Abstract
Populations of Limnodrilus sulphurensis Fend, Liu & Erseus, 2016 (Naididae, Oligochaeta) were found on sulphur bacterial mats in a stream with sulphidic and hypoxic water in Sulphur Cave, Colorado (USA). In order to reveal adaptations to the hostile conditions, we assessed physical and chemical factors in reference to anatomical and physiological details of L. sulphurensis and compared with the common L. hoffmeisteri from non-sulphidic waters. Sections and 3D reconstructions of the posterior body of L. sulphurensis showed an extensively ramified integumental blood system and an extraordinarily rich circumintestinal chloragogue tissue. This contained accumulations of sulphide and iron. These features coincided with an unusual blood physiology of these haemoglobin-containing annelids living under permanent sulphide exposure. The blood of L. sulphurensis has a high oxygen binding capacity and the binding sites seem unaffected by sulphide (sulphhaemoglobin is not formed). Exposure experiments indicated oxidation of sulphide to thiosulphate, which can be easily released. Hplc analysis of the chloragosomes showed the presence of haemin (presumably haematin), a known catalyst of sulphide oxidation. L. sulphurensis shares these features with many marine “sulfide-annelids”. For the freshwater realm these data are novel. The proposed “sulfur-haeme scenario” would explain the unrivalled existence of L. sulphurensis under sulphidic conditions.
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