Osedax

Like other siboglinids, Osedax lacks a mouth, gut, or anus, and instead depends on colonies of endosymbiont microbes housed inside a trophosome for nutrition. Unlike other siboglinids, however, this trophosome takes the form of a vascularized root system which penetrates bone. These microbes, of the order Oceanospirillales, produce enzymes which hydrolyze collagen from bones, yielding nutrition to the worms. Osedax exhibit very drastic sexual dimorphism, with females being >20,000 times larger than males. Males are paedomorphic and microscopic, inhabiting a section of the females' trunk where they produce sperm from yolk reserves. The epidermis also plays key roles in bone deterioration and nutrient uptake. This process of bone deterioration occurs through a symbiotic relationship with an endosymbiotic bacteria. The cells in the epidermis of the Osedax root region are responsible for the secretion of digestive enzymes. The epidermis also has an expanded microvillus border which increases the surface area. Osedax roots are covered by a mucus sheath that helps protect the worm's trunk. Some studies support the theory that this sheath plays a role in dissolving the bone. This sheath could also play an important role in reducing the damage to Osedax skin by absorbing harmful acid. Another potential function of the mucus sheath is that it could inhibit the breakdown of the worm's bone matrix. This is significant because the bone matrix is integral in maintaining the worm's position while in direct contact with a bone. There is ongoing debate in the literature over whether the symbiosis in Osedax roots is commensal or mutualistic. Endosymbionts The Oceanospirillales symbionts are found in the specialized roots and play a major role in accelerating the degradation process of bones, as well as facilitating nutrient uptake for the Osedax. Oceanospirillales are known for their ability to degrade complex organic compounds. Campylobacterales are abundant along the trunk of the Osedax according to a 2023 study. Different genera in this order are found in Osedax at different points during the whale's degradation: • Members of the Arcobacter genus are the primary early colonizers (140 months, and are key players in its symbiosis with the Osedax host. The Sulfurimonas genus in particular protects the Osedax worms from potentially harmful by-products produced at >140 months of the whale fall degradation. The Sulfurimonas bacteria house the type II and IV sulfide:quinone oxidoreductase genes that encode enzymes to oxidize and assimilate sulfide. These reactions prevent the host from absorbing toxic by-products across the epithelial barrier. ==Sexual dimorphism and life cycle==

Osedax males are notably smaller than their female counterparts. Between 50 and 100 microscopic dwarf males live inside the tube surrounding a single female and never develop past the larval stage; they produce sperm from yolk reserves. • Sessile lifestyle: attach to and rely on females for food, Osedax japonicus in particular has showcased an environmental form of sex determination. Life cycle Female Osedax worms have been observed spawning both in the wild and in laboratory aquaria. Osedax rubiplumus can spawn hundreds of oocytes at a time. They are already fertilized when they are released from the female worm. The worms' endosymbionts, species of bacteria in the order Oceanospirillales, were not observed in the spawned oocytes, which suggests that they are acquired after the worms settle on the bones. Osedax appears to be highly fecund and reproduces continuously. • Mature female Osedax worms spawn eggs into the mucus attached to their tubes, where the embryos develop for 3 days. • Larvae then begin to swim in the water column. This is called the trochophore stage. The larvae settle on whale bones and begin crawling. • During the trocophore stage, male Osedax settle on the tubes of the females, where they are metamorphosed into dwarf males, which can be inside or outside the female tube. • 1 day after settling on bones, larvae use two pairs of chaetae to attach to the substrate. Juvenile worms begin to secrete mucus and develop two ventral palps on the dorsal side of the prostomium. • 2 days after settling, the palps elongate and the heart starts to beat. The roots attach to the bones begin to digest. • 4 days after settling, the trunk and ventral palps elongate, where symbiotic bacteria are detected in the root. • 7 days after settlement, pinnules extend from the ventral palps. • 10 days post settlement, the juvenile worms have 4 palps with pinnules, an oviduct, and a distinct root system. ==Ecology==

'' When initially discovered, some scientists proposed that Osedax was a whale-bone specialist, a notion later disproven by the discovery of Osedax on a variety of skeletal substrates including bone fragments, reptile and bird skeletons, and even human remains. Multiple species of Osedax can co-exist in close proximity, even on the same bone. The role of Osedax in the degradation of marine vertebrate remains is important to marine vertebrate taphonomy. == Evolution ==

The oldest trace fossils on bones characteristic of Osedax are from a plesiosaur humerus from the Cambridge Greensand, England, likely reworked from late Albian (c. 100 million years old) sediments and a rib and costal plate from a sea turtle found in Cenomanian (100–93 million years ago) aged sediments of the Chalk Group, England. Further material is known from the Campanian and Maastrichtian. Following the extinction of almost all large marine reptiles at the end of the Cretaceous, Osedax likely persisted on the bones of sea turtles, marine birds, and fish. ==Species==

'' Selected species: • Osedax antarcticus Glover, Wiklund & Dahlgren, 2013 • Osedax bozoi Berman, Hiley, Read & Rouse, 2024 • Osedax braziliensis Fujiwara, Jimi, Sumida, Kawato, Kitazato • Osedax bryani Rouse, Goffredi, Johnson & Vrijenhoek • Osedax byronbayensis Georgieva, Wiklund, Ramos, Neal, Glasby & Gunton, 2023 • Osedax craigmcclaini Berman, Hiley, Read, Rouse, 2024 • Osedax crouchi Amon, Wiklund, Dahlgren, Copley, Smith, Jamieson & Glover, 2014 • Osedax deceptionensis Taboada, Cristobo, Avila, Wiklund & Glover, 2013 • Osedax docricketts Rouse, Goffredi, Johnson & Vrijenhoek • Osedax estcourti Berman, Hiley, Read & Rouse, 2024 • Osedax fenrisi Eilertsen, Dahlgren & Rapp, 2020 • Osedax frankpressi Rouse, Goffredi & Vrijenhoek, 2004 • Osedax jabba Rouse, Goffredi, Johnson & Vrijenhoek • Osedax japonicus Fujikura, Fujiwara & Kawato, 2006 • Osedax knutei Rouse, Goffredi, Johnson & Vrijenhoek • Osedax lehmani Rouse, Goffredi, Johnson & Vrijenhoek • Osedax lonnyi Rouse, Goffredi, Johnson & Vrijenhoek • Osedax mucofloris Glover, Kallstrom, Smith & Dahlgren, 2005 • Osedax nataliae Gularte, Sumida, Bergamo & Rouse, 2024 • Osedax nordenskjoeldi Amon, Wiklund, Dahlgren, Copley, Smith, Jamieson & Glover, 2014 • Osedax priapus Rouse et al., 2014 • Osedax packardorum Rouse, Goffredi, Johnson & Vrijenhoek • Osedax randyi Rouse, Goffredi, Johnson & Vrijenhoek • Osedax rogersi Amon, Wiklund, Dahlgren, Copley, Smith, Jamieson & Glover, 2014 • Osedax roseus Rouse, Worsaae, Johnson, Jones & Vrijenhoek, 2008 • Osedax rubiplumus Rouse, Goffredi & Vrijenhoek, 2004 • Osedax ryderi Rouse, Goffredi, Johnson & Vrijenhoek • Osedax sigridae Rouse, Goffredi, Johnson & Vrijenhoek • Osedax talkovici Rouse, Goffredi, Johnson & Vrijenhoek • Osedax tiburon Rouse, Goffredi, Johnson & Vrijenhoek • Osedax traceyae Berman, Hiley, Read & Rouse, 2024 • Osedax ventana Rouse, Goffredi, Johnson & Vrijenhoek • Osedax waadjum Georgieva, Wiklund, Ramos, Neal, Glasby & Gunton, 2023 • Osedax westernflyer Rouse, Goffredi, Johnson & Vrijenhoek == References ==