Early studies of Rimicaris exoculata failed to find any symbiotic bacteria within the body (endosymbiosis), a common form of nutrition in other vent organisms (e.g. tubeworms), so Rimicaris was initially presumed to be a grazer on the dense bacterial mats (fig. 7) found around hydrothermal vents (Van Dover et al, 1988). However the discovery of dense populations of filamentous chemosynthetic bacteria (fig. 8) covering the carapace, setae and mouth parts of Rimicaris (termed episymbiotic bacteria) suggested an alternative food source (Gebruk et al, 1993). Given the logistical challenges and costs (Ruth, 2006 estimated that a 30-day deep sea sampling cruise costs approximately $1 million) of directly observing Rimicaris, indirect methods were used to determine what proportion episymbiotic bacteria contributed to the diet. Results from Isotope composition and lipid analysis work strongly supported the theory that harvesting of the episymbiotic bacteria was the main food source of Rimicaris (Rieley et al, 1999). However recent work has demonstrated the direct transport of organic material between the episymbionts and Rimicaris (Ponsard et al, 2013) and the precise nature of the nutritional relationship between the two is not yet clear.
The episymbiotic bacteria were initially believed to be of one phylotype (effectively “species or family”) however recent research indicates that the episymbionts of Rimicaris are more diverse than previously thought (Durand et al, 2010). Researchers have been thus far unable to culture the episymbionts in the lab, hampering the study of these organisms however molecular and genetic studies strongly suggest that the different phylotypes are able to utilise a variety of chemosynthetic metabolic pathways (Zbinden et al, 2008). Genes for the oxidation of reduced Sulphur, Hydrogen (Jan et al, 2014) and Methane (Zbinden et al, 2008) have been found and there are strong hints (fig. 9) that the episymbionts may also be able to oxidise reduced iron (Zbinden et al, 2004).
An important finding from the aforementioned studies has been the discovery that metabollicaly distinct episymbionts co-occur within the same individual. Such an arrangement has obvious adaptive significance to the shrimp as it would allow the animal to survive and prosper under changing chemical conditions, e.g. if the sulfide concentration of the vent fluid reduced but methane concentration increased, the relative proportions of the bacterial phylotypes could change accordingly. Such a theory has yet to be demonstrated and would require comparison of the episymbiotic community of Rimicaris between sites. However the suggestion of Iron oxidising episymbionts at the unusually Iron rich Rainbow vent site provides circumstantial evidence for this theory (Zbinden et al, 2004). Given the dynamic nature of hydrothermal vents, a high degree of metabolic plasticity within the shrimps episymbiotic community could be the reason behind Rimicaris’ wide distribution along the MAR.
The chemosynthetic episymbionts depend on a constant supply of warm, sulfide rich water to grow, forcing Rimicaris to also live in hot water. Just how hot is the question the next page of this blog will attempt to answer.