Woodfall invertebrates. Credit: Craig McClain
Woodfall invertebrates. Credit: Craig McClain

The Marine Creatures That Only Live on Land Plants

In November 2006, Craig McClain sailed into the Pacific Ocean, threw 36 logs overboard, and created several new worlds.

When wood sinks to the bottom of the ocean, whether from shipwrecks, uprooted trees, or keen scientists, it is soon colonised by waves of life. Clam-like creatures called Xylophaga chisel through the wood with their own shells and feed off the liberated splinters.  Small crustaceans and predatory worms squirm through the wood. Large squat lobsters sit on the surface, tearing strips off the bark with spoon-shaped claws.

Most of these animals have special adaptations (and microbes) that allow them to digest wood, and they’re only found on woodfalls. McClain estimates that around 90 percent of these species live nowhere else in the ocean. They’re marine creatures that live only on land plants!

I first wrote about woodfalls and their weird inhabitants last year, and I was struck by how little we know about these habitats. Studying the deep ocean is already quite tough; studying woodfalls is especially so. They are temporary ecosystems that appear very suddenly, only to be consumed and decomposed over a few years. They’re a network of worlds that blink in and out of existence across the ocean floor.

To really understand woodfalls, McClain had to create them. Together with James Barry at the Monterey Bay Aquarium Research Institute, he sank 36 acacia logs to the bottom of Monterey Bay in California, in a site affectionately known as Deadwood.

The logs ranged in weight from just a few pounds to around 45 pounds. McClain wrapped each one in mesh laundry bags, lowered them 3 kilometres down onto the ocean floor with a “benthic elevator” (read: a fancy shopping trolley with weights and floats), and scattered them using a remotely-operated vehicle, or ROV (read: underwater robot assistant). Five years later, they collected half the logs.

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A woodfall log at the Deadwood site.

They found that the woodfall communities go through several stages, with different groups of animals succeeding and replacing each other. The clam-like Xylophaga are the first colonisers. By boring through the wood, they create nooks and crannies that later waves of residents can live in. “They’re ecosystem engineers like beavers or termites,” says McClain. “They alter the landscape and provide new habitats for other species. Without [them], the carbon energy in the wood would not be available to other species.”

Xylophaga is a messy eater. As it bores into the logs, it kicks out wood chips and pellets of faeces. These land on the nearby sediment and feed deep-sea bacteria, creating a white halo around the wood. Other critters come along and eat the slurry of bacteria, wood and faeces. Through their activity, they use up all the oxygen in the sediment and discolour it.  Over time, the white halo becomes a black one.

The size of the logs also influences the community of animals that eventually settle upon it. Snails, for example, only take up residence on the larger timbers. “Snails, contrary to the typical view of them, are metabolically expensive,” McClain explains. “They may need to a lot of energy to support a viable population.”

These two factors—the size of the logs, and the presence of the Xylophaga engineers—explained a lot of the variation in the different woodfall communities. But McClain also found that these communities were extraordinarily variable and bizarrely random. Even though he sank all the logs at the same time, some were in the earliest stages of colonisation and others were well into their final acts. Some had been so heavily bored that they were practically falling apart. Others were relatively untouched.

These differences didn’t reflect their location, size, or surface area. “Basically it appears that recruitment of larvae into the wood fall is a near random act even two logs are meters from one another,” says McClain.

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A woodfall log, riddled with boreholes. Credit: Craig McClain.

Why? That’s the obvious question and there is no obvious answer. And there’s still the bigger mystery of what the woodfall animals are doing when there’s no wood around. As McClain said the last time I spoke to him: “Are there just larvae hanging around in the sediment waiting for something to land, or are there chemical cues that draw them in? I have no idea. Every explanation I can come up with doesn’t really fit. You just think: How would that actually work?”

This isn’t the last we’ll hear about these fleeting worlds. McClain has more experiments planned and he still has the remaining 18 logs, which he recovered late last year the rest of them a month ago.

Reference: McClain & Barry. 2014. Beta-diversity on deep-sea wood falls reflects gradients in energy availability. Biology Letters http://dx.doi.org/10.1098/rsbl.2014.0129