 suddenly wells up from the bottom and gets drawn toward the beach. That means some West Coast urchins have spent millions of years being exposed to high-CO2 waters.

In fact, upwelling is part of the reason Northwest oyster larvae were among the world's first-known victims of acidification. Because the water already was near the extreme edge of what oyster larvae can tolerate, when man-made acidification spiked it higher, billions of shellfish were wiped out.

So Hofmann and a colleague, UCSB evolutionary biologist Morgan Kelly, mated "wimpy" Southern California sea urchins — those experiencing less CO2 exposure — with hardier males from northern upwelling zones.

The result: northern animals passed on genes more resistant to acidified water.

"The progeny, the babies — the kids — of that father were much better at maintaining the size of their body and not succumbing to the stress of high-CO2 water," Hofmann said.

At the same time, Palumbi and other Stanford researchers reared urchins in water from Southern California and from high-CO2 zones in Oregon. They found the frequency of some gene sequences shifted in response to growing up under higher CO2.

Urchins, in both cases, were showing they could evolve.

"It was really kind of surprising, and a little bit on the hopeful side," Hofmann said.

The very upwelling phenomenon that makes the Northwest an acidification hot spot could actually help some species get through it.

But which ones? And for how long?

"The truth is we don't know," Palumbi said. "The experiments don't tell us how long it will take for them to reach their limit. And it doesn't tell us the price they'll pay."

As oceans' CO2 level rises, urchins demonstrate evolution in action 11/08/13 [Last modified: Friday, November 8, 2013 6:20pm]
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