Osedax: Ancient bone eaters

 

Somewhere, in the depths of the ocean, a marine ambrosia descends from above; the body of a dead whale.  Despite the depth, the corpse won’t be in solitude long; a mini-ecosystem will soon erupt around it, one that could last a decade. The body draws hag fish, sea cucumbers, rat tail fish, brittle star fish, a host of other invertebrates and myriad species of bacteria, all come to feast on the marine bounty. And soon the ‘bone devourers’, Osedax, come too.

Osedax are a group of at least five species of marine polychaete worm, related to those found living on hydrothermal vents, who feed exclusively on the bones of other animals. They are much smaller than their hydrothermal cousins, only up to a few cm long. And, while they aren’t as terrifying as their name might suggest (Osedax is Latin for ‘bone eating’) they may once have feasted on the bones of now extinct giant marine reptiles, and, at least by our surface dwelling standards, they are weird… and a bit gross.

osedax-better

Different species of Osedax found in Monteray Bay, California

Osedax have tube like bodies coated in protective mucous, at the top of these, gently wafting in the ocean currents, are delicate looking red gills. At first sight, they resemble a small, redish flower, albeit one covered in snot; hence the name of one species Osedax mucofloris—the ‘bone eating snot flower’. They have no mouth, stomach or anus, instead, they feed through roots sent down into bone; like a tree, much of Osedax exists unseen below the surface. But without a mouth, or anything else to mechanically gnaw and wear, how they managed to penetrate bone was something of a mystery.

Instead of munching, Osedax may be using acid to drill their way into bone. High numbers of proton pumping enzymes are found in cells at the tips of their roots; acidifying the environment immediately around them. This acidic environment demineralises and breaks down bone, letting the roots penetrate and reach the more nutritious marrow. The root tip cells are also elongated, with tiny finger-like protrusions, giving them a larger surface area to both acidify the environment and absorb nutrients. Despite this alien-esque acid ability, Osedax lack a stomach, so how do they breakdown the fats and proteins from the marrow into something more usable? Simple, if you can’t do something yourself, outsource it.

Outsourcing, sex and survival

Osedax roots have specialised cells, called bacteriocytes, which house colonies of symbiotic bacteria called Oceanospirillales. In exchange for easy access to raw materials the bacteria provide the elements of metabolism needed to breakdown the fats and proteins into something more usable, and share them with their hosts.  Osedax aren’t born with these bacteria though, they must recruit them from the environment once they’ve settled on a new home.

The life cycle of Osedax is equally as weird as the way they feed. After their discovery in 2002 it was noticed that all Osedax were female. Where were the males? The answer, it turned out, was right under the researcher’s noses all along; the males were living on the females, they just didn’t develop beyond the tiny (~1mm) larval stage. It was a world of giant females with harems of ‘dwarf’ males living in the mucous surrounding them. The bigger the female, the more males there were. Females spawn eggs into the mucous, these are then fertilised by males and develop into larvae that swim out to make their own way in the oceans.

Osedax and dwarf males

Osedax and dwarf males

Once Osedax larva stumble across a potential new home, they settle on the bone, elongate their bodies and crawl about a bit until they find a spot that—for whatever reasons an Osedax likes a spot—seems right, they then start to secrete their mucous coat and develop into their adult forms. After around 6 weeks of settling down and growing, they are ready to reproduce, and they are continuously ‘at it’. This constant reproduction makes a lot of sense for something that only lives on the bones of dead animals, since finding new remains in a vast oceanic expanse isn’t easy. By releasing a huge number of offspring Osedax improve their chances of the next generation finding a home through sheer numbers.

While Osedax have only been found on whale bones—most recently in waters off Antarctica—it’s likely because whale carcases are huge and, relatively speaking, easy to spot not that they only grow on whale bones. They have been observed growing happily on tuna and cow bones placed next to whale carcases. Not specialising in whale bone would make good sense, since opportunities to feed on bones are likely to be few and far between on the ocean floor, so not being too picky gives you more options.

New, but how old?

Despite the fact that, like many deep sea species, Osedax were only recently discovered they’ve been about for a long time. Whale bone fossils, dating from 30 million years ago, show not only signs of a good gnawing by sharks (fossilised shark teeth were embedded in them) but also the characteristic signs of Osedax; small bore-holes with smaller branching holes, likely formed by Osedax’s root tendrils. In fact Osedax may be even older than that; older even than the whales we commonly find them feeding on.

Gareth Monger

The remains of a Rhomaleosaurus, whose bones are coated with (speculatively) an early Osedax species. Illustration by the excellent Gareth Monger:  www.facebook.com/DinosaurIllustrator

By examining pieces of DNA from different species of Osedax and comparing them, it’s possible to get an idea of how long ago the two species separated from a common ancestor. With more species you can build a ‘phylogenetic tree’—a kind of evolutionary scale family tree—that can gives you an idea of relationships of these stretches of DNA, and the timeframe they separated from each other. This suggested that fossils from as far back as the Cretaceous (~145-66 million years ago) should be examined for any traces of Osedax. Though no traces have been found on reptile fossils from this time yet, the fossilised remains of a flightless marine bird called Plotopteridae from the Cretaceous period showed similar markings to those found on whale fossils.

It’s possible that the ancestors of modern day Osedax, in the absence of whales, feasted on the remains of giant marine reptiles like the 7 meter long Rhomaleosaurus.

 

 

Thanks and Acknowledgments

Special thanks to two excellent scientific illustrators for this post:

Firstly thanks to Michael Rothman for use of his excellent ‘Whale fall, stage 1′ illustration used as the featured image for this post. You can see more of Michael’s scientific illustration and contact details at http://www.michaelrothman.com/

And huge thanks to Gareth Monger, for use of his excellent illustration of Osedax growing on a Rhomaleosaurus. You can find more of Gareth’s work on facebook here, or contact him at microraptor{at}hotmail.com.