Zoologger is our weekly column highlighting extraordinary animals ? and occasionally other organisms ? from around the world
Species: Ramisyllis multicaudata
Habitat: shallow seas off the north coast of Australia
Nowadays the name "Challenger" is most associated with the doomed space shuttle, but it wasn't always so. The shuttle was named after a 19th century naval vessel, which, from 1872 to 1876, was the flagship of one of the first expeditions to explore the deep oceans.
While touring the Philippines, the Challenger's crew dredged up a sponge and found that its cavities were filled by a bizarre, branching worm, which was dubbed Syllis ramosa.
For over a century, S. ramosa remained unique. But no longer. A second species has been found off the coast of Australia, and there could be many more out there.
Home sweet sponge
Christopher Glasby of the Museum and Art Gallery of the Northern Territory in Darwin, Australia, made the new find. In November 2006 he went diving in the shallow waters of Darwin harbour and, avoiding "the occasional crocodile and deadly Chironex jellyfish", collected a single Petrosia sponge.
The hollow spaces inside were filled with the branched body of a worm ? a new species, which Glasby and colleagues have named Ramisyllis multicaudata.
Worm-sponge pairs are easy to find, Glasby says, but difficult to dissect because they are fragile. As far as he can make out, each worm has a single head that lurks deep inside the sponge. "Finding the deeply-embedded head end is literally like finding a needle in a haystack," he says.
As the worm's body grows, it develops branches that follow the many channels in the sponge. Each branch can develop many sub-branches, and those in turn also develop branches, and so on.
As a result each worm can have hundreds or even thousands of extremities, many of which poke out through holes in the sponge's surface. At first glance, an infested sponge looks like it has dozens of hairy white tentacles.
Tree-like worm
Glasby doesn't know what R. multicaudata eats. It may consume parts of the sponge, but that probably wouldn't sustain it. Alternatively, its skin might absorb dissolved organic material ? in which case a branched body would be useful as it increases surface area.
We also don't know what the worm's relationship is to the host. It could be a harmful parasite, but it is also possible that the protruding branches deter predators, or that the worm's extremities stimulate the sponge's growth or improve the flow of water through the sponge.
Glasby says its branched body may help the worm produce more young. Worms reproduce by forming small buds near their tail ends. A single R. multicaudata can have over 100 tails, and in theory each could produce young.
Genetic analysis of R. multicaudata, S. ramosa and 50 other worms suggests that the two branched species are quite distantly related. That suggests they evolved the ability to branch independently, probably by modifying the budding method used to reproduce.
If two species can do it, why not more? Glasby says several specimens of branched worms have been described as S. ramosa despite appearing subtly different. "It is quite possible that the diversity of branching [worms] has been underestimated because of the difficulty of finding and extracting them intact," he says.
Journal reference: Zoological Journal of the Linnean Society, DOI: 10.1111/j.1096-3642.2011.00800.x
Read previous Zoologger columns: The bird that cares for its rival's chicks, Itsy bitsy teeny weeny chameleons , Don't bite ? how the zebra got its stripes, The only males with more brain than females , How a blurry-eyed spider pounces on target
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