In the 1870s, as the research ship HMS Challenger passed through the Philippines during its circumnavigation of the Earth, it dredged up a sponge from the gray sludge 95 fathoms (nearly 600 feet) below the surface. The scientists knew that glass sponges abounded in these waters, their bodies fragile, translucent, and curiously geometric. Sponges of this sort often house other animals, such as monogamous shrimp and various worms. So it was not exactly revelatory that the dredged-up sponge contained a worm, but the worm itself was a revelation. It had laced its body through the sponge's many passages, so much so that it might have resembled many worms. But it was just one worm whose body appeared able to branch like a tree. "The body of the annelid appears to have a furor for budding," the zoologist W. C. M'Intosh wrote in an 1879 paper describing the preserved specimen of the worm. Although M'Intosh did not comprehend this at the time, the worm had a furor for budding multiple anuses.
Maria Teresa Aguado, an evolutionary biologist at the University of Göttingen in Germany, learned about branching worms as a PhD student. She had always romanticized the Challenger expedition and imagined what it was like to explore distant oceans and glimpse the life lurking at great depths. When she came across an illustration of the branching worm found off the Philippines, now called Syllis ramosa, the worm sparked a lifelong fascination. "I remember staring at it, wondering if such a creature could really exist," she wrote in an email. "How could an animal evolve such a bizarre body plan?"
For more than a century, branching worms seemed extraordinarily rare, with only a single species described by science. There were some reports of branching worms found elsewhere around the world, in the Red, Tasman, and Arafuro Seas, all identified as S. ramosa. But in 2012, Aguado and colleagues found a second species of branching worm off of northern Australia, and then a third species off an island in Japan. Aguado began to suspect that branching worms were not so rare after all. "Perhaps there were many more species, and the old records weren’t just about one or two isolated cases, but about a widespread and diverse group," she said. She was right; in September, Aguado and colleagues published a paper in the Zoological Journal of the Linnean Society that describes 10 probable new species of branching worms and reconstructs the evolutionary history of the strange group.
A branching worm, regardless of species, is not born branched. The young worms must find their way to a suitable sponge, slip inside one of its holes, and grow. The worms then branch throughout the sponge's various canals and generate anuses galore in a process that remains mysterious to researchers. Scientists do not know what the worms eat; if they prey on the sponge or feed on scraps drifting by. Each tip of the branch can produce a stolon, which is essentially a detachable mini-worm with eyes and appendages for swimming. When the stolons mature, they break off, spawn, and die.
Scientists know very little about branching worms in part because they are so hard to find. "Unless you’re specifically looking for them, you simply won’t notice them," Aguado said. The worms that branch inside glass sponges live in the inaccessible depths of the deep sea, and their sponge hosts are difficult to collect intact. The worms that live inside shallow-water sponges are easier to detect. Divers can sometimes spot their butts sticking out of the sponge, "but only if you’re close and know what to look for," Aguado said.
The researchers did manage to discover some new species in the wild. Naoto Jimi, a worm specialist at Nagoya University who helped describe the third species of branching worm off Japan, collected glass sponges on a few deep-sea expeditions and found branching worms nested inside. And Aguado tracked down new species through other, more reliable means. She reached out to natural history collections, contacting the worm researcher Ekin Tilic in Frankfurt and the sponge specialist Nicole de Voogd in Leiden. de Voogd had a decades-old collection of sponges from the Pacific and Indian Oceans and the Red Sea, and Aguado was able to reveal the worms lurking inside using a microCT scan. She remembers her excitement upon sighting a branching worm inside a sponge from Mauritius. "To open a specimen preserved for decades and discover a branching worm inside, perfectly preserved, was deeply moving," she said.
With a glut of specimens in hand, the team sequenced the DNA of both the newly collected and long-preserved worms and found they had two distinct lineages. The DNA revealed that all the branching worms, whether they live in the shallows or the deep, were closely related and share a common ancestor. "This means that branching worms are not isolated oddities but a widespread and successful lineage, distributed across the Pacific Ocean," Aguado said. Although the worms only evolved branching once, the two distinct lineages branch in different ways: one looks more orderly, branching directly out of a segment of their old body, and the other looks more chaotic, with a branch emerging from between segments.
Although the new paper resolves many questions about these bizarre worms, some of the biggest ones remain unsolved. Aguado rattled off a list of her most burning questions, such as what the worms eat, how they protect themselves from the sponge's defenses, whether their relationship is parasitic or mutualistic, and how and why the worms branched in the first place. The researchers suspect there are more branching worms out there, an abundance of anuses lurking out of sight and maybe even out of reach, resting near the ocean floor like well-hidden booty.
