SEATTLE — It didn’t take long for researchers examining the tiny sea snails to see something amiss.

The surface of some of their thin outer shells looked as if it had been etched by a solvent. Others were deeply pitted and pocked.

These translucent sea butterflies known as pteropods, which provide food for salmon, herring and other fish, hadn’t been burned in some horrific lab accident.

They were being eaten away by the Pacific Ocean.

For the first time, scientists have documented that souring seas caused by carbon-dioxide emissions are dissolving pteropods in the wild right now along the U.S. West Coast, and damaging a potentially important link in the marine food web sooner than expected.

“What we found was just amazing to us,” said Richard Feely, a scientist with the National Oceanic and Atmospheric Administration’s Pacific Marine Environmental Laboratory, who helped collect the live samples. “We did the most thorough analysis that’s ever been done and found extensive impacts on marine life in the field from ocean acidification.”

This is the broadest and most detailed indication ever that acidification is already damaging native creatures in the wild. It raises many questions about whether other sea life, too, might already be harmed – directly by acidifying seas, or by subtle shifts in parts of the food chain.

“These changes are happening years earlier than we had projected,” said Nina Bednarsek, a research fellow with NOAA who inspected the pteropods to identify shell scarring. “It is really a first indication of what is going on in our ecosystem.”

Feely and others already had documented that sea chemistry in many areas off the West Coast, particularly in the Pacific Northwest and Northern California, was changing far faster than initially expected as oceans absorb more CO from fossil fuels. They also had shown that this chemical change already has killed nonnative Northwest oyster larvae.

Now, they’ve found severe shell damage on more than half of the pteropods they collected from waters near shore between Central California and the Canadian border. The findings were published Monday in the British Journal “Proceedings of the Royal Society B.”

The shell damage corresponds so precisely to where chemical changes have hit the marine world hardest – specific coastal hot spots in Washington and Oregon that NOAA scientists said they could clearly pinpoint the cause: atmospheric carbon dioxide.

As human activity emits more carbon dioxide from cars and power plants, about a quarter of it gets absorbed by the oceans, which lowers the pH of marine water. That change reduces the availability of carbonate ions, which creatures like oysters, mussels and pteropods need to build their shells.

These chemical changes have struck particularly hard in the Northwest. Here, when heavy winds blow in the right direction, deep, cold water wells up from the bottom and gets drawn toward the beach. This water already contains high carbon dioxide from natural processes. The addition of carbon dioxide from humans helps make it some of the most corrosive water found anywhere.

Because of this upwelling phenomenon, some pteropods near shore almost certainly saw shells dissolve even before the industrial revolution, the study says. But those incidents have doubled in the past several hundred years and could triple by 2050.

In fact, the amount of water in the top 300 feet of West Coast ocean that may be inhospitable to some shelled organisms has increased sixfold since the industrial revolution.

To calculate the shell damage, Feely and others back in 2011 gathered live pteropods from 17 locations during a research cruise and preserved them. Then Bednarsek, who had developed an elaborate technique for examining the tiny shells, spent months detailing the corrosion.

Monday’s study examined a vast section of water up and down the coast, from close to the beach to many miles out to sea, and found damage to marine snail shells here across a far greater area of water.

Outside experts said the discovery was significant.

Dave Mackas, a scientist who recently retired from Canada’s Department of Fisheries and Oceans after years of surveying pteropods, said Bednarsek’s findings were disturbing.

“I think she’s got a pretty ironclad case that damage to the shells and the extent of the damage is proportional to how corrosive the water is,” he said. “It’s an outcome you expected to happen eventually, but it’s surprising how severe it is so soon.”