Even without ears, oysters are “clamming up” when they hear too much noise in the ocean.
In response to sounds similar to cargo ships, oysters slam their shells closed, seemingly to protect their soft bodies, according to a study published Wednesday in PLOS ONE. Oysters are filter feeders, so noise pollution in the ocean may stunt growth and reduce water quality, the scientists argue.
Ocean noise pollution is a known problem for many marine mammals, which use their hearing for survival tasks like navigation and finding food. But little is known on how sound affects invertebrates, which account for the largest number of animals in the sea.
A few years ago in a bustling Spanish port, University of Bordeaux physiologist Jean-Charles Massabuau came across an underwater filmmaker. As a large cargo ship crossed the water, the filmmaker surfaced and said, “Wow, I never heard such a noisy spot,” Massabuau recalled. Man-made sounds such as offshore drilling, seismic testing for deep sea oil, and even the hum from that Spanish cargo ship permeate the ocean at ever-increasing levels.
Massabuau’s research involves learning how changes in light, temperature or salinity affect oysters. So after his exchange with the diver, Massabuau wondered, “Can the oysters hear it?”
Back in the lab, his team affixed accelerometers to thirty-two oysters to detect when their shells were open or closed. An oyster’s shell position is linked to its well-being. An open shell indicates a relaxed state, while shutting is a marker for stress.
Massabuau lowered the animals into two tanks replete with food, currents and seawater pumped from the Bay of Arcachon, France. With an underwater speaker in one of the tanks, he played a variety of sounds, including low frequencies below 200 Hertz that are typically produced by cargo ships.
Massabuau found that the oysters rapidly closed their shells with sound frequencies between 10 to 1000 Hertz. He likens an oyster’s reflexive shutting to the sharp shrug that humans do when startled by an unpleasant sound.
“They are aware of the cargo ships,” Massabuau said. “What is for sure is that they can hear. The animals can hear these frequencies.”
Many marine organisms can detect vibrations like ones produced by predators. But most definitions of hearing require an organ capable of sound perception, said University of Hull marine biologist Mike Elliott.
Oysters don’t have ears like humans, but hair cells similar to ones in the inner ear are found on the gills. These cells sense vibrations, Massabuau said, so whether people call it “hearing” or “sensing sound vibrations” makes little difference to him.
Elliott, who was not involved in the study, has conducted research similar to Massabuau’s, but with hermit crabs and mussels. Elliott said when these animals become stressed and hide inside their shells, they stop feeding and breathing, “and sooner or later they start suffering.” But Elliott said it remains unclear if sound pollution can harm these organisms in the long run.
“It is quite a big leap from detecting a response [to sound] to if the animal is being harmed by it,” Elliott said. “The big challenge is converting this into a response that denotes harm to the organism.”
Massabuau agreed. His lab is investigating if chronic exposure to unnatural sounds can disturb the growth rates of oysters. He reports signs of slow growth rates, indicating poor health in a study in the process of publication.