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'Twilight Zone' Reefs Win a Conservation Spotlight

Oceans
'Twilight Zone' Reefs Win a Conservation Spotlight
Plates of leaf-like foliose corals are common on the Philippines' Benham Bank. Oceana / UPLB

By Allison Guy

In May 2016, technical divers descended 200 feet to Benham Bank, the shallowest portion of a huge underwater plateau off the Philippines' northeast tip. As they neared the bottom, an otherworldly landscape emerged from the dim cobalt blue.

Plates of coral grew one atop the other like china at a yard sale, dotted with sea fans and sprigs of algae. Coral columns encrusted in yellow, orange and pink coralline algae looked as though they'd been splashed with rainbow paint. Colonies thrived as far as the eye could see.


The divers, part of a scientific expedition to Benham, were shocked by the reef's health and size, said Gloria Estenzo Ramos, the head of Oceana in the Philippines. "Our country is at the center of the center of marine biodiversity, but there are almost no more reefs left that we can call pristine," she said. "At Benham Bank, we found a place that still has 100 percent coral cover."

Benham lies in the ocean's "twilight zone" between bright surface waters and the permanent night below. Only in the last decade has technology advanced enough to let scientists venture this deep. Recent discoveries reveal that these little-studied deep reefs are everywhere their sunlit cousins are—and that they're often bigger, healthier and home to species no longer common in the shallows.

Lovely, Dark and Deep

Almost 190 years ago, Charles Darwin dredged up tropical, reef-building corals from 420 feet deep. It's a strange place to find typically sun-loving organisms.

Tropical corals, the kind familiar to scuba divers, all need the sun to survive. Their tissues harbor tiny beneficial algae that convert sunlight into food. Life in the twilight zone's mesophotic depths—"meso" for half and "photic" for light—stretches this partnership to its limits. Deep-dwelling corals, like those at Benham, grow in plate and lettuce-leaf shapes to expand their surface area and capture as much sunlight as possible.

Corals aren't the only photosynthetic organisms in the mesophotic zone, a band between 100 and 490 feet deep. Halimeda algae, which look like strands of flat green beads, are common at these depths. For Celia Smith, a reef algae ecologist at the University of Hawai'i at Mānoa, light-dependent organisms are just one of mesophotic reefs' many mysteries.

"How does a photosynthetic organism function in extreme low light?" she asked. "How do they thrive? We're still at the very beginning of understanding how these ecosystems work."

Frisky Fish

On May 15, Philippine President Rodrigo Duterte put almost 190 square miles of Benham's reefs off-limits to all human activities except research. A further 1,160 square miles of the Philippine Rise, where Benham is situated, were closed to destructive forms of fishing. This decision safeguards Benham's delicate corals from bottom trawling and potential threats like mining, and will help preserve the healthy fish communities necessary for reef health.

Duterte's declaration was spurred by a campaign led by Oceana and joined by ally organizations. Oceana launched its conservation efforts soon after the May 2016 scientific expedition, where the team provided technological assistance. The campaign got a boost in December 2016, when the Convention on Biological Diversity declared Benham to be of "critical ecological importance."

Mesophotic reefs like Benham aren't just important habitats for corals and algae. Their intricate topography shelters fish and other animals as well. Some fish species are familiar from the shallows, while others, like a newly identified butterflyfish from the Philippines, are unique to the mesophotic zone.

A diverse collection of fish thrives on this 250-foot-deep reef in Hawaii.NOAA

These smaller fish attract bigger fish, like groupers, snappers and sharks, many of which are important for local food security and incomes. "When you protect these reefs, you get a huge benefit in terms of fisheries species build-up," said Tyler Smith, a coral reef ecologist at the University of the Virgin Islands in St. Thomas, who is not related to Celia Smith.

So far, more than 200 fish species have been documented at Benham. The spot is the world's only confirmed spawning ground for Pacific bluefin tuna, a lucrative species that's plummeted to just 3 percent of its pre-fishing abundance.

Safeguarding mesophotic spawning sites pays off, Tyler Smith explained. In the Virgin Islands, after 20 years of protection, endangered Nassau grouper are returning to mating spots once decimated by overfishing. "We're starting to see their numbers rebound, and you now see them not so uncommonly in shallow waters," he said. "They're slowly coming back to being a viable species."

Shelter From the Warm

Benham is just one of a vast network of mesophotic reefs that could benefit from protected status—and protection begins with simple knowledge of their existence.

When Smith and his colleagues surveyed the ocean southwest of St. Thomas in the U.S. Virgin Islands, they were surprised to find that a single mesophotic reef system was "more extensive and well-developed" than all the islands' shallow reefs combined, with potentially hundreds of millions of coral colonies. On top of this, he said, the deeper ecosystems harbored a species of boulder coral that's all but vanished from the Caribbean's decimated surface reefs.

St. Thomas isn't the only haven for rare coral. In Panama and the Galapagos, a species of fire coral is thought to vanish from the shallows during the unusual warmth of the El Niño weather phenomena, only to be re-seeded from deeper-living specimens.

Scientists now debate whether mesophotic reefs can shelter shallower species from climate change, like a Svalbard Seed Vault for corals. So far, the answer has been "it depends."

During the recent nightmare bleaching that wiped out one-third of Australia's Great Barrier Reef, some mesophotic corals appeared to fare better. But in other parts of the world, mesophotic reefs are more imperiled than shallow one. That's because twilight corals aren't acclimatized to the dramatic temperature swings that can batter reefs up-top. How well mesophotic reefs fare depends on local ocean conditions, like deep, cool currents, which can mean the difference between life and death.

Deep Thinking

Colorful creatures like this hawkfish enliven the "twilight zone." Oceana / Karl Hurwood

There's no debating that twilight zone reefs have one advantage on their side: distance from us. Living at a remove from humans helps insulate them from coral-killers like bottom dredging, coastal development and agricultural runoff. It's also tougher to fish at mesophotic depths, said Tyler Smith, so these reefs often host bigger and more abundant fish.

Tyler Smith's big fear is that fishing gear and other technologies will advance to the stage that mesophotic reefs will lose their built-in safeguards. This makes legal protections, like those on Benham, all the more vital.

We might not fully understand these reefs, said Celia Smith, the algae ecologist, but that's no excuse to ignore or damage them. "It's the precautionary principle," she said. "These reefs could have a significant role in the future."


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Just in time for Halloween, scientists at Cornell University have published some frightening research, especially if you're an insect!

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.

Hoy agreed.

"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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