Corals without Boarders: It's Cold, Dark, and There's No Help from Live-In Algae

By Milius, Susan | Science News, August 7, 2004 | Go to article overview

Corals without Boarders: It's Cold, Dark, and There's No Help from Live-In Algae


Milius, Susan, Science News


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.

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[degrees]C to 29[degrees]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[degrees]C and 13[degrees]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). …

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