How Do Larvae Find a Place to Settle Down?

It’s still a mystery: How do the tiny larvae of marine animals that hatch in the open ocean find their way to coral reefs where they settle as adults? One theory is that they home in on suitable coral neighborhoods using sounds made by coral reef denizens.

But a new study by Woods Hole Oceanographic Institution researchers showed that sounds created by adult fish and invertebrates may not travel far enough for larvae to hear them.

“Based on our data, it seems unlikely that larvae would be able to use sound to locate far-off reefs,” said Max Kaplan, the study’s lead author and a graduate student in the MIT-WHOI Joint Program in Oceanography. “That was a surprise to us.”

Kaplan and his Ph.D. advisor, WHOI biologist Aran Mooney, made painstaking acoustic measurements off the Hawaiian island of Maui, placing sensors at distances ranging from zero to nearly 5,000 feet away from coral reefs.

The scientists used two different types of instruments to record two different components of sound—pressure waves (the element of sound that pushes on a human eardrum) and particle motion (the physical vibration within water as sound waves travel through it). The latter, Kaplan explained, is how the majority of fish and marine species detect sound, yet no previous studies have focused on recording it.

“Think of it like being at a loud concert—if you’re standing next to a huge speaker, you effectively feel the sound as it vibrates your skin,” Kaplan said. “Fish and invertebrates sense sound in a similar way.”

Invertebrate species such as squid can detect vibrations through their statoliths, the invertebrate version of ears. Similarly, adult fish sense them through the motion of tiny bonelike structures called otoliths inside their skulls.

Although researchers in the past have detected reef sounds from many miles away, Kaplan said that most of those studies rely on a hydrophone, an underwater microphone, which can detect only pressure waves. In their Maui study, however, the researchers recorded particle motion as well, using a sensitive accelerometer alongside a hydrophone.

“Particle motion is really the relevant cue for marine animals,” Mooney said. “When we’re measuring pressure, we’re measuring the wrong thing—it only gives a ballpark sense of what marine species hear. We think studying particle motion is a big step to figuring out how larvae find their way to a reef.”

Their experiments found that particle motion was much lower than expected, dropping rapidly below levels that most marine species can sense—even just a few feet away from the reef.

“It’s possible that larvae can still use chemical signals from other animals to locate the reef, or maybe can read the currents to move toward shore,” Kaplan said. He thinks that once larvae do locate a reef, sound may play an important role in finding a suitable location to settle down, which is key to their survival.

“In cases like that, sensing sound on the order of meters would make a big difference,” Kaplan said. “If you hear sounds of your species instead of predators, you might be more inclined to settle in a specific spot.”

Kaplan said his findings might be useful for reef conservation efforts. “To keep a reef healthy, you need a constant supply of new larvae to repopulate animals that die off,” he said. Past studies have shown that larvae are attracted to reef sounds when played through an underwater loudspeaker, so Kaplan thinks that playing recorded biological sounds could be used to steer larvae to damaged reef areas and help repopulate them.

“But you’d have to boost sound levels by quite a bit to get the response you want,” Kaplan said. “If we can figure out the hearing threshold of each species’ ability to sense particle motion, we might be able to amplify that motion to make it audible to them at a distance.”

“Recording particle motion in the field hasn’t really been done before,” Kaplan said, “but now that we’ve worked out these methods, we can start to expand our research.”

The research, published in August 2016 in the online journal Scientific Reports, was supported by the Woods Hole Oceanographic Institution Ocean Ventures Fund, the PADI Foundation, and the WHOI Access to the Sea Fund.