From mind-controlled ants to zombie worms. Although the ‘zombie’ tag is a bit misleading as these worms do nothing any more sinister then feed on the dead flesh of whales at the bottom of the ocean. This specialist genus of worms, properly known as Osedax, was first discovered by science in 2002 when a remotely operated vehicle from the Monterey Bay Aquarium Research Institute discovered them feeding on the bones of a dead whale at a depth of 2,893 metres. They are certainly strange creatures, related to more familiar earthworms and yet specially adapted to life in the deepest oceans, and since their initial discovery they have been documented several times scavenging on dead whales. Now evidence has emerged which suggests that these worms many be far more ancient then previously suspected.
A paper published in Biology Letters last month (April 2015) documented the discovery of evidence of Ostedax from the fossil record. And not just any fossils but the flipper of a giant marine predator – a plesiosaur, and from the shell of an ancient turtle. Plesiosaurs were large swimming reptiles which died out at the end of the Cretaceous alongside the dinosaurs, and various other large reptile groups, but during their reign they were represented by dozens of different species. They were generally large, and although there were long necked and short necked varieties all had four large fins which propelled them through the water. They first emerged during the late Triassic and came to dominate the oceans during the Jurassic. This particular specimen was 100 million years old and its bones showed clear traces of Osedax feeding.
These tiny worms have no mouth, gut or anus but instead rely on symbiotic bacteria to help them bore into the bones of the whales they feed on, and use a combination of feathery ‘gills’ and a bizarre root-like appendage to absorb nutrients. It is this ability to drill into the bones themselves to feed which led to their scientific name ‘Osedax‘ which means ‘bone-eating’. The process leaves characteristic marks on the bones and it is these marks which the researchers were able to identify. In order to examine the fossils more closely they took 3D scans of the bones, using equipment at the Natural History Museum, London. This allows for the production of a computer model of the fossil which reveals far more information then available from the physical specimen. Such techniques are becoming increasingly common in palaeontology as the technology improves and, as in this case, it can produce some exceptional results.
This discovery changes our understanding of how and when these bizarre little animals evolved. It had been thought, given their rudimentary physiology, that Osedax evolved from another group of worms which feed exclusively off chemicals in the water. By absorbing the nutrients directly they do away with the need for a mouth or gut but this split of lineages was only thought to have happened around 45 million years ago. Now we know that that hypothesis was probably wrong this this fossil puts their evolution back to at least 100 million years ago. This discovery also raises an interesting answer to one of the other major mysteries which has surrounded Ostedax since their discovery. If they feed only on whales and yet whale carcasses are quite rare then how do Ostedax survive between feeds? Are they entirely limited to whales and therefore have to have some method of surviving periodic famines or are they more generalists and feed on other animal remains? Now the answer seems clear. Ostedax evolved as generalists, able to feed on a variety of reptilian remains, and since turtles survived the Cretaceous extinction they likely still feed on them as well.
Reference: Silvia Danise & Nicholas D. Higgs. Bone-eating Osedax worms lived on Mesozoic marine reptile deadfalls. Biology Letters, published online April 15, 2015; doi: 10.1098/rsbl.2015.0072