Loudspeakers Can Help Bring Degraded Reefs Back to Life, Study Shows
A healthy fish population can help fight the degradation of coral reefs, the study's authors explained in Nature Communications Friday, but damaged reefs don't attract as many fish because they don't smell or sound like healthy reefs.
"Healthy coral reefs are remarkably noisy places – the crackle of snapping shrimp and the whoops and grunts of fish combine to form a dazzling biological soundscape," senior author and University of Exeter professor Steve Simpson explained in a press release received by EcoWatch. "Juvenile fish home in on these sounds when they're looking for a place to settle."
A loudspeaker on a coral reef. Tim Gordon / University of Exeter
So a team of researchers from the University of Exeter and the University of Bristol in the UK, and Australia's James Cook University and the Australian Institute of Marine Science spent October to December of 2017 in the Great Barrier Reef trying to see if they could replicate these healthy reef sounds in damaged environments.
The Washington Post explained their process:
At the start of fish recruitment season, when fish spawn and mature, the team built 33 experimental reef patches out of dead coral on open sand about 27 yards from the naturally occurring reef. They then fixed underwater loudspeakers to the center of the patches, angling them upward to ensure the sound was distributed evenly in all directions.
Over 40 nights, the team played recordings from a healthy reef in some of the patches. In other patches, they used dummy speakers that emitted no sounds, and they left a third group of patches untouched.
Tim Gordon deploys an underwater loudspeaker on a coral reef. Harry Harding / University of Bristol
The result? The reef patches that broadcasted the healthy reef sounds attracted double the number of fish as the other patches, and the fish species drawn to them were 50 percent more diverse. That diversity included species from every section of the food web, from plankton eaters to predators, which is important for reef health.
"Reefs become ghostly quiet when they are degraded, as the shrimps and fish disappear, but by using loudspeakers to restore this lost soundscape, we can attract young fish back again," Simpson said in the press release.
The scientists called this process "acoustic enrichment" and think it could be one tool for helping reef ecosystems recover more quickly.
"Of course, attracting fish to a dead reef won't bring it back to life automatically, but recovery is underpinned by fish that clean the reef and create space for corals to regrow," Australian Institute of Marine Science fish biologist and study author Dr. Mark Meekan said in the press release.
However, the researchers urged that it is also important to tackle reef threats like the climate crisis or overfishing directly. And in this they were backed up by scientists not involved in the study.
"Using acoustic enrichment to help recolonise degraded reefs with essential reef fish is a novel tool which can add to the reef conservation toolbox," Dr. Catherine Head of the Zoological Society of London and the University of Oxford told The Guardian. "Our biggest tool in the fight for coral reefs is the 2016 Paris climate change agreement to curb global CO2 emissions, and we must continue to put pressure on governments to fulfil this agreement alongside doing our bit to reduce our own carbon footprints."
The ghoulishly named ogre-faced spider can "hear" with its legs and use that ability to catch insects flying behind it, the study published in Current Biology Thursday concluded.
"Spiders are sensitive to airborne sound," Cornell professor emeritus Dr. Charles Walcott, who was not involved with the study, told the Cornell Chronicle. "That's the big message really."
The net-casting, ogre-faced spider (Deinopis spinosa) has a unique hunting strategy, as study coauthor Cornell University postdoctoral researcher Jay Stafstrom explained in a video.
They hunt only at night using a special kind of web: an A-shaped frame made from non-sticky silk that supports a fuzzy rectangle that they hold with their front forelegs and use to trap prey.
They do this in two ways. In a maneuver called a "forward strike," they pounce down on prey moving beneath them on the ground. This is enabled by their large eyes — the biggest of any spider. These eyes give them 2,000 times the night vision that we have, Science explained.
But the spiders can also perform a move called the "backward strike," Stafstrom explained, in which they reach their legs behind them and catch insects flying through the air.
"So here comes a flying bug and somehow the spider gets information on the sound direction and its distance. The spiders time the 200-millisecond leap if the fly is within its capture zone – much like an over-the-shoulder catch. The spider gets its prey. They're accurate," coauthor Ronald Hoy, the D & D Joslovitz Merksamer Professor in the Department of Neurobiology and Behavior in the College of Arts and Sciences, told the Cornell Chronicle.
What the researchers wanted to understand was how the spiders could tell what was moving behind them when they have no ears.
It isn't a question of peripheral vision. In a 2016 study, the same team blindfolded the spiders and sent them out to hunt, Science explained. This prevented the spiders from making their forward strikes, but they were still able to catch prey using the backwards strike. The researchers thought the spiders were "hearing" their prey with the sensors on the tips of their legs. All spiders have these sensors, but scientists had previously thought they were only able to detect vibrations through surfaces, not sounds in the air.
To test how well the ogre-faced spiders could actually hear, the researchers conducted a two-part experiment.
First, they inserted electrodes into removed spider legs and into the brains of intact spiders. They put the spiders and the legs into a vibration-proof booth and played sounds from two meters (approximately 6.5 feet) away. The spiders and the legs responded to sounds from 100 hertz to 10,000 hertz.
Next, they played the five sounds that had triggered the biggest response to 25 spiders in the wild and 51 spiders in the lab. More than half the spiders did the "backward strike" move when they heard sounds that have a lower frequency similar to insect wing beats. When the higher frequency sounds were played, the spiders did not move. This suggests the higher frequencies may mimic the sounds of predators like birds.
University of Cincinnati spider behavioral ecologist George Uetz told Science that the results were a "surprise" that indicated science has much to learn about spiders as a whole. Because all spiders have these receptors on their legs, it is possible that all spiders can hear. This theory was first put forward by Walcott 60 years ago, but was dismissed at the time, according to the Cornell Chronicle. But studies of other spiders have turned up further evidence since. A 2016 study found that a kind of jumping spider can pick up sonic vibrations in the air.
"We don't know diddly about spiders," Uetz told Science. "They are much more complex than people ever thought they were."
Learning more provides scientists with an opportunity to study their sensory abilities in order to improve technology like bio-sensors, directional microphones and visual processing algorithms, Stafstrom told CNN.
"The point is any understudied, underappreciated group has fascinating lives, even a yucky spider, and we can learn something from it," he told CNN.
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