Deep-sea denizens go years without food with clever biological fix

By Marta Serafinko

June 19 (Reuters) - A pill bug dwelling under a garden pot curls its body into a tiny armored ball as self-defense. Far below the ocean surface, some of its much larger relatives face a harder problem: how to stay alive when the next meal may ‌not come for years.

Those creatures are called deep-sea isopods, a group of crustaceans with flattened and segmented bodies that, as new research reveals, have resolved ‌the dilemma with a multifaceted biological fix.

The realm they inhabit is a cold, dark desert far below the ocean surface where food falls only as rare "snowflakes" of dead organic matter from above, according to crustacean ​biologist Jianhai Xiang of the Institute of Oceanology, part of the Chinese Academy of Sciences, one of the authors of the study published in the journal Cell.

"It is a world of perpetual night and crushing pressure, yet life finds a way," Xiang said.

Thanks to their unique biology, these creatures can survive more than five years without food. New research indicates that their solution is partly anatomical and partly genetic - a huge stomach and very low metabolism complemented by the work of a gene that helps control bodily energy production.

These creatures ‌are bottom-dwelling scavengers with 14 jointed legs and a hard ⁠exterior exoskeleton, thriving in the Atlantic, Pacific and Indian oceans. Some are more than half a meter (20 inches) long. Like pill bugs, they too can curl into a ball for protection.

The new findings focus on two species: Bathynomus doederleini, a giant isopod found at ⁠about 300 meters (985 feet) below the sea surface, and Bathynomus jamesi, a supergiant isopod found at about 900 meters (2,950 feet) under the surface.

"Deep-sea isopods have a clever 'earn more, spend less' survival strategy," said Jianbo Yuan, a professor at the Institute of Oceanology and lead author of the study.

In the deeper-dwelling species, the stomach occupies about two-thirds of the body cavity, allowing the animal ​to ​store a large meal when food appears.

Yuan compared it to a "food warehouse" that slowly releases energy ​while the body runs on "standby mode." With drastically lower metabolism, slower ‌digestion and extremely efficient nutrient use, Yuan said, "they can make that one meal last for years."

Their stomach microbes may also help the isopods. In the deeper-dwelling species, bacteria called Chlamydiae - often known for causing disease in humans and other animals - were linked to fat storage, potentially giving the isopod slow-release energy and the bacteria a stable home.

"This is a win-win," Yuan said.

The energy control of isopods also may depend on ND1, a gene that appears to have once belonged to a symbiotic bacterium that lived inside the bodies of these animals before becoming part of their own genome. That process, known as horizontal gene transfer, means DNA moves between distantly related organisms rather than ‌being passed from parent to offspring.

Yuan said the isopod appears to have "borrowed" or "hijacked" a bacterial gene ​that helps control energy production.

"This is surprising because bacteria and animals are very different, and such transfers ​are extremely rare," Yuan added. "The gene gives the isopod an extra tool to ​fine-tune its energy use, especially when it needs to slow down."

Because deep-sea isopods are difficult to study alive, the team tested ND1 ‌in the laboratory in zebrafish, nematode worms and human cells. The ​gene increased metabolism at normal temperatures but, ​under cold conditions, helped save energy and extend starvation survival.

Yuan said ND1 works like a metabolic switch, speeding up or slowing down energy use depending upon the environment.

Study co-author Kahou Chu, a professor emeritus at the Chinese University of Hong Kong, said horizontal gene transfer can give organisms a faster route to ​new traits than ordinary inheritance alone, helping some organisms out-compete ‌others in extreme settings.

The deep sea, Xiang said, is "Earth's largest living space," and the unusual evolutionary adaptations of its denizens can offer ideas for ​medicine, robotics and conservation.

Understanding how animals survive extreme food scarcity, Xiang said, also helps researchers think about resilience in a changing planet, including food-web ​disruptions and climate change.

(Reporting by Marta Serafinko in Gdansk, Poland; Editing by Will Dunham)