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Osedax worms are marine annelids closely related to the deep sea vestimentiferan worms. The sessile (i.e., fixed in one place) females bore into the bones of whale carcasses--and possibly bones of other vertebrates, as suggested by their documented colonization of experimentally submerged cow bones and of non-cetacean bones that had apparently been tossed off a boat as waste (Vrijenhoek et al. 2008). Sex in these worms is believed to be environmentally determined, with undifferentiated larvae that settle on bones developing as females and subsequent larvae that settle on females transforming into dwarf males. Mature females in at least some species maintain male "harems" and the number of males present grows rapidly as female size increases. (Vrijenhoek et al. 2008). The first Osedax worms were discovered by Rouse et al. (2004), who described two unusual new annelid species found on the bones of a dead Gray Whale (Eschrichtius robustus) off the coast of California (U.S.A.) at a depth of nearly 3000 meters. These worms were very abundant and each had four pinnule-bearing palps and a long oviduct on a trunk enclosed in a transparent mucous tube extending out of the bone tissue. Reaching into the bone marrow were vascularized “roots” extending from an ovisac that was filled with oocytes (developing eggs). The “roots” were packed with symbiotic bacteria of the order Oceanospirillales, a group of bacteria known for heterotrophic degradation of complex organic compounds. These unusual endosymbionts are likely responsible for the nutrition of Osedax worms (Goffredi et al. 2005). All visible worms were females, with trunk widths ranging from 0.2 to 0.5 mm. Associated with the trunk but attached to the mucous tubes of these 2 to 7 centimetre-sized Osedax females were dwarf males that were several orders of magnitude smaller than the females. The tubes of individual females contained numerous microscopic males--as many as 100 or more, with a male: female sex ratio of 17:1. (Rouse et al. 2004, 2008) Rouse et al. (2009) report on the first observations of spawning in Osedax (based on both field and laboratory studies) and describe the zygotes, embryogenesis, and larval development for two species from Monterey Bay, California. Cytological and molecular analysis of the spawned eggs of these Osedax revealed no evidence of the bacterial endosymbionts that female worms require for their nutrition, suggesting that the bacteria must be acquired later from the environment, as they are in other worms in the family Siboglinidae (Rouse et al. 2009).
As of 2009, the total number of known Osedax species was 17 or 18 (most of these not yet formally described). These species are distributed in both the Atlantic and Pacific Ocean basins and are found not only in the deep sea but also on whale carcasses as shallow as 34 meters (Pleijel et al. 2009 and references therein; Vrijenhoek et al. 2009). Vrijenhoek et al. (2009) present a phylogenetic analysis of all known formally described and putative Osedax species based on molecular data (from two mitochondrial and three nuclear genes). They also summarize key morphological features for each taxon and demonstrate close concordance between molecular analyses and patterns of variation in morphological traits, lending support to their conclusions about relationships within this group.