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At one time, people thought that planktonic larvae floated passively through the ocean, drifting wherever the currents took them. That, after all, is how they got their name: planktos is Greek for wandering. But beginning in the early 1990s, biologists began to realize that many planktonic animals can detect differences in light, temperature, and salinity—and that they use those cues to navigate their world. Now, new research led by marine biologist Heidi Fuchs at Rutgers University in New Jersey shows that some planktonic organisms can also detect different types of motion in the water.
In her lab, Fuchs and her team reared two different sea snail species: eastern mud snails, which live in the intertidal where turbulence caused by crashing waves is high, and threeline mud snails, which live on the continental shelf where they are subject to the regular oscillations of passing waves.
In tanks designed to simulate the water movements in both regions, Fuchs and her colleagues tested how the larvae of both species reacted to deduce whether they were using water motion as a navigational aid.
They found that both snail species responded to turbulence from crashing waves by diving more frequently. Because the snails responded in a similar manner, they probably don’t use this type of water motion as a navigational aid. When snails in the earliest stages of life were put in conditions akin to passing ocean waves, both responded by swimming harder. But as the snails aged, their behaviors diverged. Older larval eastern mud snails started to ignore the waves. Threeline snail larvae, however, exerted as much as 40 times more propulsive force when they felt oscillating waves than they did in calm water.
Fuchs thinks that by reacting to how the water moves, these larvae can more effectively find the appropriate habitat for their transition into adulthood. In other words, the two types of larvae wind up moving in opposite directions within the water column when they’re exposed to passing waves, each one advancing toward its preferred adult habitat and away from the other. These contrasting responses help keep these otherwise similar species isolated to their own geographic areas.
“It’s a cool story with really big ramifications for how we think about larvae,” says marine ecologist Craig McClain, executive director of the Louisiana Universities Marine Consortium. “In the big wide open ocean, where we think everything is the same, these species cue on differences even at a very early life stage.”
Yet other work going on in Fuchs’s lab suggests that these simplified navigational mechanisms could backfire in the face of ongoing climate change. The threeline snail larvae’s tendency to cue on wave motion has served them well so far, but now changes in water temperature may be causing them to hatch earlier in the season, when waves are bigger and upwellings are weaker. This combination could cause them to wind up settling in shallower water that is far closer to the coast and far warmer than they need. “Their best strategy might be backfiring because of climate change,” Fuchs says.
Eventually, this basic navigational misfiring may cause the snails to settle in water that is too hot for them to survive. While faster swimming species, like fish, can overcome this, relocating entirely likely exceeds the rudimentary swimming skills of these tiny wanderers.