A review of: Thurber AR, Jones WJ, Schnabel K (2011) Dancing for Food in the Deep Sea: Bacterial Farming by a New Species of Yeti Crab. PLoS ONE
Symbiosis is a way of life for many creatures and a survival technique for others. We heard about the symbiosis between V. fischeri lighting up its host squid in Colins lectures and have seen many examples of corals becoming a home for a large collection of bacteria. The symbiosis here is shown between the newly described, Kiwa puravida, a methane - seep crab and the bacteria that grow on its chelipeds (claws). Although there has been many studies completed on epibiont symbiosis, exactly how the collection of bacteria is attained remains a mystery and their benefit to the host is unclear.
Firstly the bacteria found associated with K. puravida was investigated. Interestingly it seems that much of the bacteria found to be growing on this new species are similar to that found on other decapods whose habitat includes vents and seeps, only two were unique to K. puravida. This backs up hypotheses that suggest horizontal transmission events by vent species epibionts have occurred. As well as this, organisms found at vents and seeps do not often co – exist (although similar species occupy both as the same chemical reactions seem to fuel the vents) suggesting the symbiotic bacteria have free living stags which allow them to disperse among communities.
K. puravida were seen to be waving its chelipeds at methane seeps which have a high nutrient content. Carbon isotopes provide evidence that the crab uses the bacteria as its main food source. The carbon isotope composition found in K. puravida is lighter than that found in phytoplanktonic production and so the energy source for the crab does not come from filtered phytoplankton. The fatty acid composition of the crabs muscle tissue reflected that which is also found in the bacteria which cover the crab’s setae and the abundant fatty acid 16:2 was not found in the phytoplankton sample. This however does not completely reject the idea that some nutrition may be scavenged by the crabs. Shrimp found in the same habitat are known to scavenge free living microbes from surrounding rocks. Although the rocks where Kiwa was found were not tested, rocks from the same seep did not contain any 16:2 fatty acids either; this plus the lack of scavenging behaviour support the hypothesis that the main food source for K. puravida comes from the bacteria found on its setae. The morphological features and behaviour of the crab shows us how it has become perfectly adapted to its habitat. It has specific mouthparts which allow it to scrape the harvested bacteria from its setae to its mouth. It also facilitates the growth of its epibiotic symbionts by waving its chelipeds to remove boundary layers which form around active bacteria and can stunt growth by reduced productivity. The waving allows the bacteria to access oxygen in the water column and the sulphide or methane it needs from the seeps, therefore increasing the chemoautotrophic yield and in turn the food source for the crab. It is farming its epibiotic bacteria.
The role of epibionts remains unclear. There doesn’t seem to be a regular pattern of behaviour between the bacteria and their many hosts found around the vents. Rimicaris exoculata has not been seen farming its bacteria and does not have the correct features to do it either, others do not have similar fatty acid composition to the bacteria found covering it. Kiwa puravida has been shown to farm its bacteria and it consumes it with adapted mouthparts whilst ensuring its symbionts get the nutrition need to produce a large yield. It’s a new and interesting form of symbiosis. It would be interesting to see if the rocks where Kiwa were found contain any fatty acid composition similar to what is found in its muscle tissue, and if not, scavenging for food could be eliminated as a source of nutrition for this crab.
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