As described above, when gutless oligochaetes were first discovered it was assumed that they gained their nutrition by uptake of dissolved organic compounds from the environment. At the Bermuda site where I. leukodermatus occurs in high abundances, concentrations of dissolved free amino acids and organic carbon, as well as total carbohydrates were higher than in other Bermudian carbonate sediments (Giere et al. 1982). However, in uptake experiments with radiolabeled substrates, organic carbon was taken up much more slowly than inorganic carbon by I. leukodermatus. In contrast, Liebezeit et al. (1983) argued that dissolved organic carbon may contribute significantly to the nutrition of I. leukodermatus. At the Elba site in the Mediterranean, the worms are most abundant in sediments around sea grass beds, indicating that organic substrates from the sea grasses may play a role in the distribution of these worms. Clearly, well-designed uptake experiments are needed in which net uptake (and not only total uptake as in previous studies) of dissolved organic compounds and CO2 are examined in more detail including a differentiation between uptake by the symbiotic bacteria and the host itself.
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