A baby king crab hangs onto its nanny sea pig. Credit: ©2011 MBARI

On the muddy grounds of the deep ocean, sea cucumbers are playing nanny to young king crabs. But are they being compensated?’

These sea cucumbers, commonly known as sea pigs, are bottom-dwelling creatures that look like grapefruit jelly with legs and could fit in the palm of a hand. Juvenile King crabs are around half an inch long, less than one-sixth the size of a sea pig.

Off the coast of Monterey Bay, remotely operated underwater vehicles (ROVs) videotaped juvenile king crabs clinging to sea pigs. Researchers had never seen this behavior before, and think that this relationship could be protecting the crabs from predators.

“It’s like looking for a port in the storm,” said James Barry, ecologist and lead author of the study at the Monterey Bay Aquarium Research Institute (MBARI) in Moss landing. Sea cucumbers are the ports or the biggest buildings to hide next to in an otherwise empty area.”

Whether nanny, port, or protective building, sea cucumbers seem to be playing an important role in the survival of these juvenile crabs.

The researchers published their results in Marine Ecology in October.

The scientists from MBARI and the Monterey Bay National Marine Sanctuary sent two ROV missions down to the bathyal continental slope around 4,000 feet deep off Monterey Bay in California. This area is devoid of seaweed and rocks, which offer protection for the crabs.

But the juvenile crabs seem to have found a way around this problem.

Barry and his team found a total of 600 juvenile crabs, 96 percent of which were either clinging onto sea cucumbers or hanging around right next to them. Sometimes the crabs were upside down holding onto the belly of the sea pig and other times they were crawling on its side. In some cases, the researchers found more than one crab on a sea cucumber. Of the nearly 2,600 sea cucumbers videotaped, 22 percent had at least one juvenile crab clinging to them. Although symbiotic relationships—those that involve two separate species living together—are not uncommon in the sea, this was the first time this relationship had been observed.

The researchers, however, are unsure why this relationship exists and whether both nanny and crab benefit.  “It could be that the sea pig is thinking, ‘I’ve got another crab on me, how do I get rid of this thing?’,” said Barry. “But it could also be that the crab is crawling around and pinching things off of the sea pig and cleaning it.”

“It is possible that the crabs are feeding on potential parasites that might settle on the cucumbers,” said Dave Pawson, an emeritus senior scientist at the Smithsonian National Museum of Natural History who studies sea cucumbers and who is not part of the study. “But the cucumbers don’t seem to be pestered by many forms of parasites.” According to him, it wouldn’t be surprising if the cucumbers weren’t getting anything out of this relationship.

Barry and his colleagues suggest that sea pigs may be helping the crabs find food. Sea pigs can “smell” their environment with legs at the top of their head that look like antennas, according to Pawson. Researchers think these structures help them sense the seafloor and help them move toward tasty sediments and decaying marine animals such as whales. “Perhaps the crabs might benefit directly from being transported to richer environments,” he said.

The researchers have not seen any adult crabs in the area. This suggests that the juvenile king crabs may only be protected until they grow too large for the sea pig and are eventually eaten anyway by fish in the deep sea. “I think their larvae may have landed in the wrong spot,” said Barry. In fact, both adults and juvenile king crabs usually dwell in rockier terrain. Once they are too big for the sea pigs, they are still small enough to be predated, he added. “This may be a last attempt to survive on their part; we don’t know.”

The only way to understand this relationship between crab and pig is to do some experimental work, according to Barry. Next steps could be to put juvenile crabs on a seabed near sea cucumbers and put others where there are no sea cucumbers. Then, the researchers would periodically check on them to see if they die or disappear. “I think that predators would munch down those smaller crabs that don’t have the protection of the sea pigs,” said Barry.

Millions of sea cucumbers in the deep sea constantly feed on sea floor sediments and almost as continuously poop, so they play an important part in recycling nutrients and filtering sediments in the sea, according to Pawson. “The cucumbers themselves are crucially important in the deep sea and thus, it becomes important to learn about the roles that associated animals play—such as juvenile king crabs.”

“There are many species that engage in some sort of refugian protection,” said Barry. “This is just one case when the species wants to hide, but there is nowhere to hide.”

This post first appeared online on the Scientific American Guest Blog on December 26, 2016.