#boneworms

Boneworms (Osedax spp.)

Family: Siboglinid Worm Family (Siboglinidae)

IUCN Conservation Status: Unassessed

First discovered in February of 2002, the roughly 26 species of typically deep-sea dwelling worms in the genus Osedax (known commonly as boneworms, bone-eaters, bone-eating worms or zombie worms) are noted for the extremely specific ecological niche they occupy: when a whale dies and the blubber that aids it in controlling its buoyancy begins to break down, its remains will sink to the sea floor and (depending on where the whale died and the circumstances of its sinking), they may sink to extreme depths where most marine scavengers cannot find them (an event known as a whale fall), with the whale’s soft tissues providing food for a variety of unique deep-sea scavengers for decades at a time. Once the soft tissues have been consumed the whale’s skeleton is left largely intact, at which point the larvae of female Boneworms come to settle on it and develop into their adult forms: small, pale worms with no mouth or digestive tract, but with a network of superficially root-like structures at their base which allows them to anchor themselves onto the skeleton and gradually bore into it. The “roots” of a female boneworm house unique bacteria that live in a mutualistic relationship with the worm, living in the worm’s tissues and secreting substances capable of breaking down the bones of the whale, releasing nutrients into the water that the boneworms absorb directly across their epidermis (the outermost layer of their body,) which is lined with tiny structures similar to the villi in the small intestine that maximizes the worm’s surface area and allows for more efficient absorption of nutrients. The body of a female boneworm is lined with a protective mucus that prevents the substances excreted by the bacteria in their roots from damaging their tissues, and within this protective mucus layer lives a “harem” consisting of hundreds of tiny males, tens of thousands of times smaller than the female they inhabit, which fertilize the clutches of ova (egg cells) that she frequently releases into the water in large numbers, allowing them to develop into new larvae that can later colonize new skeletons. A single whale skeleton may come to support a large number of boneworms (with multiple species sometimes coexisting on a single skeleton), and by boring through the hard outer layers of the bones boneworms expose the softer and more nutritious inner regions (such as bone marrow) to other scavengers such as crabs, amphipods and gastropods (with some, such as the sea snails of the genus Rubyspira, relying on whale bones that have been “softened up” by boneworms as their primary source of food.) Despite the very specific conditions in which they live it seems that boneworms are abundant and surprisingly diverse (with species so-far having been found in the Atlantic, Pacific and Southern oceans), and while wild boneworms have only ever been found living in the bones of whales it has been observed that they will inhabit the bones of other animals, such as cattle, if provided with them, suggesting that they feed on whale bones less out of necessity and more because whale bones are simply the easiest large bones to find at great depths. The presence of marks that closely resemble the holes left by boneworms in the 100 million year old fossilized bones of a plesiosaur and extinct sea turtle found in Cambridge, England seems to suggest that boneworms, or at least their ancestors or relatives, have been around for considerably longer than the whales they now rely upon for food.

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Boneworms may be a little bit eerie, but they’re also a very cool example of just how specialized animals can get, and how even the most bizarre species live complex lives and are part of wider ecosystems. The first image above shows the full body of a female of the northeastern Pacific species Osedax rubiplumus, while the second image shows several females of the southern Atlantic species Osedax braziliensis boring into a whale skeleton.