Spawning and metamorphosis of Osedax boneworms

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Abstract

We report observations on spawning and early development in bone-eating worms of the genus Osedax. Individual female Osedax rubiplumus were observed at 1820 m depth spawning hundreds of oocytes, and females of an undescribed species, Osedax "orange collar", were observed spawning in laboratory aquaria. Cytological and molecular analysis of the spawned oocytes of two Osedax species revealed no evidence for the bacterial endosymbionts that the female worms require for their nutrition, suggesting that the bacteria must be acquired later from the environment, as they are in other siboglinids. Individual O. “orange collar” females released an average of 335 (± 130) per day. Fertilization rates of the spawned oocytes varied from 0 to 100%, though most females showed nearly 100% fertilization rates. Oocytes spawned in the laboratory at 4-6°C were negatively buoyant. If fertilised, these oocytes extruded polar bodies and then after at least four hours cleaved unequally. Subsequent cleavages ioccurred in a spiral pattern at roughly 2-h intervals, resulting in free-swimming larvae after 24 h. These lecthotrophic trochophores swam for 9–16 days before settling with several hooked chaetae, similar to those of dwarf Osedax males. The larval life span in the Osedax species studied in the laboratory appears to be shorter than in closely related Vestimentifera. Osedax rubiplumus, on the other hand, has much larger oocytes and so may have greater dispersal potential than these other Osedax species. The high fecundity and continuous reproduction of Osedax boneworms permits the opportunistic exploitation of sunken vertebrate bones.