Osedax - Drilling for nutrition

The recently discovered genus Osedax is a member of the marine polychaete family Siboglinidae, well known for living in obligatory symbiosis with endosymbiotic bacteria in a variety of reducing habitats. While siboglinid larvae are aposymbiotic and most possess a functioning digestive system, adult Frenulata, Sclerolinum and Vestimentifera house chemoautotrophic gammaproteobacterial symbionts in a specialized organ the trophosome. The worms deliver reduced substances like sulphide and/or methane and oxygen to their symbionts, which in return sustain their hosts by providing organic carbon through the fixation of carbon dioxide. This nutritional relationship is widely accepted and has become a textbook example for chemoautotrophic symbiosis between invertebrates and marine microbes today. However, Osedax differ from known siboglinids by exploitation of vertebrate bones sunken to the ocean`s floor in a variety of depths worldwide. Osedax primary oocytes and larvae are aposymbiotic, but the possession of a digestive system is not yet adequately studied. Adult females house heterotrophic bacteria of the order Oceanospirillales. Even though data from stable isotope analysis shows a reliance on bone-derived organic carbon and fatty acid analysis points to incorporation of bacterial products into the worm`s metabolism, the route of nutrients from bone to worms and the specific roles of the symbiotic partners are virtually unknown today. Recent morphological analysis shows, that adult females possess a trophosome integrated into the body wall of the roots and ovisac region. Also the epidermis overlying this organ shows different ultrastructural organisation in the ovisac and the roots region, presumably in relation to bone degradation, nutrient uptake and transfer to the symbionts. In the proposed study, the anatomy and organization of Osedax larvae, as well as the bone demineralization, nutrient uptake and transport with respect to the different symbiotic partners in female Osedax adults will be investigated using ultrastructural, molecular, histochemical and immunohistochemical techniques to elucidate not the physiology of this specific symbiosis, but also to extend our knowledge on nutritional relationships within Siboglinidae and microbial symbiosis in general.