One Head, 1,000 Rear Ends: The Tale of a Deeply Weird Worm

If you had 1,000 butts, what would you do with them? The marine worm Ramisyllis multicaudata is one of only two known animals to find itself in this rather awkward situation (shopping for pants must be a nightmare)—and it isn’t yet telling. But given that that many booties doesn’t “just happen” to a worm, there must be a pretty good reason, and a new anatomical study has offered up some tasty clues.

The story starts off normally enough. Ramisyllis is a bristle worm that lives inside the water passages of a sponge called Petrosia in a shallow reef off the coast of northern Australia. Its lone, unremarkable and rather lethargic head is buried deep in the sponge. Shortly after that things get weird.

Its body begins to branch repeatedly and without pattern. The legion resulting posteriors may protrude into the seawater through natural holes in the sponge and amble along its surface. One “small” sponge observed by scientists was festooned with more than 100 crawling worm fannies, sometimes more than 10 to a single opening. Although sponges are many remarkable things, sentient is not one of them, and that must surely be counted as a win here.

Further, each branch contains its own set of internal organs. According to the first detailed anatomical study of these worms, published this year in the Journal of Morphology by a team from Spain, Australia and Germany, these organs are in no way different from that of the unbranched juvenile. They further found that the worm’s gut is continuous throughout the entire labyrinthine animal—but conspicuously empty. No sponge tissue has ever been found inside, nor food particles of any kind.

Yet they also found the worm’s hind gut is covered in cilia and microvilli, little fingerlike extensions that maximize the surface area available for nutrient absorption (your own gut is covered in a similar velvety lining of villi and microvilli). That implies their gut could still function, although how the sprawling animals could survive on invisible food that enters only through their woefully inadequate regulation-sized mouth remains a mystery.

What makes these worms particularly interesting to me is that they appear to be an animal that has adopted a fungal lifestyle. Look at a fungus under a microscope, and you will see a system of branching tubes with a strong resemblance to Ramisyllis. And this similarity suggests what these worms might be up to in their sponges.

Fungi are absorptive feeders. They tunnel into their food, secrete digestive enzymes and then resorb the resulting goo. The reason their highly branched, filamentous bodies put the emphasis on surface area is that rather than having a long intestine crammed into a small body as we do, their entire body is an intestine, inside out. In this setup, the more body you have, the more food you can eat.

It’s been known for a while that soft-bodied marine invertebrates can absorb dissolved organic matter (a.k.a. liquid food) directly from seawater through their “skin.” But Ramisyllis may have taken this to the next level: the anatomy team discovered the worm’s body is also suspiciously covered in long microvilli. Given the strong emphasis on square footage in the Ramisyllis body plan—and the lack of emphasis on producing heads or mouths commensurate with the situation—one must strongly suspect that, like fungi, they have converted their outsides into insides.

If their highly branched bodies aren’t suggestive enough of fungi, allow me to present Exhibit B: their bonkers reproductive system.

The first clue that to their extremely alternative lifestyle is the fact that Ramisyllis is never going to go on a date. Once you’ve crammed your thousands of tentaclelike branches into the water passages of a Petrosia sponge, you’ve made a commitment to a house, not a relationship (or even a hookup). The usual solution is to simply boot your millions of cheap gametes directly into the water, wave bye-bye, and turn on some must-see TV. Corals and sea anemones are notable practitioners of this enviable reproductive art.

But this is not the route Ramisyllis and many other syllid polychaete worms took. At the back of their bodies sits a little tail called a pygidium (trilobites also had this cute butt flap). Just in front of it lies the polychate worm version of the apical meristem in plants: a place where stem cells continuously generate new body parts called the posterior growth zone. Polychaete worms have these in order to make new segments. But it is an unusual situation for animals, and it has led to some unusual results.

Sometimes, instead of making a new standard segment, these regions start building a head containing a rudimentary brain and four eyes. After the head come more body segments stuffed with gametes, and before you know it there’s a sexy little hot rod attached to the mother ship, to be jettisoned when the time is ripe. These stripped-down clones (botanically termed “stolons”; strawberry runners and other horizontal plant stems are also called stolons) are armed with paddles, driving directions, a libido and little else.

In short, Ramisyllis makes autonomous gonads that lie in that hazy middle ground between detachable penis and college freshman. The group to which these worms belong—the syllids—are perhaps unique among bilaterally symmetrical animals in this bizarre reproductive strategy, termed “gemmiparous schizogamy.” Certain insects, of course, do something similar in that they produce ephemeral adults whose sole aim is to knock extremely tiny, extremely urgent boots, but they generally live as larvae for a much longer period. And they do not bud from existing insects. That’s a very mycological way of doing things.

Indeed, the image of a Ramisyllis stolon amidst the branches of its generative worm is strikingly similar to photographs of the fungus Fusarium bearing its distinctive boat-shaped spores. Stolons of other nonbranching syllid species can also be made in bunches or chains, just like fungal spores.

It may be this very reproductive habit is what allowed syllid worms to grow multiple-choice bodies. The ability to make a branch bearing a sex-seeking clone may only be a few mutations away from substituting the regular bits instead.

Still, something about this story bugs me. If their whole bodies can absorb dissolved food, why is there such an emphasis on all the myriad backsides reaching the surface of the sponge? In one specimen dissected by scientists, bunches of worm butts were found stuffed into sponge cul-de-sacs. The scientists interpreted this as the thwarted attempt of said backsides to reach the surface. The tails also contain a bright white pigment of unknown function that they make whether or not they reach topside.

Why is it so vital the tails find an exit? Is the dissolved organic matter really that much tastier outside the sponge? And why are they wearing the equivalent of reflective highway paint? Is it just for sunscreen? Or is there some other use?

Even though Ramisyllis is apparently doing what I would do with a thousand booties—shake them—exactly what that it is really doing with them remains a mystery.

This is an opinion and analysis article, and the views expressed by the author or authors are not necessarily those of Scientific American.

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