63.2 Jan. 7 The dwarf males of Osedax (Siboglinidae, Annelida). ROUSE, G.W.*; WORSAAE, K.; GOFFREDI, S.K.; VRIJENHOEK, R.C.; Scripps Institution of Oceanography; Zoological Museum, Copenhagen; California Institute of Technology; Monterey Bay Aquarium and Research Institute email@example.com
All Osedax spp. discovered to date in Monterey canyon show marked sexual dimorphism, unlike all other siboglinids. Osedax males are found living in the lumen of the gelatinous tubes inhabited by the much larger females. The males appear to be arrested larval forms and are filled with all stages of spermiogenesis, with mature sperm accumulating in an anterior duct that opens the surface of the head. We have studied the anatomy of the males using light, confocal laser scanning and electron microscopy. Larval features appear to be the retention of an anterior ciliary ring (prototroch), two chaetae-bearing segments posteriorly, a typical annelid larval nervous system and the presence of yolk. The sperm structure is very similar to that of other siboglinids such as Riftia and Siboglinum, but there is no evidence of sperm being packaged as spermatozeugmata, or in spermatophores. The chaetae are long-handled hooks with curved teeth that are opposed in some cases, as seen in other siboglinids. There is no sign of the development of a gut, or any features typically seen in females such as the four anterior palps. Bacteria are present on the surface of males but there are no internal bacteria as seen in females. We present data showing the gradual accumulation of males by females as they grow and discuss the evolution of this life history.