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The range of blue mussels has shrunk considerably as the ocean has warmed and changed how their larvae are dispersed by currents. 

Andrew J. Martinez/Science Source

Ocean warming has seafloor species headed in the wrong direction

As the world warms, many species of plant and animal will have to find new—often cooler—places to live. But things are trickier for sedentary marine creatures like snails, worms, and clams, according to a new study. It finds that in the Northwest Atlantic Ocean, many species are spawning earlier in the year, when currents take their larvae southward and into warmer waters—the wrong direction. For some of them, including the sand dollars beloved by beachcombers, this means their range is shrinking.

Earlier spawning is a serious threat. "For species that can't move effectively, it's going to enhance the likelihood that they're going to get really rare and potentially be driven extinct by climate change," says Steve Gaines, a marine ecologist at the University of California, Santa Barbara, who was not involved in the research. A key question, he says, is whether these species will likely evolve to spawn later or tolerate warmer water. And if they don’t, should biologists try transplanting them to more suitable environments?

Many biologists have assumed marine species can shift habitat fairly easily if their environment changes, but the new study shows things are more complicated for species distributed only via drifting larvae. "The big take-home message is, you can't just assume critters will spread to wherever the climate is congenial for them," adds James Pringle, a physical oceanographer at the University of New Hampshire, Durham, who was not involved with the new work.

This isn't the first study to find that some marine species are shifting their ranges into inhospitable habitats. But those findings have often been dismissed as “noise,” Gaines says. And there have been no explanations for the counterintuitive patterns, says Heidi Fuchs of Rutgers University, New Brunswick, who studies how marine snails—such as the threeline mud snail—spawn and how their larvae drift in currents along the Northwest Atlantic continental shelf.

Many relatively sedentary marine creatures release microscopic larvae, such as this threeline mud snail.

Heidi Fuchs

A few years ago, Fuchs noticed the range of a snail species had shrunk significantly over the decades, and another had disappeared from the outer part of the shelf. Digging up data for other bottom-dwelling marine invertebrates—known as benthic organisms—she found a similar, odd pattern among many species in the same area: Some of these cooler-water species were moving south, but overall their range was contracting because they were also moving from deeper to shallower water. Their new habitats tended to be warmer—and perhaps hazardously hot. "It was pretty clear that they were going in the wrong direction," Fuchs says.

But why? Fuchs knew benthic species tend to release their tiny larvae when the water warms to a certain temperature. That's usually in the late spring or early summer along the mid-Atlantic Ocean. Over decades, the average ocean temperature there has risen about 2°C. That means temperature thresholds are reached sooner in the year and species are releasing their larvae about a month earlier than before.

Fuchs teamed up with colleagues, including Rutgers oceanographer Robert Chant, to look at how ocean currents along the continental shelf change during the year. They realized the currents are faster in the early spring and slow down after that. If a species spawns too early, its larvae might be whisked too far down the coast.

The scientists checked to see whether this pattern was widespread along the mid-Atlantic coast. They gathered records of 50 species from a public database and mapped their ranges—including where and when they probably spawned. They then added information about how temperature has changed in each location over the decades. Finally, considering how changes in spawning time might affect larval transport, they calculated long-term changes in the range of the species.

Overall, they found that the extent of possible habitat increased for most species; as the ocean warms, cooler waters in the north become more habitable for species that live farther south. Yet species' actual ranges have shrunk by about 10% on average compared to the period spanning the 1950s to the 1980s, they report today in Nature Climate Change.

"It's pretty worrying that so many benthic species that used to be really abundant have disappeared from the outer shelf," Fuchs says. The species that are shrinking in distribution the most—anywhere from 30% to 50%—include the common sand dollar as well as the blue mussel, an economically important species and a keystone member of intertidal communities.

Fuchs and others note that it's hard to generalize these results to other parts of the world. What happens to the geographic ranges of sedentary species will depend on when they spawn and the patterns of regional currents. For the benthic species in the Northwest Atlantic Ocean, it's possible that some might adjust their spawning to a time when the currents are more favorable, or evolve to tolerate warmer water.

But if that appears unlikely, biologists might need to think about transplanting species to more favorable environments, Gaines says. “I think it really raises the question of whether we need to be thinking strategically about when and for what kinds of species do we do this kind of assisted migration.”