Zoologger: Wood-eating shrimp from the ocean's depths

Zoologger is our weekly column highlighting extraordinary animals ? and occasionally other organisms ? from around the world

Species: Hirondellea gigas
Habitat: deep-sea trenches of the western Pacific, including the deepest part of the ocean ? the Mariana Trench

Earlier this year, James Cameron took a submersible about 11 kilometres below sea level, to the bottom of the Mariana Trench. In doing so he became only the third human being to visit the deepest point of the ocean.

Not that there was much to see in the murky depths. "The only free swimmers I saw were small amphipods," Cameron told National Geographic. If he'd looked closely, Cameron might have seen those amphipods tuck into a surprisingly woody meal.

Amphipods are shrimp-like crustaceans. We don't know what species Cameron saw, but it might well have been Hirondellea gigas. It is one of the few animals known to plumb such extreme depths. To do so, it has evolved unique molecular tools, which allow it to digest food that most animals would spit out. This could be the key to the species' survival in the wasteland of the deep sea.

Deep places of the world

Amphipods are one of those luckless animals that hardly anyone takes an interest in, despite the fact that they are spectacularly diverse and widespread. They live in virtually every sizeable body of water on Earth, whether fresh or salty. Most are just a few centimetres long, but giant amphipods, 10 centimetres long, have been found beneath Antarctic ice sheets, and "supergiants" twice that length are sometimes found in the Pacific.

Hirondellea gigas is less spectacular, ranging from 2 to 5 centimetres in length. Its pale, almost bleached, skin makes it look like an anaemic woodlouse.

But it must be pretty special, because it can survive in the Mariana Trench. At the Challenger Deep, the Trench's deepest point, the pressure is 1000 times that at the surface, and the water is pitch-dark and just 2 ?C above zero.

In 2009, Hideki Kobayashi of the Japan Agency for Marine-Earth Science and Technology in Yokosuka, Japan, and his colleagues lowered a camera and four baited traps into the Challenger Deep. They captured 185 animals in 3 hours, all of them H. gigas. Clearly, the animal flourishes in the depths. Kobayashi studied their digestive systems to find out what they eat.

Sawdust om nom nom

He found that H. gigas could carry out many of the same digestive reactions as other animals. In particular, it could break down polysaccharides ? complex organic molecules found in plants ? into simpler sugars.

More impressively, in the lab H. gigas could also digest sawdust. Being made from wood, sawdust is rich in a compound called cellulose that is notoriously tricky to digest. Ruminants like cows, and termites, can digest it but only with the help of symbiotic microorganisms living in their guts. Human guts, meanwhile, pretty much treat it as roughage.

Kobayashi managed to isolate the enzyme that H. gigas uses to digest cellulose. It didn't look like any cellulose enzyme we know of. Instead, it was closely related to a family of enzymes that separate sugars from other molecules. That suggests H. gigas has evolved a unique enzyme.

Chunks of wood, and similar materials such as coconut shells, are sometimes found deep in oceanic trenches. These sunken pieces of vegetable matter may form a big part of the amphipod's diet.

That may not sound very appetising, but if you live in a high-pressure, pitch-black desert, you take what you can get.

Journal reference: PLoS One, DOI: 10.1371/journal.pone.0042727

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