Zombie worms don’t crave brains: instead they seek bones. The 1 to 3 inch (2 to 7 centimeter) Osedax worms were first discovered living in the bones of a rotting gray whale on the deep sea floor, nearly 10,000 feet (3,000 meters) deep, in 2002. Since then, more Osedax species have been discovered: there are 26 according to the World Register of Marine Species.
Zombie worms don’t eat mineral bones directly; instead, they digest fats within the bone. However, their style of “eating” is quite different from ours because they don’t have a mouth or a stomach! They secrete an acid from their skin that dissolves bone, freeing up the fat and protein trapped inside. Then, symbiotic bacteria living in the worms’ bodies digest the fat and protein. How Osedax acquire nutrients from the bacteria isn’t known: they may simply digest the bacteria, or nutrients are somehow transferred to the worm.
While Osedax are best known for extracting some of the final nutrients from whale skeletons fallen to the deep, they don’t discriminate among bones. They’ve also been found on fish bones, and have colonized cow bones dumped from a ship.
They hold onto whatever bones they can find by drilling in with roots, which contain the symbiotic bacteria. Feathery plumes splay from the other end of their bodies, which act as gills to extract oxygen from the seawater. Zombie worms can retract these plumes into the body when they’re disturbed.
If all this isn’t strange enough, the only worms doing any drilling are female: the microscopic males live inside their bodies. One study counted 111 males inside just one female zombie worm! This eliminates the pesky step of having to search for a mate because the eggs and sperm are right next to each other. Then the worms can disperse many fertilized eggs far and wide, hoping that they land near some recently-fallen bones.
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Editor's Note: Updated in 2019. The number of Osedax species was updated from five to 26 to reflect the new species discovered since the publication of this article.