杏吧原创

Species: Ramisyllis multicaudata
Habitat: shallow seas off the north coast of Australia

Nowadays the name 鈥淐hallenger鈥� is most associated with the doomed space shuttle, but it wasn鈥檛 always so. The shuttle was named after a 19th century naval vessel, which, from 1872 to 1876, carried out one of the first expeditions to explore the deep oceans.

While touring the Philippines, the Challenger鈥檚 crew dredged up a sponge and found that its cavities were filled by a bizarre, branching worm, which was later 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 in Darwin, Australia, made the new find. In November 2006 he went diving in the shallow waters of Darwin harbour and, avoiding 鈥渢he occasional crocodile and deadly Chironex jellyfish鈥�, collected a single .

The hollow spaces inside were filled with the branched body of a worm 鈥� 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. 鈥淔inding the deeply embedded head end is literally like finding a needle in a haystack,鈥� he says.

As the worm鈥檚 body grows, it develops branches that follow the many channels in the sponge. Each branch can develop many sub-branches, in turn which 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鈥檚 surface. At first glance, an infested sponge looks like it has dozens of hairy white tentacles.

Tree-like worm

Glasby doesn鈥檛 know what R. multicaudata eats. It may consume parts of the sponge, but that probably wouldn鈥檛 sustain it. Alternatively, its skin might absorb dissolved organic material 鈥� in which case a branched body would be useful as it increases surface area.

We also don鈥檛 know what the worm鈥檚 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鈥檚 extremities stimulate the sponge鈥檚 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. 鈥淚t is quite possible that the diversity of branching [worms] has been underestimated because of the difficulty of finding and extracting them intact,鈥� he says.

Journal reference: , DOI: 10.1111/j.1096-3642.2011.00800.x

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