The skin of Osedax (Siboglinidae, Annelida): an ultrastructural investigation of its epidermis.

The symbiotic polychaetes of the genus Osedax living on the bones of whale carcasses have become known as bone-eating worms. It is believed that whale bones are the source of nutrition for those gutless worms and that fatty acids are produced by their symbionts and transferred to the host. However, the symbionts are of the heterotrophic group Oceanospirillales and as such are not able to synthesize organic carbon de novo. Also, they are not housed in close contact to the bone material. We studied the ultrastructure of the integument overlying the symbiont housing trophosome in the ovisac region and the roots region and of the symbiont-free trunk region of Osedax to investigate the host's possible contribution in feeding for the whole symbiosis. The epidermis differs conspicuously between the three regions investigated and clearly points to being correlated with different functions carried out by those regions. The ultrastructure of the integument of the root region changed towards the ovisac region and corresponds with the change of the ultrastructure observed in the Osedax trophosome. We suggest that the epidermis in the root region is tightly linked to bone degradation and nutrient uptake. The trunk region possess two types of unicellular gland cells, at least one of which seems to be involved in secretion of the gelatinous tube of adult Osedax females.