The surprising life of a piece of sunken wood

wood-boring clam

Wood-boring clams are among the first creatures to settle on wood underwater. They have jawlike shells that help them to burrow and gut bacteria that assist in the digestion of cellulose.

Craig McClain © 2012

The ocean is full of unique communities. Hydrothermal vents along deep ocean ridges feed chemosynthetic bacteria, specialized tubeworms and bacteria-farming shrimp. Sharks, worms, mollusks and more feed off dead whales as the carcasses fall to the ocean floor and disintegrate into nothing. Microbes grow on plastic waste floating through salty waters, creating their own “plastisphere.”

All the wood that makes its way into the world’s oceans is also a potential home to sea creatures. Every log or piece of lumber washed away by flooding or dumped by humans has the potential to host its own community, and the diversity of those communities can be surprisingly different from log to log. Even two pieces of wood of the same species left under the same circumstances can end up with dramatically different worlds of organisms living on them, researchers report April 9 in Biology Letters.

Back in 2006, Craig McClain of the National Evolutionary Synthesis Center in Durham, N.C., and James Barry of the Monterey Bay Aquarium Research Institute in Moss Landing, Calif., sunk 36 acacia logs to the bottom of the northeast Pacific Ocean to a depth of 3,203 meters. Every year, the scientists sent a video camera down to check on their logs, and in 2011, they brought 18 back to the surface. Each log was weighed and measured, and each critter living on them was picked off, preserved and identified.

Researchers used a remotely operated vehicle to retrieve bundles of acacia wood that had been left more than 3,000 meters underwater for five years. © 2012 MBARI
No two logs were the same. Some had few creatures living on them. Just tens of meters away, other logs had been colonized by lots of species — wood-boring clams, bacteria, fungi and more. The pattern of colonization appeared to be random, not driven by the size of the log or its placement.

When a piece of wood managed to get a community of life going, it was first colonized by the clams. Like termites, wood-boring clams are able to digest cellulose with the help of specialized bacteria. That frees up some of the carbon energy stored in the wood; other creatures access that carbon by eating clam poop or the clams themselves. The clams also make lots of little holes that other animals can hide in.

The next stage of community succession is marked by the arrival of tiny, shrimplike crustaceans called tanaids. They’re probably eating the clams or the clam feces, the researchers think.

The resulting mix of organisms seen after five years underwater may have something to do with the density and amount of wood available to the marine creatures. Larger logs ended up with more diverse communities, the researchers found. More wood, and more dense wood, they reason, provides more energy for those creatures to harvest and a bigger variety of niches for them to inhabit. A larger wood fall can thus support creatures like deep-sea snails that require more energy.

Past investigations have found that wood-boring clams grow at different rates depending on the species of wood involved. Teak on the Titanic wasn’t ever colonized by those clams, even after 70 years. Wood treated with clam-unfriendly chemicals may likewise resist colonization. And then there’s those acacia logs that randomly never got colonized by clams. It seems, then, that every tree, every branch, every piece of wooden debris washed into the ocean has the chance to host its own unique deep-sea community, or perhaps no community at all.

Sarah Zielinski is the Editor, Print at Science News Explores. She has a B.A. in biology from Cornell University and an M.A. in journalism from New York University. She writes about ecology, plants and animals.

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