By Matt Walker
Editor, Earth News
M. andamanica, a wood-muncher
Deep under the ocean, there is a species of crab that eats trees.
The crab survives by eating wood that has sunk to the ocean floor, comprising trunks and leaves swept into the sea, as well as the odd shipwreck.
Inside the stomach of the crab, also called a squat lobster, are bacteria and fungi that help digest the wood.
The discovery, published in the journal Marine Biology, adds to evidence that these so-called 'wood falls' help support special underwater communities.
"At first sight, it seems improbable," says PhD student Caroline Hoyoux of the University of Liège, Belgium.
"Munidopsis andamanica is a species only found in the deep sea and yet it eats 'terrestrial food'," she says.
Sunken wood degraded by wood-boring bivalves
Ms Hoyoux and colleagues based at the University of Liege and at the Natural History Museum and Pierre and Marie Curie University in Paris, France made the discovery while studying which animals colonise wood falls.
Among worms, bivalves and a host of crustacean species they found Munidopsis andamanica, a species known as a galatheid crab, or squat lobster.
Further investigation of the crabs mouthparts and gut contents revealed they feed exclusively on wood.
"We were surprised, because crustaceans are often regarded as predators or scavengers. The fact I found M. andamanica consistently feeding on vegetal remains, especially wood, instead of eating molluscs or [worms] breaks with the general a priori about the diet of squat lobsters," says Ms Hoyoux.
The importance of wood falls and the communities they host are only just being appreciated.
Although first discovered in the late 19th century, it was not until the late 1970s that scientists began to study the animals that colonise them.
Until this century, these were mainly thought to be wood-boring molluscs.
"However, crustaceans are the second most important group, according to the number of species and individuals," says Ms Hoyoux.
Another species of galatheid crab clings to a piece of wood
She is studying wood fall crustaceans for her PhD thesis and as part of the international DiWOOD project, which seeks to learn more about animals colonising deep-sea wood.
"The wood falls that we study are principally natural tree debris that have sunk and reached the deep sea floor. They consist of real wood as well as plant fragments like leaves, seagrass, coconuts etc."
To collect the animals, Ms Hoyoux and colleagues mimic real wood falls by immersing mesh boxes baited with wood.
The mesh is wide enough to allow crustacean larvae to colonise the wood, but too small the allow the animals to escape as they grow.
After a year, these boxes are brought to the surface and the animals collected.
Among those found are 15 species of decapod, one species of isopod and one amphipod, including hermit crabs, shrimp and galatheid crabs of the genus Munidopsis and Munida.
The squat lobster is thought to bite off small splinters of wood which it then passes through a 'gastric-mill' of strong teeth used to grind the wood down.
A branch or trunk trawled from hundreds of metres underwater
The crab's gut then contains bacteria and fungi that produce enzymes that help digest the cellulose in the wood.
The ecological importance of wood falls rivals that of whale falls, where highly specialised communities of deep-sea animals colonise the bodies of dead whales and dolphins that drift to the sea bed.
"Although they are not as quantifiable as whale falls, they could be more important," says Ms Hoyoux.
"It is strongly assumed that these vegetable debris constitute an important and significant contribution of food to the deep-sea fauna."
They could even be important stepping stones in the colonisation of more extreme deep-sea environments such as hydrothermal vents, say the researchers.