-- 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 lowly 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 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 as uni.

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 loved.

“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.