Updated: Mar 25, 2019
Superlatives are over-used these days, and stories of 'weird creatures' on the internet should usually be taken with a grain of salt. But sometimes a creature really is just gobsmackingly strange, and Ramisyllis multicaudata must be a strong candidate for the world's weirdest.
The name says it all, really. Ramisyllis multicaudata means the 'many-tailed branching syllis', and R. multicaudata is just that. It's a worm in the family Syllidae with one head and up to a hundred or so tails. To do this it does something that virtually no other animal does - it branches.
The Syllidae are a remarkable family in their own right. These are polychaete worms, a class of segmented marine worms with bristle-like appendages along the body. If you're a keen fisherman you may well have baited your hooks with various species of polychaetes dug out of sandy or muddy beaches. Syllids are a family of small-to-medium-sized polychaetes, with around 165 species from 34 genera in Australia (and with new ones discovered and described regularly). They have a remarkably diverse range of sexual strategies (of which more later).
Ramisyllis multicaudata was discovered in Darwin Harbour in 2006 by worm taxonomist Chris Glasby from the Museum and Art Gallery of the Northern Territory (and subsequently named by Chris and colleagues in 2012). Sponges were being collected from the turbid, nutrient-rich waters of the harbour as part of the Museum’s bioprospecting program. A spinoff for Chris was that sponges are good habitat for various marine worms. One of them, a Petrosia sponge, held a surprise.
Petrosia sponges have numerous, branching channels running through their bodies, each opening to the exterior by a pore (an osculum). The sponge that Chris collected had a series of odd, filamentous structures poking out of each osculum. Careful dissection back in the lab revealed Ramisyllis multicaudata - deep inside the sponge was a single, fairly normal worm-like head, behind which the body of the worm branched and branched again, one branch occupying each of the branching channels of the sponge, with a single tail at each osculum.
Now animals don't branch. Plants branch. Fungi branch. Animals don't. Most plants (and fungal mycelium) are indeterminately modular - that is, they are made up of modules (branches) that can make more modules (more branches) more or less without stopping. Most animals, however, are unitary - they comprise a single module. At most, this may comprise a set of segments (think insects and most worms), but even so the set is determinate, and doesn't branch.
So Ramisyllis multicaudata is an animal doing something that plants do. That's weird.
There's a very particular reason why Ramisyllis is able to do this, and it's all down to sex in syllids.
Many syllids do what many other worms do: they hatch, grow, become sexually mature, have sex, produce eggs, then die. Many of these species live on the seabed until they're ready to mate, at which time they change in form, swim up into the water column for a mass spawning event, then die after laying eggs.
While this is clearly a good strategy for many species, it's risky for others, which have instead opted to stay on the seafloor as adults and send bits of themselves up for the have-sex-lay-eggs-then-die bit. Each year these syllids grow small side-branches called stolons (a term usually associated with branching root-like stems in plants). The stolons develop gonads, then break off the "parent" and swim away with stolons from other individuals, to reproduce.
Ramisyllis has modified this system to produce its remarkable morphology. Each branch of a Ramisyllis is a stolon, but instead of developing gonads and breaking away it remains attached and produces more branches. Eventually, when the branches reach the osculum of the sponge they do produce sexual stolons that probably swim off much like in any other syllid. But the worm that remains in the sponge is unlike any other.
Well, not quite. It turns out there is one other branching worm, another syllid called Syllis ramosa (the species epithet again means 'branching'). Three specimens of this are known, collected in the 1870s in the Philippines by scientists on H.M.S. Challenger, a naval research vessel and one of the first expeditions to explore the deep oceans. The Challenger’s crew also dredged up a sponge with its cavities filled by a bizarre, branching worm. This and R. multicaudata are the only known examples of this remarkable lifestyle.
Syllis and Ramisyllis pose more questions than they answer. Think about it. With a body intricately branching through a sponge, these worms can't move. How would you move anyway with one head and a hundred tails? The worm lives its whole life locked in its sponge. So how do they feed? The head remains in one place deep inside the sponge, with the gut branching along with the body. Perhaps the worm draws its nutrients from the sponge somehow, but with the head remaining fixed in one place that seems like a bad idea. Perhaps the worm simply absorbs nutrients from the water stream passing through the sponge. We have no idea how this all works. The taxonomy's done, now we need some basic biology to understand how the world's weirdest worm pulls off its remarkable trick.
In nature, it's safest not to try to make up rules, because the strongest rule is that pretty much anything that can happen, will. And even some things that shouldn't be able to happen, probably will. Ramisyllis multicaudata shouldn't exist, but it does, just offshore from the restaurants and apartment blocks of Darwin Harbour. The world is indeed a wonderful place.