Deep beneath the waters of Costa Rica, dozens of crabs are waving their claws in unison, in what seems to be a rhythmic performance. It’s almost as if these crabs are locked in a ritual dance. But these charming crabs are not dancing. They are farming.
The hairy claws of these crabs are covered with bacteria. With every swing of their arms, they mix up the water column and provide their homegrown bacteria with additional nutrients. The submersible team that discovered this new species shot this amazing video of the gardening crabs in action:
These white and hairy crabs are a new species of ‘Yeti crab’. The species received the formal name of Kiwa puravida in PLoS ONE article last week, meaning ‘pure life’, which is a common saying in Costa Rica.
The first Yeti crab (Kiwa hirsuta) was discovered in 2006 off the coast of Easter Island. The team that discovered this crab already noticed that its bristled claws were covered with bacteria. They only collected a single specimen however, limiting the opportunities for a thorough investigation of the association between bacteria and crab. The nature of their relationship remained a mystery.
Until now, that is. Not long after these first Yeti crabs were found, more Yeti’s revealed themselves, over 6,500 kilometres away from Easter Island. This new species was discovered thanks to one submersible pilot. “Gavin Eppard is one of the pilots of the ALVIN submersible. He was in the sub when he spotted the new species of Yeti crab, standing on a carbonate block waving their claws back and forth”, says Andrew Thurber, one of the authors of the recent paper. “Gavin was on the original cruise that discovered the first Yeti. He immediately recognized that this was something new to science.”
The submersible team returned to collect more dancing crabs after this initial discovery. All the crabs were found waving their arms near cold seeps, where methane and hydrogen sulfide escapes from the ocean floor. You might think such environments are inhospitable places for life, but several species of bacteria thrive near such seeps. They liberate energy from methane and hydrogen sulfide by stripping the electrons from these molecules and passing them on to oxygen.
These species can form dense mats around cold seeps, but they also grow on the Yeti’s claws. Thurber and his colleagues found DNA belonging to two bacterial families that eat methane and hydrogen sulfide, respectively. Thurber thinks that the crabs perform their dance to make sure that the bacteria always have access to both oxygen from the ocean water and methane or sulfide from the seep. If the crabs would stand still, the symbiotic bacteria growing between its bristles could locally deplete either resource. But by waving their arms, Yeti crabs mix water and seepage, keeping bacterial productivity high.
The symbiotic bacteria of the Yeti crab were most similar to bacteria that live near hydrothermal vents and on the creatures that live there, such as the vent shrimp. Hydrothermal vents are similar to cold seeps, so Thurber suggests they disperse through the oceans using vents and seeps as stepping stones.
While these findings indicate that Yeti crabs grow their own food, Thurber and his colleagues also show that the Yeti’s harvest it. Thurber didn’t observe them snacking on bacteria in the wild, but he did film captured crabs that used their mouth parts to feed from their claws. “I initially put them in the aquarium to see if I could get them to dance. They wouldn’t, making me think that they sway their arms in response to the movement of water or a chemical queue. Instead they ended up feeding off their bacteria, which I was lucky enough to catch on film”, he says. Without seepage to farm in, this poor fellow probably went hungry:
The Yeti crabs themselves also contain traces of feeding symbiotic bacteria in the wild. Carbon comes in a heavy (C13) and a lighter (C12) variety. The enzyme that plants and bacteria use to derive energy from sunlight selects the lighter form of carbon slightly more often than the heavy form. However, the enzymes of microbes that consume methane or sulfide have a very strong preference for C12 over C13. As a result the ‘carbon signature’ of methane and sulfide munchers will be lighter than that of bacteria that obtain energy from sunlight. The carbon profile of the Yeti crab matched that of its symbionts, indicating that they are its main food source. The fatty acid distribution of Yeti crabs mirrored that of its bacteria in a similar manner.
All in all Thurber et al have made a compelling case that Yeti crabs grow and harvest their own bacteria. But don’t these crabs ever get tired from dancing? Thurber: “The crabs have to use energy to swing their arms back and forth – so by doing so they must gain more energy through their symbionts than they expend by waving their arms. I don’t think they get tired.”
The dancing yetis also seem to have more than enough energy to engage in some yeti wrestling from time to time. The ALVIN team captured a video of what seems to be two Yeti crabs fighting for a nice spot in the seep. The challenging crabs had recently molted, so perhaps it wanted a good position to regain its bacterial covering. But Thurber points out that this confrontation could also be a mating display, as crabs are known to mate after molting. Strife or love, you decide:
The Yeti crab’s rise to internet fame was swift. Proof: a compilation of crabs dancing to different pieces of music.