A new study has found that blue mussels in the Northeast, including Lond Island Sound, are changing in response to warming seas, as they're building more porous shells.

Climate change is threatening the ability of shellfish to grow a dense shell, the study out of the American Museum of Natural History shows. As the climate warms, a complex stew of high temperatures, increasingly acidic oceans and other factors may be causing softer shells to grow on Mussels and other marine life.

Leanne Melbourne, a postdoctoral fellow at the museum, chose mussels to study because they are common along the Northeast U.S., and the museum had a strong collection of historical samples.

Melbourne compared mussel shells collected in the past to contemporary shells collected from the same locations. The mussels were gathered all over Long Island Sound, Massachusetts and near New York harbor.

"What we're seeing here is this increasing porosity through time," said Melbourne. "We're led to the point where we believe that the main driver is temperature."

She examined the shell thickness, weight, mineral layering, size and porosity of the shells. The shells collected from the early 1900s were significantly denser and less porous than those collected now.

The effect was most pronounced in places where the mussels were exposed to the biggest change in spring and summer ocean surface temperature since the early 1900s.

"The largest increase in temperature affects them while they're actually calcifying," said Melbourne. "In the winter they're not calcifying, they're not growing. So the change in temperature might affect their internal bodies ... but not the actual calcification of the shell."

Melbourne said that more porous shells are a concern because strong shells are what make mussels the environmental engineers that they are. Mussel beds and reefs serve as buffers against storms, and offer shelter for small fish. As the water warms, there may be a point in the future where the change in porosity may affect the structural integrity of the shells, making them weak or brittle.

"The idea would be to grow some mussels under extreme warming to see if the porosity keeps getting worse, and then do mechanical studies to see where the breaking point is," said Melbourne.

She said that depending on how warm the water got, blue mussels might be lost in parts of their habitat. Right now, blue mussels can be found from the Canadian Maritimes south to North Carolina. If the water grows too warm, that range may shift further north.

Veronica Padilla Vriesman a paleontologist and professor at Oberlin College who studies climate change's effect on mussels, said that as the sea warms, cold-adapted shellfish would be lost in certain areas and in some cases, hot summer and low tides may outright kill them from overheating.

"I've done a lot of my work on the west coast where we've seen pretty significant declines," said Vriesman. "You see this narrowing or thinning of mussel beds. Where they used to be expansive, now they're just a thin band throughout the most ideal zone."

She said that warming, ocean acidification and changes in local conditions can have multiple, competing effects on mollusk growth.

"If I had to characterize the impacts of climate change on bivalve growth ... I would say it's extremely variable," Vriesman said that it was still worth trying to pin it down so we could figure out which species in which areas were being hit hardest.

Vriesman said that mollusks, the family of animals that include mussels, oysters, snails and other shellfish, had survived multiple mass extinction events in the past. At the end of the Permian period, 251 million years ago, carbon dioxide levels spiked, the temperature rose and 90 percent of life on earth died.

Scientists aren't totally clear on what caused the Permian extinction. But mollusks, including the ancestors of mussels, survived and thrived afterward.

"Mollusks kind of took off because they were able to replace the organisms that had been wiped out," said Vriesman. "Now we live in a mollusk dominated marine world."

Vriesman said that it was hard to tell how the evolutionary history of mollusks would come into play during climate change. She said that at minimum, for mussels to survive they had to adapt faster than how quickly the climate was changing.

"The things I worry about are the rates of change," said Vriesman. "On top of that, the direction of the change can be variable when we're talking about climate change because variability is increasing ... that kind of unpredictability and increased variability isn't great for their capacity to adapt."