Shhhh! Baby oysters are always listening

We need oysters, and not just to improve our moods.

The types we eat are among the most nutritious foods – rich in protein, zinc, iron, calcium, vitamins A and B. Oysters provide a habitat to serve and feed other sea creatures; their reef structures stabilize shorelines; and each of the mollusks can filter 40 to 60 gallons of seawater a day.

Now there’s new potential to learn more about oysters and ocean reef environments. For the past four years, Ashlee Lillis, a Ph.D. candidate in marine science at N.C. State, has literally dived into a little-explored aspect of oyster settlement: how the sounds of an undersea ocean reef can attract larval (baby) oysters to a permanent home.

To her knowledge, “no one works with effects of sound on oyster larvae,” said Lillis, the project’s lead researcher. “The other work being done is all in coral reefs and on fish. So no one in this country or hemisphere is looking at the influence of sound on marine invertebrates.”

She said her team found an increased settlement rate – in both the laboratory and the wild – when the larvae were exposed to reef sounds. Determining precisely how the noise affects the baby oysters could lead to strategies for establishing new oyster beds, she said, as well as for monitoring the health of the undersea reefs.

“One of the things we’re interested in is, what is a healthy soundscape versus an unhealthy soundscape, and then, does that matter?” she said.

The oyster crawl

Oyster larvae are so small that a million can fit in a human hand. So they can’t swim across or against currents, but they can move up and down within columns of water. As they mature, they grow a “foot” that helps them feel for the ocean floor terrain.

“One of the things they’ll do is crawl around, and if the texture is appropriate, that might be where they’ll attach,” Lillis said. “They often settle on something hard, like other oyster shells.”

Finding the right landing spot is difficult for the little critters.

“Oysters and other animals that need to attach to a hard substrate or hard bottom have a particular challenge in that way because these habitats, such as oyster reefs, or in tropical systems coral reefs, are not widespread,” she said. “The larvae are potentially transported far from where they were hatched, and across large areas that are just sand or mud without appropriate settlement structure.

“Because the suitable areas in which these organisms need to settle are patchily distributed, but they disperse over many kilometers of unsuitable habitat, they have adaptations to help increase their chances of encounter with adult habitat. Responding to sounds associated with adult habitat might be one of them – as suggested by my study – and larvae are also known to use other cues such as turbulence, chemicals exuded by adults, and water properties to give them information about the environment as they are transported around by the currents.”

Testing the theory

The sounds of an undersea ocean reef are loud, like a busy city. Lillis hypothesized that if this environment is noisy enough to be heard by scuba divers and snorkelers, there might be enough vibration to affect the larvae – even though oysters don’t have ears and can’t hear sound, per se.

With help from her adviser David Eggleston, professor of marine sciences, and N.C. State geophysicist Del Bohnenstiehl, she made underwater sound recordings of the louder oyster reefs and the quiet open seafloor.

“We use what’s called a hydrophone, which is an underwater microphone,” Lillis said. “It’s attached to an acoustic recorder and then can be anything from a digital hand-held recorder that journalists may use. There are some more sophisticated long-term recorders that will take recordings at scheduled times, so we can leave those out a little bit longer.

“So we deploy these usually when scuba diving and (attach) them to the seafloor.  We’ve also had some drifting hydrophone studies, which are very interesting, to see how the sound changes as the recorder moves through the environment. There are lots of ways to do it, but the basic technology is a hydrophone and a recorder.”

Then they tested these oysters in the laboratory and in the wild to determine whether settlement rates increased when they were exposed to reef sounds versus those from farther out. They found the larval oysters preferred settling in substrates with the reef sounds.

Learning and educating

Growing interest in Lillis’ research is reflected in grants she has received. There were none at first, she said, but then “we had several small awards to me early on from the National Shellfish Reef Association. Also, the American Academy of Underwater Scientists gave me a small grant for the pilot work.

“Then once we had some good data, we received a larger National Science Foundation grant. That’s what we’re using now to continue for the next couple years to do some extensions of what I did for my Ph.D.”

Many questions remain. Besides determining what’s a healthy soundscape and what’s not, it’s not known precisely which sounds oyster larvae are attracted to, or whether human noise (such as from boat engines) affects oyster reefs.

Meanwhile, Lillis works to educate the public outside of the science community via a cartoon website that she developed with an illustrator: “It’s an attempt to do some public outreach and explain some of these concepts in a simpler way,” she said.