Sea cucumbers bloat themselves to zoom around the ocean

Sea cucumbers, long confined to the slow-moving category of marine critter, are actually high-speed travelers, new research reveals.

When they fancy a change of scene, the sea creatures balloon up and hitch a ride on ocean currents, floating and bouncing around like tumbleweeds of the sea.

Sea cucumbers (also dubbed sea slugs, because of their unhurried nature) were thought to be capable of swimming long distances only during their larval stage, like most bottom-dwelling sea creatures. After becoming adults and settling down, they either crawled, or—if a predator was lurking around—crawled slightly faster.

But it seems they've been hiding a far more efficient mode of transport. By flooding their bodies with water, they reduce their density until they’re buoyant, detaching from the floor and opening themselves up to the mercy of the sea. (And at least one, Enypniastes eximia or informally the “headless sea chicken,” has been spotted using winglike fins to swim.)

“They take up water from all the orifices they can, including through the anal opening,” says Annie Mercier, a marine biologist from Memorial University of Newfoundland in Canada and coauthor of the study, published January 12 in the Journal of Animal Ecology. The animal's respiratory system, which uses the anal opening to move water in and out of its body, sucks in water and becomes flooded. Some sea cucumbers then went bottoms-up, with the heavily dilated anus floating around to the top spot.

Mercier, who has been researching sea cucumbers since the 1980s, investigated a trail of anecdotal evidence about cucumber bloating. She and her colleagues observed two species of sea cucumber in the lab and field: Cucumaria frondosa, a cold-water species living throughout the North Atlantic and Arctic oceans, and Holothuria scabra, from tropical waters in the Indo-Pacific.

In the lab, the researcherstested the cucumbers' response to salinity levels, proximity to others, and the levels of sediment—re-creating storms, strong currents, or after trawling activities on the sea floor. When salt levels dropped too low, or sediment levels grew too high, the sea cucumbers would flee the scene. Within minutes, some cucumbers increased their water-to-flesh ratio by over 700 percent, ballooning up and drifting away quickly.

Videos taken from a boat showed bloated C. frondosa tumbling through the ocean, and analysis suggested some were traveling at speeds of up to 56 miles (90 kilometers) per day, faster than they can swim as larvae.

Steve Purcell, a marine biologist at Southern Cross University, once scoffed at fisherman discussing H. scabra floating around New Caledonia. He says that the study “gives credit to this phenomenon,” but he cautions against extrapolating too far from these results. “If this behavior only lasts for five minutes, the dispersal potential is different than if it lasts for five hours.”

Ocean cleaners

Sea cucumbers are both ecologically and commercially important, and can be found covering coastlines and lining abysses from equator to pole. They vacuum up sand and poop it out cleaner, providing crucial services to ecosystems.

They are also important hosts for other marine organisms: Some fish even live inside their rear ends. (How a fish survives in a sea cucumber’s bum.)

Many scientists are interested in their remarkable regeneration abilities, too. Some sea cucumber species have been known to jettison their internal organs to satisfy lurking predators, before regrowing them in safety elsewhere. (Watch a video of sea cucumbers fighting with their guts.)

Turtles and larger fish consider sea cucumbers a delicacy. So do humans. The sea creatures have been harvested as food for hundreds of years in Asia, where rampant demand and overfishing for the luxury seafood has directly contributed to 16 species gaining a place on the IUCN's Red List of Threatened Species. With Asian stocks depleting and demand rising, commercial operations are booming in other parts of the world, threatening other species. (Learn more about how sea cucumbers are being overfished.)

Potential for migration

Sea cucumbers’ newly discovered abilities may affect efforts to save them. “The research rightly highlights the potential implications for fisheries, and indirectly also for marine protected areas, given that assumptions of being sedentary are apparently incorrect,” says Andy Allen, a wildlife ecologist from the Swedish University of Agriculture. As sea cucumbers are capable of moving in and out of management areas, he adds, this affects population estimates, harvest quotas, and conservation efforts.

He also foresees several future avenues of research given their potential active mobility, wondering, for example, if they can take advantage of nutrient dynamics brought about by seasonally changing ocean currents. "Are they capable of migration?” he asks.

The scientists noticed that one species, H. scabra,would bloat and float more during full moons, and at night. This suggests the behavior is deliberate, and not random, explains Mercier. “It does tell us probably that it's a means of dispersal,” she adds.

Other bottom-dwelling species—sea stars, sea anemones and even some soft corals—have been observed bloating too, says Mercier. “It's possibly more widespread than we would think.”