Dancing for Food in the Deep Sea: Symbiosis in a New Species of Yeti Crab

A review of: Thurber A.R., Jones W.J., Schnabel K. (2011) Dancing for Food in the Deep Sea: Bacterial Farming by a New Species of Yeti Crab. PLoS ONE 6(11): e26243. doi:10.1371/journal.pone.0026243

Hydrothermal vents and cold seeps are a good place to look for examples of chemoautotrophic bacterial symbiosis. As understanding of these reducing systems widen, new species are being constantly revealed. In 2005, a new family of crab were discovered at a hydrothermal vent. The individual was named Kiwa hirsuta and it had chelipeds, or claws, that were covered in dense setae with epibiotic bacteria which led to the common name, ‘Yeti crab’. In 2006, a second species of the Yeti crab, Kiwa puravida n. sp, was discovered, in a methane seep in Costa Rica, which is formally described in this paper. However, I will focus on the interesting example of symbiosis that is described in this paper.

Despite these discoveries, how epibiont bearing crustaceans harvest their symbionts has remained largely elusive. Only the crab, Shinkaia crosnieri, has been observed scraping off its epibiotic bacteria which it transfers to its mouth. Yet, even the importance of this is unknown. A key aspect of this type of mutualistic symbiosis (farming) is the direct transfer of energy from the symbionts to the host. This can be shown through biomarker analysis, and carbon isotopic and fatty acid biomarker analysis are used in this paper.

Morphological and molecular data suggest that Kiwa puravida n. sp is a new species of the Kiwaidae family with high similarity in 18s rRNA sequence (98%) to the related Kiwa hirsuta. Additionally, 16s rRNA bacterial gene sequences were collected from Kiwa puravida n. sp including ε, δ and γ-proteobacteria. γ-proteobacteria were similar to the other epibionts collected from Kiwa hirsuta, Shinkaia crosnieri and Rimicaris exoculata (shrimp), being 97% and 98% similar across the four taxa. Phylogenetic analysis of the proteobacteria found that two clades of ε-proteobacteria were unique to Kiwa puravida n. sp and that, interestingly, phylotypes of some ε-proteobacteria were more similar to phylotypes of other species than those from the same host.

Phylogenetic analysis found that there is an epibiotic fauna that specialises in these reducing systems and the presence of closely related ε and γ-proteobacteria on a variety of hosts suggests multiple horizontal transmissions by these epibionts. Additionally, as the hosts are likely to inhabit more than one site, there is the suggestion that these similarities mean free-living bacterial stages of these symbionts.

This new species, Kiwa puravida n. sp was not observed scavenging food as was suggested for its relative Kiwa hirsuta. Biomarker analyses found an abundance of 16:1 FA (monounsaturated fatty acids) alongside the presence of 16:2 FA which along with an isotopic composition that indicates chemosynthetic nutrition, suggests that Kiwa puravida n. sp ’s main food source is its epibiotic bacteria.

Kiwa puravida n. sp has both morphological and behavioural adaptations to harvest its symbionts. A set of specialised comb row setae on its 3^rd maxilliped, a mouth appendage, is used by the crab to scrape bacteria off the whip-like barbed setae which adorn its chelipeds, sternum, and pereopods (legs) and transfer them to its mouth. But, a species must also facilitate the growth of its epibionts in order to farm them. Kiwa puravida n. sp does this by providing an attachment substrate in the form of these setae but it also moves its chelipeds continually which increase the epibiont productivity. How? Chemoautotrophic symbionts require access to oxygen and reduced compounds such as sulphide or methane from the seep. During carbon fixation, a boundary layer is formed resulting in the depletion of one or more of these solutes, limiting productivity. The behaviour of the crab moving its chelipeds removes these boundary layers and so restores productivity.