Regulation of gutless annelid ecology by endosymbiotic bacteria

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Abstract
In studies on invertebrates from sulphidic environments which exploit reduced substances through symbiosis with bacteria, experimental ecological results are often underrepresented. For such studies the gutless oligochaete Inanidrilus leukodermatus is suitable due to its mobility and local abundance. It contains endosymbiotic sulphur-oxidizing bacteria and inhabits the sediment layers around the redox potential discontinuity (RPD) with access to both microoxic and sulphidic conditions. By experimental manipulation of physico-chemical gradients we have shown that the distribution pattern of these worms directly results from active migrations towards the variable position of the RPD, demonstrating the ecological relevance of the concomitant chemical conditions for these worms. Their distributional behaviour probably helps to optimize metabolic conditions for the endosymbiotic bacteria, coupling the needs of symbiont physiology with host behavioural ecology. The substantial bacterial role in the ecophysiology of the symbiosis was confirmed by biochemical analyses (stable isotope ratios for C and N; assays of lipid and amino acid composition) which showed that a dominant portion of the biochemical fluxes in the symbiosis is bacteria-based. It appears that the distributional and nutritional ecology of the gutless annelids is strongly influenced by their symbiosis with sulphur bacteria.

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