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As the world warms and climate zones shift, species that are able may alter their range to try to keep themselves at a comfortable temperature. Although the oceans are generally warming more slowly than the land, migrations of marine species are well documented already. For organisms like fish, mobile adults can dynamically track suitable conditions. But many seafloor-dwelling critters primarily move in their wandering youth—as larvae that (mostly) passively ride the currents. Changing your destination isn’t so easy when you’re not in the driver’s seat.
Unfortunately, some of those species have been seen migrating the wrong way, toward even higher-temperature waters rather than away from them. We’ve seen this happen in the coastal northwest Atlantic, including in some commercially harvested species like clams. Seeing these species shift requires something unusual, since they're adapted to their current-catching lifestyle. So what gives?
A team led by Heidi Fuchs at Rutgers University wanted to test the hypothesis that earlier spring warmth could explain things. These organisms take their spawning cue from warm temperatures, so long-term warming can push that springtime cue earlier and earlier. If the currents are different in early spring, that could lead to larvae drifting toward new locations simply because they’re ahead of schedule.
To look for this activity, the researchers used species location data going back to 1950 for seafloor invertebrates between North Carolina and Nova Scotia. Most species shifted their range—a few kilometers per year—southward along the coast while also moving upslope to shallower water. About two-thirds of species have seen the average temperature of their range increase. Critically, their ranges have warmed faster than the water—because they’ve migrated the wrong way.
If you estimate the timing of spawning each year, the trends make more sense. The researchers calculated the timing that waters rose above temperature thresholds for spawning behavior, finding that each is generally being crossed a couple days earlier per year. And comparing these sliding dates to currents shows that larvae should be drifting farther if they spawn earlier. In this region, spring winds drive a stronger current southward along the coast. At the same time, greater flow from rivers discharging freshwater into the sea also boost that southward current.
Some of these species have responded a little differently simply because their spawning behavior is less sensitive to temperature. The researchers note that clams and mussels, for example, spawn at a pretty low temperature and very clearly fit the wrong-way migration profile. But scallops spawn within a wider temperature range and haven’t migrated much at all.
Because these species are migrating the wrong way, their ranges are effectively shrinking. It’s like being trapped on a conveyor belt that is pushing you toward an approaching danger.
This all depends on the currents and trends in a specific place, so it won’t have the same effect everywhere. But at least in some places, climate change can both create the need for a species to migrate and prevent it from doing so.