Invertebrates
Zombie Worm
Osedax · also called bone-eating worm, bone worm
When a whale dies and sinks, its skeleton becomes an ecosystem that lasts for years. The last animals to arrive are small red worms that carpet the bones and slowly eat them from the inside. They have no mouth, no gut, and no eyes, and the males are microscopic and live inside the females.
Overview
Osedax, the zombie worms, live on the deep-sea floor wherever the bones of whales and other vertebrates come to rest. An adult has no mouth, stomach, intestine, or anus. It feeds through a green, branching, root-like mass that bores into the bone, secreting acid to dissolve it and relying on symbiotic bacteria in the roots to digest the fats and protein released. A plume of red gills waves above to take up oxygen.
Dwarf males
The visible worms are all female. The males never grow up, staying at a larval size a fraction of a millimetre long, and dozens or hundreds live inside a single female’s tube, existing only to fertilise her eggs. Whether a larva becomes a female or a dwarf male depends on where it settles.
An ancient recycler
First found on a grey whale skeleton in 2002, Osedax also eats the bones of fish, seabirds, and cattle, and its distinctive borings have been found in the fossil bones of a plesiosaur, so bone-eating worms may predate whales by 100 million years.
Status
Not Evaluated — apparently common and worldwide, but only found by chance on whale falls or on bones scientists deliberately sink.
Zombie Worm: common questions
How does an animal with no mouth or stomach eat?
The zombie worm has outsourced digestion. Its rear end grows into a green, branching mass of tissue that bores into and spreads through a bone, and this tissue is packed with symbiotic bacteria. The worm secretes acid from these roots to dissolve the mineral of the bone, releasing the fats, oils, and protein trapped inside, and the bacteria then process those compounds into forms the worm can absorb through its own cells. There is no ingestion, no gut passage, and no waste in the ordinary sense; the whole feeding system is a partnership between the worm's root network and the microbes it farms.
Why is it called the zombie worm?
The formal genus name, Osedax, is Latin for bone-devourer, which captures what it does. The common name zombie worm comes partly from the eerie, bloodless way it consumes a skeleton, hollowing out bone from within, and partly from the image of a whale carcass on the sea floor slowly being reanimated as a writhing red field of these worms. The plume-topped, root-anchored body also looks strange and plant-like rather than obviously animal, which adds to the effect.
What is going on with the male zombie worms?
The zombie worm has one of the most extreme size differences between the sexes of any animal. The worms you can see, with their gill plumes and bone-boring roots, are all females. The males never grow up. They remain at essentially a larval stage, a fraction of a millimetre long, still carrying a yolk sac, and they live inside the gelatinous tube that surrounds a female, sometimes more than a hundred crammed into one tube. Their only function is to fertilise the eggs the female produces. Which sex a larva becomes seems to depend on where it lands, on a bare bone it becomes a female, and if it lands on a female already established there it becomes one of her dwarf males.
Did zombie worms exist before whales?
Almost certainly, in some form. Genetic clock estimates put the origin of Osedax well before the modern whales evolved, and the worms readily consume the bones of fish, seabirds, and other animals, not just whales, so they did not need large whales to get started. Supporting this, a 2015 study found the distinctive borings that Osedax roots leave in bone in the fossilised skeleton of a plesiosaur and a sea turtle from the age of dinosaurs, suggesting bone-eating worms were recycling the sunken bones of marine reptiles more than 100 million years ago, long before the first whale.