Symbiont Garden: Epibionts on Kiwa hirsuta

Most symbiotic relationships in deep sea hydrothermal vent systems are between an organism and endosymbionts which reside within bacteriocytes in the epithelium of the organism, however in decapods crustaceans the symbionts are epibiotic, living on the outside of their host. These usually occur on the setae of the decapods. Epibionts are particularly useful to their host as they can provide all stages of the life cycle with protection against pathogenic organisms and predators; this is done through the epibiont secreting compounds which facilitate the protection. In this article the authors investigate the trends between epibionts and decapods interactions and also investigate the possibility of epibionts that may be indigenous to invertebrates. They do this by taking samples of the epibionts from the setae of Kiwa hirsuta, a decapods crab found in the cracks along the Pacific Antarctic ridge where the temperature is relatively lower and also found within the mussel beds around black smokers where the temperature is tolerable to them, and using a comparative 16SrRNA analysis coupled with Fluorescent in situ Hybridisation analysed the phylogenies of the bacteria which were thought to be sulphur oxidising bacteria. The use of electron microscopy in order to assess the structure of the setae seemed to prove very insightful as it was then found that the bacteria only attached themselves to the chitinous outer layer of the setae and mostly at the distal end. The main morphologies of the bacteria included thick and thin filamentous communities, though it was noted that there was a distinct absence of structures which would allow for attachment of the filamentous communities to attach to the setae. This paper uses a variety of genes which are involved in the reductive Tricarboxylic Acid (rTCA) as well as the genes involved in sulphite oxidation and sulphate reduction to identify the potential function that the epibionts might perform for K. hirsuta. This does, I feel grant the papers results more power in determining the phylogeny of these epibionts and how they interact with the host organisms. As decapods crustaceans grow most need to undergo ecdysis to shed their rigid cuticle and grow a cuticle more appropriate to the new size of the organism, this raises the issue of how does the organism end up with new bacteria as the old onew would still reside on the cast off cuticle. The authors mentioned that it is possible that after ecdysis free living bacteria could colonise the newly moulted organism. They address the issue of recognition between the bacteria and the organism with the theory that the host could exude an exopolysaccharide which would facilitate the binding of epibiont to the host. They do admit that many questions remain about this process which should prompt the study of this mechanism more closely. The paper found that many of the bacteria residing on the setae of K. hirsuta are a species of Proteobacteria that possesses the correct enzymes to fix CO2 for the rTCA cycle and has the capacity to cycle sulphur species through the oxidative and reductive pathways, perhaps these bacteria are merely hitching a ride for the nutrition from the vents and in return the compounds that they create offer protection to their host? 

Goffredi SK., Jones WJ., Erhlich H., Springer A., Vrijenhoeck RC., 2008, Epibiotic bacteria associated with the recently discovered Yeti crab, Kiwa hirsuta, Environmental Microbiology, 10, 2623-2634