Bone Boring Worms

posted by Perrin Ireland / on May 25th, 2010 / in Annelids, Development, lifecycles

In 2002, while out roaming the depths in Monterey Bay Canyon with the remote operated vehicle (ROV) Tiburon, MBARI scientist Robert Vrijenhoek stumbled upon a whale carcass on the ocean floor, and noticed that it had its own little ecosystem. When a whale has died, its skeleton drops to the ocean floor, creating a habitat island in the depths. Creatures apparently gather from far and wide to use the whale carcass’ nutrients and living space.

Scientists have categorized four stages of whale carcass ecosystems- first the “mobile scavengers” show up, such as sharks, crabs, hagfish. These guys pick away at what luscious meat remains. Snails, slugs and worms show up next to make use of the nutrient-rich poo (in science speak, “organically rich sediment”) the larger scavengers have left behind. The third stage is comprised of animals that rely on hydrogen sulfide gas emitted from the decomposing bones and organic sediments. These animals, like vesicomyid clams, depend on symbiotic bacteria that live inside their cells to make energy for the animal from sulfur based compounds. Free-living bacteria that also live off sulfur form in mats that coat the bones. The final stage of a whale bone’s community succession is the reef stage, when most of the nutrients the whale bone can provide have been exhausted, and the minerals remaining in the bone provide a surface for suspension and filter feeders, who rely on the ocean currents to bring food their way.

When Vrijenhoek and his colleagues were at depth checking out whalebone world, they noticed little red worms that they were unable to identify all over the remaining whalebones. They collected a sample and send the worms to worm expert Greg Rouse, who informed them they had discovered a new species. Related to tube worms that live at the mouths of hydrothermal vents, Osedax grow at their longest to be about the length of your index finger, and as thick as a pencil. Penetrating deep into the marrow cavities of the whalebones are their elaborate root systems. These roots house bacteria that help the worms extract and digest nutrients from the bone, as they lack stomachs and digestive tubes.

Perhaps most bizarre and enticing about the Osedax worm is that all the worms the scientists first discovered appeared to be reproductive females, with no males in sight. Eventually they found the tiny males living in tubes along the female’s trunk. An Osedax female essentially has a harem of up to fourteen males that do nothing else but provide sperm for the eggs she produces. Osedax males feed for their entire lives on yolk provisioned by the egg from which they hatched, like forty year olds living at home on Mom’s meatloaf. The males look strikingly similar to Osedax larva, suggesting that they are larva in arrested development that began producing sperm.

Most of the eggs exiting the female are already fertilized. But how do those little guys lying along her trunk scoot their sperm up to catch the eggs as they’re on the way out? And how, then, do larvae being flung into the dark beyond know whether to become male or female? It could be possible that sex determination depends on whether a larva lands on bone or lands on another female. Perhaps similar to hydrothermal vent worms, a juvenile becomes a male if it lands on a female and she releases a chemical, enticing it into her little harem, to do her reproductive bidding.