Corals without Boarders

It's cold, dark, and there's no help from live-in algae

Tony Koslow is one of the authors of a new United Nations report on cold-water corals, yet he says he wasn’t giving them much thought as recently as a decade ago. These aren’t the coral reefs in vacation paradises of warm, sunny beaches, but the species that grow where sunlight can’t penetrate and temperatures stay downright chilly. Koslow’s interest came from studying a fish, the orange roughy.

SEAMOUNT. The craggy surface of an extinct volcano about 1000 meters deep in the waters around Tasmania bristles with cold-water corals. CSIRO Marine Research
SECRET GARDENS. Diverse cold-water species abound off the Aleutian Island chain. A. Linder/NOAA Fisheries
WHAT GOES AROUND. In the Gulf of Alaska, lingcod swim around hydrocoral. Corals offer shelter to young fish of many species and serve as breeding grounds for adults. Commercial fishing with trawls, however, can destroy cold-water corals. V. O’Connell

Advances in technology had opened deep fishing grounds for the prized delicacy in the Tasman Sea, and fleets were making fortunes by dragging big, elongated trawls over the crags of extinct volcanoes, or seamounts, more than 600 meters below the surface.

In 1997, Koslow, who directed deep-sea biological research for Australia’s governmental science agency, CSIRO, and a team of 25 taxonomists organized a cruise to explore the effects of trawling on roughy habitat. They dangled a camera from the ship and dropped gear that pulled up samples from the seamounts.

“We had no idea what was there,” says Koslow. The researchers found that the seamounts frequented by trawlers yielded “mostly rubble,” as Koslow puts it—broken, dead coral. However, when he saw the first pictures from the untouched seamounts, “it was an incredible moment,” he says. There were soft corals on top of stony corals, with other creatures living among them. “Some of these seamounts looked as if they could have been warm-water, tropical shallows,” Koslow recalls.

During the same period, investigators in the northern Atlantic Ocean were also discovering coral wonders of the depths. The burst of exploration has mapped previously recognized, but generally ignored, deepwater reefs and discovered new ones.

In July, the U.N. Environmental Program (UNEP) released the report Koslow helped write. It concludes that the planet has a surprising abundance of cold-water corals in a great variety of places. Biologists have now found cold-water corals in waters off 41 countries. UNEP says that this abundance needs protection from several menaces, foremost of which is Koslow’s original concern, trawling.

Chilly paradise

The postcard image of coral that most people have comes from shallow waters at pleasantly swimmable temperatures, 23°C to 29°C. Sunlight filters through the water to power the specialized algae, the zooxanthellae, that make their homes inside these corals. The algae use carbon dioxide released by the coral polyps and return products of photosynthesis, which the corals use for growth.

Corals can live at temperatures between –1.8°C and 13°C in environments very different from tropical shallows. For at least 2 centuries, though, scientists have realized that corals can survive in deep water, says Les Watling of the University of Maine in Walpole. Bits of coral have snagged in the gear of fishermen working deep waters, such as those off the coast of northern Europe. In fact, in the 18th century, Carolus Linnaeus, the founder of modern taxonomic classification, “named a species or two,” Watling notes.

Cold-water corals survive in water below the reach of sunlight, so no zooxanthellae provide extra nutrition. Instead, the corals make do by catching edible tidbits that slosh by on ocean currents.

By now, taxonomists have recognized 672 species of stony corals that don’t need algal partners and therefore are candidates to live in deep water. Some of these species, in fact, cohabitate with symbiotic algae in the shallows but survive on their own in water 40 m or deeper. From the perspective of marine biologists, that’s not impressively far down, so the term deepwater corals seems to be giving way to cold-water corals. Most cold-water corals do live farther down in the oceans. The deepest report comes from 6,328 m.

Besides the stony corals, three other major groups of corals include species that thrive in cold water: true soft corals (Octocorallia), calcifying lace corals (Hydrozoa), and black corals (Antipatharia). Biologists had studied them from time to time decades ago, but since the late 1940s “they sort of passed out of everybody’s mind,” says Watling.

Off the shelf

Interest in cold-water corals ignited during the 1990s, says Watling. In the northern Atlantic Ocean, Norwegian oil explorers had been taking advantage of new technologies to survey the continental shelf. Their remotely operated vehicles and other tools revealed that a raised spine called the Sula Ridge, long known to have some corals, turned out to have an abundance of them.

Biologists undertook their own cruises to get a good look at this marvel. A band more than 13 kilometers long bristled with pale, crisscrossing branches of a stony coral, the ivory tree coral Lophelia pertusa. Some of these corals reach 35 m in height. This thicket ranked as the largest Lophelia reef then known.

“What really got attention in Norway was that [the reef] was being destroyed—well, smashed to smithereens,” says Watling. The scientists’ cameras showed swaths of rubble or just bald ocean floor as a result of fishing fleets dragging weighted nets through the area.

These images of destruction were particularly upsetting because researchers calculated that the Sula Ridge coral structure represents some 8,000 years of growth.

Meanwhile, a 1998 survey of the seafloor north of Scotland revealed two zones of hundreds of hills of sand, now called the Darwin Mounds. Although the larvae of Lophelia generally settle on surfaces firmer than sand, the tops of these mounds bristle with it.

On the Darwin Mounds, scientists have found that the Lophelia have unusual neighbors, little-known creatures called xenophyophores. A xenophyophore looks like a grimy version of the irregular sponges sold in bed-and-bath shops. The xenophyophores, however, actually are unusually large, unicellular organisms, and the residents of the Darwin Mounds rank as giants among giants, single-cell organisms that can reach 20 centimeters in diameter.

Another survey—in the Lofoten archipelago off the Norwegian coast—revealed the Røst Reef, a stretch of deepwater Lophelia coral much larger than even the Sula Ridge. The Lofoten band, which is 3 km wide, extends some 40 km at depths of 300 to 400 m. The discoverers reported that the Røst Reef was still largely intact.

Another suite of corals thrives in the deep waters off Alaska. Sometimes called gardens instead of reefs, these communities feature octocorals. Their polyps typically wave eight feathered tentacles and cluster in colonies, many in Easter-hat colors. The gardens provide homes for other creatures, such as snails and crustaceans.

Seamount sites

On the other side of world, Koslow and like-minded researchers were finding their own marvels. After the 1997 cruise turned up the rich coral ecosystems on the Tasman Sea’s submerged peaks, Koslow’s team compared notes with the team of Bertrand Richer de Forges of Centre IRD de Noumea in New Caledonia. The group was studying seamounts just 1,000 km away in the ocean south of New Caledonia.

Together, the two research groups had logged some 850 species of invertebrates and fish. More than 200 of the creatures weren’t just new to seamounts, but also new to science.

“What ended up being the biggest surprise was that there were no species in common,” between the reports of the two teams, says Koslow.

Currents often sweep ocean organisms considerable distances, so finding such diversity in a relatively small piece of the world’s oceans startled the researchers. “It was the Galápagos of the deep,” he says.

And like the abundance of terrestrial life forms found only on the Galápagos Islands, the seamount communities set researchers worrying about preserving species. The findings have “really enormous implications for conservation,” argues Koslow.

Mercantile fishing has pushed many marine species to commercial extinction, where harvesting is no longer profitable, but “it’s fairly rare that we worry about species going [biologically] extinct,” says Koslow. If seamounts typically host creatures with narrow ranges, however, repeated trawling could indeed snuff out species.

Scientists are now investigating whether the richness of narrow-range species typical of Tasman Sea seamounts turns up elsewhere, says Koslow. Biologists previously had focused on the problems of species endemic to small areas of land, rather than on those of oceans.

Checking for diversity from a different angle, Marie Le Goff-Vitry of the University of Southampton in England is looking at a species that biologists have regarded as cosmopolitan. This ivory tree coral grows in all the world’s oceans except the polar seas. Le Goff-Vitry and her colleagues analyzed genes in bits of the coral collected from 10 sites off European coasts and in Scandinavian fjords.

Even that part of the range doesn’t function as a single population, the researchers reported in the March Molecular Ecology. Distinct subgroups showed up. Conservationists need to take care in planning how to preserve this genetic diversity, the team suggests.

Similar patchiness is showing up in preliminary results of a study of red precious coral. Civilizations from the ancient Greeks to the Polynesians treasured the octocoral genus, Corallium, for its vivid color. Species that look widely distributed sometimes have subpopulations that only genetic analysis can detect, according to Tim Shank of the Woods Hole Oceanographic Institution in Massachusetts.

Bottom line

Increasing signs of diversity among cold-water corals fuel concerns about their health. The UNEP report, which was prepared by several marine biologists, including Koslow, addressed eight human maritime activities that worry coral biologists. These include fishing with deep-sea trawls, laying communication cables, and dumping ship and industrial pollution.

Although the corals aren’t the targets of fishing fleets, they suffer collateral damage (SN: 7/26/03, p. 59: Catch Zero). Orange roughy swim near deep seamounts, Koslow says. Fishing for orange roughy, which prompted Koslow’s 1997 seamount cruise, increased as improved global-positioning equipment and bottom-scanning devices sharpened precision in finding seamounts. Fishing crews can lower a net precisely onto an aggregation of fish such as orange roughy, herd them to the bottom, and scoop them up.

Orange roughy, which can live a century or so, don’t reproduce quickly, so fleets exhaust a seamount in less than a decade and have to find the next great spot. “There’s been a bit of a gold rush,” says Koslow.

Rockfish, cod, redfish, some crustaceans, and other deepwater prizes are also drawing fleets into the depths. The UNEP report is pessimistic about the effects.

Some countries have already moved to protect deepwater reefs. For example, within a year after surveyors found the big Røst Reef, Norway closed the area to bottom-dragging nets.

UNEP is now calling for extensive international cooperation to find, monitor, study, and protect cold-water corals. Exploration in the past decade may have revealed new wonders at a rapid pace, but, the agency cautions, without ways to protect cold-water corals, “many of the most spectacular examples discovered so far could be gone in less than a generation.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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