Atlantic shellfish are going the wrong way

Some species can save themselves from climate change simply by moving. Although organisms that are adapted to Earth’s coldest climates may be left with nowhere to go in a warming world, those from temperate and tropical zones might find new homes in cooler regions: uphill for land species, in deeper water for aquatic ones, and toward the poles for both. Even if individual organisms, such as plants, can’t migrate under their own power, their population as a whole can still shift, as offspring dispersed in cooler directions are more likely to survive.

But just because cooler homes exist doesn’t mean species can or do reach them. Habitat destruction might obstruct their paths (see Physics Today, September 2019, page 16 ). Or warming might be too rapid for a species to keep pace. Now Rutgers University’s Heidi Fuchs and colleagues have identified yet another mechanism that not only blocks species from reaching cooler habitats but actually pushes them into warmer ones.

The Rutgers study concerned 50 species of bottom-dwelling invertebrates, such as the blue mussels in the photo, that inhabit the Atlantic Ocean’s continental shelf off the east coast of North America. The researchers’ analysis of 60 years of ocean surveys showed that many species’ ranges are shifting in the wrong direction: toward the warmer waters to the south and closer to shore.

Like plants, the marine species are mobile primarily between generations. Their adults are mostly or entirely sessile. The newborn larvae can swim, but not strongly, so they drift at the mercy of the current for a few weeks before settling into their permanent homes.

The current patterns along the northwest Atlantic continental shelf haven’t been altered much by climate change—at least not yet. But they do vary with the seasons just as they always have. Seasonal patterns in weather and in river discharge result in a southward current along the continental shelf that’s strongest in the spring. Currents at other times of year are gentler.

Bottom dwellers usually spawn in the summer, when food for the larvae is plentiful. But the warming oceans may be confusing them into reproducing up to a month too soon—early enough for the larvae to get caught in the strong springtime current. And the cycle reinforces itself: As populations are pushed further south, they spawn earlier still.

Continuation of that pattern may spell doom for the populations. Transplanting the organisms back to cooler regions would be difficult, expensive, and only a temporary solution. If there’s good news to be had, it’s that the mechanism Fuchs and colleagues identified is specific to the geography of the northwest Atlantic. Blue mussels also live in the waters off Europe, which, as far as anyone knows, are not affected. (H. L. Fuchs et al., Nat. Clim. Change, 2020, doi:10.1038/s41558-020-0894-x .)