These seemingly oblivious shellfish are highly sensitive to sounds, which could help them monitor incoming tides, hear thunder and spot approaching predators
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Oysters are the latest creatures shown to “hear” the world around them. It suggests that hearing might be commonplace in molluscs and other simple marine creatures, so the impact of noise pollution from ships and exploration might be far greater than currently assumed.
Jean-Charles Massabuau at the University of Bordeaux in France and his colleagues placed 32 Pacific oysters (Crassostrea gigas) in tanks and exposed them to a range of sounds of varying loudness with frequencies between 10 and 20,000 hertz. Oysters tend to close their valves when stressed or threatened, so they were fitted with accelerometers to track whether they moved.
The team found the oysters responded to frequencies between 10 and 1000 hertz. Massabuau says they don’t hear as we would, but instead probably perceive the vibrations created by sound waves using an organ that registers movement and vibration: the statocyst.
There are all sorts of potential advantages to being able to hear, says Massabuau. For instance, the oysters might hear the arrival of tidal water, which carries food. “Our results show that in shallow waters, they must be able to hear breaking waves and water currents,” he says. They could then open up ready for the tide’s arrival.
Thunderstorms would be audible too, which could explain previous observations that oysters spawn during thunder and lightning. “It could be a trigger to synchronise spawning,” says Massabuau.
Oysters might also hear the sound of currents created when predators approach. “Lobsters or fish, which feed on young oysters, produce sounds in the oyster hearing range, if they’re close enough,” says Massabuau.
The oysters were most sensitive to low frequencies between 10 and 200 hertz. These are typically produced by shipping, exploration with explosives, seismic surveys, pile-driving and wind turbines.
“All these noises can muddle the normal oyster sound landscape,” says Massabuau. As a result, they may mistake ship noise for tides or thunderstorms and open up or spawn at the wrong time, he says. The noise might also mask the approach of predators.
Massabuau says people tend to think about underwater noise pollution solely as a threat to cetaceans like whales. “What we show here is that the problem is possibly a very real one for many more animals than we imagined,” he says.
“The findings in oysters confirm that, potentially, any marine organism that perceives sound can be negatively affected by human-produced sound sources, implying that ocean noise pollution is a global issue that concerns the whole food web,” says Michel André at Barcelona Tech. In 2011, he discovered that shipping noise pulps the sound-sensing organs of squid and octopuses. “The challenge is to determine if the noise and vibration is causing harm to the organisms,” says Mike Elliott at the University of Hull, UK. His team previously discovered that hermit crabs and marine mussels can sense sound vibrations.
“If mussels and oysters keep their shells closed because of the vibration from human-produced noise, then they may be unable to feed, leading to starvation, poor reproduction and other impacts.”
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