Numerous marine invertebrates (molluscs, annelids and nematodes) have exhibited the ability to gain nourishment from organic molecules dissolved in water and many hydrothermal vent organisms are able to assimilate energy and nutrients from chemosynthetic epibionts (Van Dover et al., 1988; Ponsard et al., 2012). Despite previous uncertainty, the direct transfer of molecules from bacteria to R. exoculata is believed to be an important source of nutrition, perhaps even the greatest source for this species (Hugler et al., 2011; Ponsard et al., 2012). This much debated topic has encouraged recent research into investigating the processes taking place between the shrimp and bacteria. Investigations into the composition of fatty acids and stable isotope signatures have provided compelling support for the epibiotic bacteria being a major supplier of organic material for the shrimp, as opposed to the grazing of bacteria that live on the walls of vent chimneys (Hugler et al., 2011; Ponsard et al., 2012). The bacteria inhabiting the shrimp’s gill chamber are devoid of scrape marks that would otherwise suggest they were being scraped off and ingested whole (Ponsard et al., 2012). Directly transferring energy, in the form of organic molecules, across the integument of the gill chamber would be a far more efficient method of metabolising energy as opposed to the ingestion of grazed or filtered microbes (Jannasch, 1985).
The morphology of the gill chamber lining (integument) is also suggestive of a direct transfer of molecules taking place (Ponsard et al., 2012). The distance between the water and the blood is much reduced in the gill integument of R. exoculata; the cuticle lining the integument is very thin, only 30 µm. Furthermore, infolding of the tissue lining and an increased amount of mitochondria within the cells suggest the capability for the uptake of dissolved molecules and the transportation of fixed carbon from the bacteria across the integument of the shrimp (Ponsard et al., 2012). The recent experiment by Ponsard et al. (2013), unlike previous studies, involved testing for the transfer of molecules on live shrimp and then analysing tissue removed from the gill chamber. The results were positive and demonstrated the transfer across the integument from bacteria to shrimp. This was the first time this capability was recorded in a decapod crustacean and provided supporting evidence for the mutualistic relationship between epibiotic bacteria and R. exoculata (Ponsard et al., 2012).
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