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Zombie Sponge Reefs Are Lurking at the Bottom of the Sea

The resurrection of glass sponge reefs proves extinction doesn’t have to be eternal.

Kim Conway was examining the sea floor off the coast of British Columbia when he found them: strange mounds that showed up as hazy…By Sarah DeWeerdt

Kim Conway was examining the sea floor off the coast of British Columbia when he found them: strange mounds that showed up as hazy “ghosts” on his sonar imaging data. The year was 1984, and Conway was a member of a survey team working for Natural Resources Canada. He was expecting a sea floor with solid hills and valleys, and thought at first that the mounds might be a false bottom, maybe created by an old landslide or a blister of gas venting underneath the sediment. A follow-up mission several years later used cameras and coring instruments to reach a very different conclusion: The bumps were alive.

They were sponge reefs, made from the skeletons of glass sponges. That explained the “ghosts” in Conway’s images: Unlike coral reefs, which are solid rock, sponge reefs are held together with soft sediment, so the reefs are semi-transparent to sonar. Also known as hexactinellids, glass sponges are an ancient group of animals that harvest dissolved silica from seawater, turn the silica into 4- or 6-pointed spikes called spicules, and weld the spicules together into cups, fingers, and fans. At the top of the reef, densely packed living sponges are anchored to the skeletons of previous generations and surrounded by a riot of crabs and octopuses, spot prawn and young rockfish. A reef “was kind of the last thing we thought they’d be,” says Conway.

The coelacanth was believed to have gone extinct 65 million years ago—until fishermen hauled it up in their nets from the depths off the coast of South Africa in the 1930s.

That’s because sponge reefs were thought to have gone extinct 40 million years ago, and were known to mankind only through the fossil record. Glass sponges appeared in the early Cambrian Period around 540 million years ago, and began forming reefs about 300 million years later. The largest fossil sponge reefs are from the Jurassic Period—glass sponge heyday. Between 155 and 145 million years ago, sponge reefs stretched for 7,000 kilometers along the northern margin of the Tethys Sea, which covered much of present-day Europe. Today, the fossilized reefs can be seen in cliffs and rocky outcrops that rise up to 200 meters high and run from Portugal through France and Germany to Romania, representing the largest biotic structure ever created on Earth.

At the end of the Jurassic Period, continental shifts, changing currents, and a large-scale reorganization of marine ecosystems touched off a dramatic decline in sponge reef populations. Corals rose to become the world’s principal reef-forming organisms, and a type of algae called diatoms increasingly outcompeted sponge reefs for dissolved silica. Today’s youngest sponge reef fossils are 40 million years old, marking what paleontologists thought was the end of this unique structure.

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That doesn’t mean that glass sponges themselves went away. About 600 species of hexactinellids still exist today, in cold, mostly deep ocean waters. They are usually solitary, spaced apart by meters to hundreds of meters. In some areas with particularly favorable habitats they gather in dense aggregations called sponge gardens—but they were not thought to be capable of producing sponge reefs, in which generation after generation of sponges grow on the skeletons of their dead. The sudden appearance of a sponge reef was a bombshell. “I couldn’t believe it,” says University of Stuttgart paleontologist Manfred Krautter, who had built his career on studying fossil sponge reefs in Europe. “A big door to a dark room was opened for me and there I could see the living ones.” He recalls a sense that the geologists hadn’t grasped the full significance of their discovery at first, as if a herd of dinosaurs had been found roaming around by people who didn’t know that dinosaurs died out long ago.

Sitting Pretty: Glass sponges like this one have begun making reefs again, after a hiatus that is believed to have lasted 40 million years.Image courtesy of Brooke et al., NOAA-OE, HBOI.

When an organism thought to be extinct is rediscovered—either in living form or in the fossil record after a gap of millions of years—it is known as a Lazarus taxon. In this sense, glass sponge reefs are a kind of Lazarus ecosystem. Lazarus taxa are often illusions created by incomplete fossil records. The coelacanth, for example, was abundant in fossils but believed to have gone extinct 65 million years ago. That is, until fishermen hauled it up in their nets from the depths off the coast of South Africa in the 1930s.

But the extinction, then resurrection, of glass sponge reefs is not an illusion. It’s very unlikely that they existed anywhere on Earth for tens of millions of years. Scientists have a good understanding of where coastlines were during that period, and where the corresponding depths of water appropriate for glass sponge reefs would have been—and no sponge reef fossils have been found there. There also weren’t mountains with a high enough silica content near the kinds of waters that sponge reefs like (cold, not too rough or calm).

Things began to change about 14,000 years ago, when glaciers scoured the North American continental shelf, leaving behind troughs filled with coarse gravel, rocks, and boulders. As the ice sheets receded and sea levels rose, these troughs became glass sponge-friendly seedbeds: patches of hard substrate at depths that would usually be sand and mud. By 9,000 years ago, according to radiocarbon dating of reef cores, some lucky glass sponge larvae had found themselves in the right place at the right time, and settled on the glacial moraines and tills crisscrossing the Hecate Strait, which runs between the mainland and the islands of Haida Gwaii off the coast of British Columbia.

“The ecosystem itself has never really changed [in] 200 million years,” Krautter says—except for the fact that it spent a bit of time not existing.

It was a good place to be a glass sponge. Rivers eroded the feldspar of the nearby coastal ranges, producing extremely high dissolved silica concentrations in the waters offshore. Nutrient-rich upwellings and tidal currents brought abundant bacteria and organic particles for the filter-feeding sponges to eat. The elements necessary for sponge reefs to form had spent millions of years in latent storage in the deep ocean, probably the most stable environment on earth, and were now made active.

Today scientists have documented over 700 square kilometers of sponge reef in the Hecate Strait. A handful of smaller reefs have also been found in the Georgia Strait, which lies between the Canadian mainland and Vancouver Island. Another reef complex may exist about 30 miles off the coast of Washington State, near Grays Harbor, and one was recently documented near Juneau, Alaska.

The sudden reconstruction of an ecosystem not seen for tens of millions of years has been a boon for a variety of organisms, highlighting the importance of paying attention, not just to the extinction of individual species, but to the extinction of the structures and behaviors they are responsible for. “We should conserve [these phenomena] because they’re interesting and inspiring, and because they can perform an important function in an ecosystem,” says Robin Naidoo, a conservation biologist with the World Wildlife Fund. Commercially important species like rockfish and spot prawns are using the reefs as nursery habitats. Many of the reefs’ other inhabitants—including single-celled foraminifera, terbellid worms, bivalves, brachiopods—are very similar to those on the Jurassic reefs. “The ecosystem itself has never really changed [in] 200 million years,” Krautter says—except for the fact that it spent a bit of time not existing.

The return of living sponge reefs is allowing scientists to pursue old questions in new ways. These questions include how glass sponge larvae locate the skeletons of old glass sponges to settle on in order to build a reef; whether new sponges on a reef grow steadily or in fits and starts; how the sponge reefs interact with other species in the ecosystem; how ancient sponge reefs influenced the larger marine ecosystem, especially the chemical composition of continental shelf waters; and how glass sponges multiply. “We still have only a vague idea of how and how often they reproduce,” Krautter says.

Having come back, glass sponge reefs face a set of challenges unknown in the Jurassic era: Soon after the Hecate reefs were discovered, investigators found evidence of scars from heavy fishing gear dragged across them. “When they get smashed by the bottom trawlers and other things, they just get kind of pulverized,” says Sabine Jessen, director of the oceans program at the Canadian Parks and Wilderness Society. “And a new sponge can’t really grow on top of that.” Jessen’s group has helped secure fishing closures around the Hecate reefs, and is working to have them declared a marine protected area. With protection, says glass sponge expert Henry Reiswig of the University of Victoria, “they should be ok for the next thousand years or so.” Which, considering the potential life span of a sponge reef, isn’t very long at all.


Sarah DeWeerdt is a freelance science journalist in Seattle specializing in biology, medicine, and the environment.

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