Osedax, the bone-eating worms
Like so many scientific discoveries, this one was made by accident. On February 6, 2002, MBARI biologist Robert Vrijenhoek was using ROV Tiburon to search for deep-sea clams in Monterey Canyon, when he came upon the decaying flesh and bones of a dead whale. "It looked like a rocky outcropping on the sonar," Vrijenhoek says, "but when we approached to take a closer look, we realized that it was a whale carcass about 30 feet long. There was no tail or skin left. There were just bones and a bit of soft tissue, and a wonderful group of organisms living on and around the carcass."
Vrijenhoek continues, "One of the first things we noticed during that first dive were large white mats of bacteria that were decomposing tissue and other parts of the carcass. A little octopus had taken up residence in a ‘cave’ created by the hole in the back of the whale’s skull. Below the octopus’ lair was a pile of crab legs and other crab parts. He was having a great time picking off the bright red lithodid crabs that were crawling all over the carcass, bringing the crabs back to his home, and dropping the debris on his doorstep. So this dead whale had become a little self-contained ecosystem, complete with predators, decomposers, and bacteria."
"But the other thing we noticed was the proliferation of these little red worm-like creatures. They were all over the bones. They were growing like crazy, carpeting the remaining whale bones. The worms had short trunks topped by red plumes, and were about an inch or two in height. There were thousands of them waving in the current. It was really fascinating to watch."
Using the ROV, the scientists collected a sample of the bacterial mat next to the whale's head and then collected a few of the animals living nearby. They also picked up bits of baleen, soft tissue, and a vertebra from the whale's tail, some of which were almost completely covered with the red worm-like animals. When the ROV came to the surface several hours later, the researchers were relieved to discover that the samples were not nearly as putrid as expected. Within a week after returning to shore, research technicians had used DNA analysis to determine that the bones belonged to a juvenile gray whale.
To learn more about the red worms, Vrijenhoek consulted a worm expert, Dr. Greg Rouse, a researcher at the South Australia Museum in Adelaide, Australia. who was already collaborating with MBARI's midwater biologists on the discovery of another unusual worm that lives in the midwater environment. Vrijenhoek sent Dr. Rouse some of these worms, and a few days later he received an email that said ”Bob, I’ve got good news and bad news. The bad news is I don’t think they are annelids at all. They’re not like any worms I’ve ever seen before, anywhere in the world. The good news is that we’ve really got something new and exciting!"
Shana Goffredi, then a research associate with Vrijenhoek’s team, conducted DNA analysis on these unusual animals. She confirmed they were indeed annelid worms, and showed that they were related to the tubeworms that live around deep-sea hydrothermal vents. Like the vent worms, the whale worms had bright red plumes that acted as gills, collecting oxygen from seawater. But what was really strange and unusual was that the whale-fall worms had green, root-like structures that were penetrated the whale bones and branched out in the marrow cavity.
These roots turned out to be "the business end of these worms," as Vrijenhoek put it. The worms have no mouths or stomachs, but they used these roots to digest fats and proteins from the whalebones. Microscopic analysis along with DNA studies revealed that the roots contained specialized bacteria that can break down the oils and proteins in the whalebone.
But, according to Vrijenhoek, "That was not the end of the weirdness. In looking at the worms under a microscope, we discovered that every one of them was a female. We didn’t find any males until I got another call from Greg Rouse. He said, 'Bob, it’s worse than you think.' I said, 'What now, Greg?' He said 'There really are males, but they are microscopic. They are dwarfs!'"
Sure enough, living within the tube that enclosed each female were 30 to 100 microscopic male worms, each only about a millimeter long. Not only that, but the male worms were still in a larval stage of development. They were making sperm in one part of their bodies, while other parts of the bodies still contained the yolk droplets. As Vrijenhoek put it, "These males don’t feed. A male lives its entire life off the yolk that was provisioned by the egg from which it hatched. This is one of the few cases in the animal world where sexually reproducing individuals are barely more developed than eggs. It’s weird."
Asked why these worms might have evolved such an unusual method of reproduction, Vrijenhoek suggests, "These worms appear to be the ecological equivalent of dandelions—a weedy species that grows rapidly, makes lots of eggs, and disperses far and wide." This strategy makes sense when you consider that after a whale skeleton has been consumed, all the worms at that site will die off. Before this happens, they must release enough eggs or larvae so that some tiny proportion will be transported by the ocean currents until they can find and colonize another whale carcass.
Eventually, Vrijenhoek, Rouse, and Goffredi determined that they had found at least two new species of worms. In 2004, they published a paper naming these two worms Osedax rubiplumus and Osedax frankpressi. The new worms seemed to have different roles, with O. rubiplumus colonizing whales soon after they arrived on the seafloor, and O. frankpressi arriving somewhat later and surviving longer. But that was not the end of story for the weird worms.
In October of 2004, a dead blue whale washed up on Del Monte beach in downtown Monterey. Vrijenhoek and his team took advantage of this windfall create their own "artificial whale fall." With the help of Moss Landing Marine Laboratories, the MBARI marine operations staff towed the whale carcass off the beach and sank it in about 1,000 meters of water (much to the relief of the nearby homeowners). Within a year, they found other new species of Osedax worms growing on this dead whale, which they nicknamed "Frank."
Over the next three years, MBARI researchers sank five more carcasses (all gray whales) in Monterey Bay at depths ranging from 400 to 1,800 meters. Altogether now, they have discovered ten additional species of Osedax worms growing on and around these whales, with different species living at different depths. They also discovered that some of the worms would grow on bones of other mammals in addition to whales.As Vrijenhoek put it, "It's amazing, for something that didn't even have a name until three years ago, we now know of twelve species just in Monterey Bay. Since our initial discovery, new Osedax species have also been found living near Sweden and Japan. Others surely exist elsewhere, but it takes someone with the time and equipment to look for them." He adds "This is a great example of one MBARI’s most significant assets. We have the ships and submersibles, and most importantly, the institutional flexibility to pursue totally unexpected things that crop up during the course of our planned research efforts."
Far from being just a biological curiosity, Osedax worms have become a cultural phenomenon, inspiring everything from hand puppets to childrens’ books and the name of a new rock band. As Vrijenhoek notes with some amusement, "These ghoulish, bone-eating worms with dwarf males have stimulated people's imaginations in ways I would never have appreciated. They seem to bring out that sense of wonder and curiosity... in adults as well as kids. That's why we're scientists, after all. It's not the stuff we know, it's the stuff we don't know that's exciting. You go out there and start looking around and you never know where it will lead."
MBARI contributors to whale fall research: Bob Vrijenhoek, Shana Goffredi, Joe Jones, Shannon Johnson
- Background article on whale falls
- MBARI news release on Osedax worms
- More photos of Osedax worms
- Log book from a whale-fall research cruise