Declines in shellfish species on rocky seashores match climate-driven changes

Two decades of data from a study of Swan’s Island in Maine co-led by biologists Peter Petraitis of the School of Arts & Sciences and Steve Dudgeon of California State University, Northridge, document a slow and steady dwindling of mussels, barnacles, and snails.

snails and barnacles on a rock on the seashore
Dogwhelks feed on barnacles on the shores of Swan’s Island. New research documents slow and steady declines in these and other intertidal species that make up an important part of the area’s food chain. Climate change is a suspected culprit. (Image: Jonathan A. D. Fisher)

The waters of the Gulf of Maine are warming faster than oceans almost anywhere on Earth. And as the level of carbon dioxide rises in the atmosphere, it’s absorbed by the oceans, causing pH levels to fall. Ocean acidification makes it difficult for shellfish to thicken their shells, their primary defense against predators.

In a new study in the journal Communications Biology, researchers Peter Petraitis, a professor emeritus of biology in Penn’s School of Arts & Sciences, and Steve Dudgeon, a biology professor at California State University, Northridge, who completed a postdoctoral fellowship with Petraitis at Penn in the 1990s, show that the changing climate is taking a toll on Maine’s sea life. A dataset collected for two decades, including numbers of five species of mussels, barnacles, and snails, shows that all have been experiencing declines—some slow, some more rapid—in part owing to climate change.

“These species are often overlooked because of how common they are,” say Petraitis, who taught at Penn for 34 years. “They’re just everywhere across the rocky shores. People don’t think anything is going to happen to them. If they decline by about 3% a year that’s a relatively small change, so you might not notice it for a while. But one year, people are going to suddenly look around and say, ‘Where are all the snails, mussels, and barnacles?’”

These species “form the core of an iconic food web” in the Gulf of Maine, says Dudgeon. “Concurrent declines of five species, including both native and non-native, is proportionally large and may cause profound changes in the ecology of coastal oceans in the region.”

In the 1990s, Petraitis and Dudgeon set up a long-term experiment on Swan’s Island in the Gulf of Maine to study the ecological principles of multiple stable states. A focus of Petraitis’ research and the subject of his 2013 book, “Multiple Stable States in Natural Ecosystems,” the concept encapsulates the idea that an ecosystem can switch quickly between entirely different compositions of organisms, given the right environmental perturbations. 

For shellfish on Swan’s Island, one such perturbation occurs when periodic powerful winter storms cause sea ice to scrape off all the organisms attached to rocks on the shore, forcing the communities to rebuild from scratch the next year. 

In 1996, Petraitis and Dudgeon simulated a single massive ice-scouring event by scraping the rocks to see what would happen as the shore recolonized. Since then the researchers have been making an annual trip to their 60 study plots on Swan’s Island, counting the incidence of organisms living not only in the scraped areas but also in areas left in their natural state, the control plots.

The current work took advantage of these control plot counts, looking at five common shellfish species: the tortoiseshell limpet (Testudinalia testudinalis), common periwinkle (Littorina littorea), dogwhelk (Nucella lapillus), blue mussel (Mytilus edulis), and barnacle (Semibalanus balanoides). 

“We didn’t expect to see much change in the control plots,” says Petraitis, “but we were surprised to see these populations declining.”

Using abundance data from 1997 to 2018, the researchers found that very young mussels were in the sharpest free fall, declining almost 16% a year, while the other four species were dwindling by 3-5% each year. During that time period, limpets, periwinkles, and dogwhelks declined in total number by 50%, contractions the researchers describe as “sobering.”

Scientist in winter gear stands on seashore next to large masses of ice
Severe winter storms can bring massive amounts of ice to the shorse of Swan’s Island. Peter Petraitis (above) and Steve Dudgeon’s study mimics how this ice can scour rock surfaces of their animal life, requiring ecological communities to build up again from scratch. (Image: Bazartseren Boldgiv)

To get at the question of why, the researchers looked to data on ocean temperature and chemistry. They found that the downward trajectory of mussels and common periwinkles matched up with increasing summer ocean temperatures collected from a nearby buoy. 

Meanwhile declines in populations of limpets and dogwhelks corresponded with increases in the aragonite saturation state, a measurement that tracks with ocean pH. This was unexpected, since lower levels of aragonite saturation are associated with more acidic ocean waters, which make it harder for shellfish to build up their shells. “This may be indicative of other conditions at nearshore areas that vary with aragonite saturation state,” Petraitis says. 

Changes in barnacle numbers did not correspond with changes in ocean temperature, pH, or aragonite saturation state, suggesting other factors are at play in their decline.

All five of these species play critical ecological roles in the Gulf of Maine. 

As filter feeders, mussels and barnacles remove phytoplankton from the water column, “digesting them, pooping them out, and fertilizing the shore,” Petraitis says. Limpets and periwinkles feed on algae and seaweed, so smaller numbers could lead to algal blooms and “greener” nearshore areas.

Since all five species serve as prey for a variety of animals, shrinking populations will reverberate up the food chain, affecting humans as well.

“Without animal consumption transferring organic matter up the food web,” says Dudgeon, “production in coastal oceans will be increasingly shunted directly through pathways of decomposition by microbial organisms, rather than to support populations of species that humans fish and on which coastal economies depend.”

Petraitis also notes the common periwinkle, now emblematic of the coast, was introduced to the Gulf of Maine from Europe sometime in the middle of the19th century. “Now it’s the most common grazer on the shores; they feed like goats,” he says. “Before 1860, the shore without periwinkles probably looked a lot greener than it does now. As they decline, we may see the shore revert back to its state in the 1850s.”

While presenting these findings at conferences in the last couple of years, Petraitis says he’s heard anecdotes from other scientists about similar disappearances of mussels across the North Atlantic, suggesting the phenomenon is not isolated to the Gulf of Maine.

Snails and barnacles on a rock surface
Common periwinkle sea snails feed on a microscopic algal film dwelling on the surfaces of rocks and attached barnacles. The new study linked common periwinkle declines to warming ocean waters. (Image: Courtesy of Peter Petraitis) 

Peter Petraitis is a professor emeritus of biology in the Department of Biology in the School of Arts & Sciences at the University of Pennsylvania.

Steve Dudgeon is a professor of biology at California State University, Northridge.

The study was supported by the National Science Foundation (grants OCE-9529564, DEB-0314980, DEB-1020480, and DEB- 1555641.) 

Video: Courtesy of the National Science Foundation