Deep-sea, shrimp-like crustaceans get big by munching on sunken wood, even from shipwrecks, according to a new study of amphipods.
The specimens of Hirondellea gigas were collected in 2009 in the Mariana Trench's Challenger Deep, Earth's deepest point and the same location of National Geographic explorer and filmmaker James Cameron's record-breaking dive in March 2012.
During his dive, Cameron also saw H. gigas amphipods, which, at up to two inches (five centimeters) long, are huge among their kind—more than twice the size of their common beachside relatives, the sandhoppers.
H. gigas live in swarms at depths of or below 30,000 feet (10,000 meters), where very little food makes its way down from the surface. This had scientists stumped—how do the amphipods get the nutrition to become relatively giant?
Analysis of the Mariana Trench amphipods revealed the scavengers harbor powerful wood-busting enzymes that can digest "wood fall"—tree and plant debris swept into the ocean that occasionally sinks.
"They are relying on sunken leftovers as their food," study co-author Hideki Kobayashi, a marine biologist from the Japan Agency for Marine-Earth Science and Technology, said in an email.
To collect the amphipods, the scientists baited traps partly made from recycled plastic bottles, Kobayashi said.
Lowered into the Challenger Deep trench for three hours, the traps caught almost 200 amphipods. (Video: How sound revealed that Challenger Deep is the deepest spot in the ocean.)
Back in the lab, the research team identified the wood-digesting enzymes, which worked even better when the scientists re-created the high-pressure conditions of the deep ocean.
Similar digestive enzymes have been found in the guts of other plant-eating animals, such as termites. But unlike other deep-sea species, H. gigas does not seem to cultivate fungi or bacteria to aid its digestion.
"We think the amphipods make the enzymes themselves in their gut," said Kobayashi, whose study appeared August 15 in the journal PLoS ONE.
The team also detected the byproducts of wood digestion inside amphipod tissues, further supporting the wood-eating theory.
That product, cellobiose, "is a component of cellulose in plants and is never synthesized in animals," Kobayashi said.
University of Aberdeen marine biologist Alan Jamieson agreed that the amphipods are using the enzymes to feed on wood.
"They wouldn't waste energy creating that capability for nothing," he said.
Jamieson added he isn't surprised by the finding, since it's already known the amphipods "eat almost anything."
"We know that they're capable of starving themselves for a long time," Jamieson said.
"When the opportunity to feast arrives, they will literally gorge themselves to the point where they're about to burst."
Study co-author Kobayashi also noted that H. gigas are opportunists.
For instance, if a ship happened to sink into the Mariana Trench, "Hidonellea gigas would gladly eat it," he said.
"In fact, a few of them bit into the wooden parts of ASHURA, the camera system on our baited traps."
Amphipod Discovery May Boost Ethanol?
In addition to shedding light on amphipods, the discovery may one day help produce ethanol for biomass energy, Kobayashi said.
At room temperature, one of the newfound wood-eating enzymes, a type of cellulase, broke down a sheet of plain paper into the simple sugar glucose, which can then be used to make ethanol. (Read about biofuels in National Geographic magazine.)
The amphipod enzyme "can produce glucose from biomass of trees, weeds, straw, as well as paper," he said.
This could be a method of producing industrial ethanol without corn or sugarcane, which puts pressure on the global food supply.