Happy as a (Newly Discovered) Clam

Aboard a research ship in 1997, Janet Voight was amazed when she examined a small log that researchers just happened to trawl up from the bottom of the sea: It had been colonized by a lush community of snails.

“These snails offered a tantalizing glimpse,” she said, and immediately got her thinking. What else was out there eating and living on pieces of wood from trees washed out to sea, wooden shipwrecks, and other sunken debris? Could animal communities really find a niche on such random flotsam, fallen to  the seafloor? And if so, what role did these animals and these deep-sea oases play in the ecosystem of the deep?

Voight, an expert on deep-sea life at The Field Museum in Chicago, knew those would be hard questions to answer. Since the 1970s, she had made eight visits to the seafloor in the submersible Alvin and taken close looks at the ocean bottom using video cameras on robotic underwater vehicles—and she never once saw wood.

So instead of searching for a needle in a haystack, she opted to place needles in a haystack. Trying to track down pieces of sunken wood would be an expensive, time-consuming proposition with little chance of success, so she decided to put down her own wood then wait for months to see if anything settled on it.

That still left a daunting challenge: Finding your needles again in the haystack—in this case, pieces of fir and oak placed on the pitch-black seafloor more than a mile below the sea surface and 255 miles from the Pacific Northwest coast—is no walk in the park.

She placed the fate of finding her experiment in the hands of one of just a few people in the world with a knack for navigating robots around ocean depths, a quiet engineer at Wood Hole Oceanographic Institution named Tom Crook.

Little margin for error

Crook is a member of the engineering team that operates the deep-sea robot Jason, via a fiber-optic cable that relays video images and other data up to the ship and commands down to Jason. A 33-year veteran of WHOI, he has sailed on research vessels worldwide and is no stranger to tracking down lost or rare seafloor objects. On Sept. 1, 1985, he was on early morning watch aboard the research vessel Knorr when researchers solved one of the biggest maritime mysteries in modern history: the location of the wreck of the Titanic.

Six years ago, Crook sailed with Voight on an expedition to the North Pacific, and she approached him with her proposal to leave wood on the seafloor and come back for them months later.

“I thought it was a little odd,” he said. But then he got to work to make it happen.

Other scientists on board, however, shook Voight’s confidence. “Every time we left a bag (of wood) on the bottom, I felt a pang of doubt as my colleagues gleefully reminded me of the problems I would face later.”

“I couldn’t use signaling devices (to find the samples) because batteries can’t last for two years at seafloor temperatures of 2°C (36°F),” she said. “Direct use of the ship’s GPS was impossible because satellite signals don’t go through water.”

Lights on deep-sea robots and vehicles help guide scientists, but at the bottom of the ocean even the largest, brightest bulbs penetrate the blackness by only about 10 feet (3 meters).

“We couldn’t be off by more than that,” she said. “It would probably be easier to do our science in the middle of the night on a country road using car headlights to find our way and do our work.”

An undersea pathfinder

In the ocean, there are no street signs or maps to follow. Instead, engineers like Crook help find the way by creating their own seafloor “guide posts,” using sound-transmitting instruments called transponders that are sent overboard and anchored to the seafloor. The ship circles at the surface, receiving sound signals from the transponders, while it determines its own position using GPS. In such a way, the transponders’ positions in the ocean and on the seafloor can be determined to an accuracy of about 33 feet (10 meters).

But to get the transponders to work well, researchers can’t just dump them over the ship’s rail. Slopes in the seafloor; being in the shadow of an underwater mountain; thick, mushy seafloor sediment—all these obstacles can interfere with sound transmission.

“Unless they are in the right places, they are useless,” said Dan Fornari, a marine geologist and director of Deep Ocean Exploration Institute at WHOI. Crook had developed such skill for placing transponders that Fornari nicknamed him “Mr. Acoustics.”

“You know how some people just have this instinct for directions when driving around town?” said Stace Beaulieu, a WHOI biologist who has sailed three times with Crook. “That’s what Tom has for navigating robots on the seafloor. He’s definitely the guy you want in the driver’s seat when you’re looking for things on the seafloor.”

After situating the transponders, Crook and the Jason team positioned 17 of Voight’s mesh bags of wood in a line extending over 575 feet (175 meters) on the seafloor.

Small wonders

Ten months later, Voight returned to the site. Crook’s original positioning allowed the Jason team to relocate her wood-sample bags easily. But when Jasonbrought several back to the surface, Voight was disappointed to find that few animals had established residence on the wood. Maybe not enough time had passed.

She returned 14 months later, this time with Alvin to collect nine remaining samples. Once again, Voight said Crook’s original positioning made them easy to find.

“Once Alvin located the wood, the pilot easily loaded the bags into the 35-pound lidded boxes I had commissioned to bring them to the surface,” she said. “If the wood had ascended through the water column unprotected, its tiny colonists would have washed off,” wiping away any samples and months of work.

When she examined the wood on deck, Voight said she found “so many wood-boring clams that the wood was falling apart. My cheeks actually hurt, I was smiling so much.”

Tiny, brown, squiggly clams that eat wood may seem a peculiar choice of study. But Voight said their unusual ability—to find rare wood pieces and use them both to live on and consume for food—have changed the way she sees the world.

Worldwide, there are 49 known species of wood-boring, deep-sea clams. But before Voight’s work, only one species had been identified off the North American coast, from Vancouver Island in British Columbia to Santa Barbara, California. With such lush forests in the coastal Northwest, she theorized there must be more clam species living on sunken tree limbs and trunks.

Once clam larvae find the wood, they attach and grow, boring into the wood fibers using toothed ridges on their shells. They eat the wood that they occupy, relying on bacteria that live in their gills to aid in digestion.

“We might think of something that washes out to sea as being gone, but these clams rely on that resource and keep the nutrients that had been tied up in the wood circulating,” she said.

Ecological exceptions

Incredible to her is how so many species of clam can share space on a single piece of wood; in one case, Voight found that up to five species of clams could occupy the same chunk of wood. It has made her revisit her beliefs about the way animal life on Earth competes and coexists.

“If you think of a chunk of sunken wood as a limited resource, and think of wood-boring clams as consumers of that resource, common sense tells you that there should not be five species in such a small area,” she said. “Yet there they are.”

Because these competitors live in such close contact, she said, they violate one of the laws of ecology—the competitive exclusion principle, which states that two species competing for the same resource cannot stably coexist.

“By studying these guys, and their predators, we learn how the world works: what maintains biodiversity, and why all these species can survive together,” she said.

Should anyone dismiss the clams as unimportant, Voight points out the role they play supporting the base of the food chain within a marine ecosystem.

“One would think that they simply must pump out all sorts of planktonic young with extremely long lives to make it possible for the species to survive,” she said. Most of this horde of young will never make it to wood. Instead, she said, they simply become enormous quantities of food for other animals.

By hook or by Crook

In the end, Voight described six different species of clam that had made homes on her pieces of wood—all of them previously unknown. In a paper published November 2007 in the Journal of Molluscan Studies, she bestowed names for the new species.

She named one Xylophaga oregona, after Oregon, near the clams’ seafloor home. Three others she named for their unique body parts: One with a crown-shaped flap near the hinge of its shell became Xylophaga corona (Latin for “crown”). Another sported a water intake valve resembling a small hand. It was named Xylophaga microchira (from micro, or small, and chir, Greek for hand).

For the last two, she thought of personal heroes, people who helped launch her biology career and support her research. One clam, Xylophaga zierenbergi, she named for a scientist mentor. The last, Xylopholas crooki, shenamed for Crook “in recognition of his years of service to science, specifically his superlative efforts during the 2002 cruise, the last before his retirement from WHOI, which allowed the deployments to be relocated and these species to be discovered.”

Told about the species named after him, Crook was characteristically modest, saying only that he “was surprised and honored.”

But Voight wouldn’t understate his role. “We literally would have been lost without him,” she said. “He was our guide to the seafloor.”

The National Science Foundation funded this research.