Ramisyllis multicaudata is an animal that seems to have adopted a fungus lifestyle

If you had 1,000 butts, what would you use them for? The marine worm Ramisyllis multicaudata is one of the only two known animals to find themselves in this rather awkward position (buying pants must be a nightmare), and there is no result yet. But considering that many loot did not "just happen" to worms, there must be a good reason, and a new anatomical study provides some interesting clues.

The story started normally. Ramisyllis is a mane worm that lives in the waterways of a sponge named Petrosia in a shallow reef off the northern coast of Australia. Its lonely, unremarkable, and rather sleepy head was buried deep in the sponge. Before long, things got strange.

Its body began to branch repeatedly, without regularity. The resulting large amount of rear may extend into the seawater through natural holes in the sponge and move slowly along its surface. Scientists observed a "small" sponge decorated with more than 100 fan-shaped worms, sometimes with more than 10 openings. Although the sponge is a lot of great things, the sentient is not one of them, and it can definitely be counted as a victory here.

In addition, each branch contains its own set of internal organs. According to the first detailed anatomical study of these worms published this year by a team from Spain, Australia and Germany in the Journal of Morphology, these organs are no different from those of unbranched larvae. They further discovered that the worm's intestines are continuous throughout the labyrinth of animals-but obviously empty. No sponge tissue was found inside, and no food particles of any kind were found.

However, they also found that the hindgut of the worm is covered by cilia and microvilli. Small finger-like extensions maximize the surface area available for nutrient absorption (your own intestine is covered by a velvet-like lining of villi and microvilli). This means that their intestines can still function, although how these huge animals can survive on invisible food that can only be accessed through their sadly undersized mouths remains a mystery.

What is particularly interesting about these worms to me is that they seem to be animals that adopt a fungal lifestyle. Looking at the fungus under a microscope, you will see a branched tube system, very similar to Remisilis. This similarity indicates what these worms may have done in their sponges.

The fungus is an absorptive feeder. They enter the food, secrete digestive enzymes, and then absorb the resulting mucus. Their highly branched filamentous bodies emphasize surface area because they do not stuff their long intestines into a small body like we do, but the whole body is an intestine, from the inside to the outside. In this setting, the more body you have, the more food you can eat.

For some time, people have known that soft marine invertebrates can directly absorb dissolved organic matter (also known as liquid food) from seawater through their "skin". But Ramisilis may have taken it to a new level: the dissection team found that the worm's body was also suspiciously covered with long microvilli. Given the high emphasis on square footage in Ramisyllis' body plan—and the lack of head or mouth accents commensurate with the situation—one must strongly suspect that they, like fungi, have transformed the exterior into the interior.

If their highly branched bodies are not enough to suggest fungi, please allow me to show Chart B: Their crazy reproductive system.

The first clue to their extreme alternative lifestyle is that Ramisilis will never go on a date. Once you squeeze thousands of tentacle-like branches into the waterways of the Petrosia sponge, you make a promise to a house, not a relationship (or even a hookup). The usual solution is to simply drop millions of cheap gametes directly into the water, wave goodbye, and turn on some must-see TVs. Corals and sea anemones are famous practitioners of this enviable art of reproduction.

But this is not the route taken by Ramisyllis and many other syllid polychaete worms. Behind their bodies is a small tail called tail whip (trilobites also have this lovely hip flap). Just in front of it is the polychaete version of the apical meristem of the plant: stem cells continuously produce new body parts in this place, called the post-growth zone. Polychaete worms have these to create new parts. But this is an unusual situation for animals and has led to some unusual results.

Sometimes, instead of making new standard fragments, these areas begin to construct a head that contains a basic brain and four eyes. Behind the head are more body parts, which are filled with gametes. Unknowingly, a small sexy hot rod is attached to the mothership, which will be abandoned when the time is right. These streamlined clones (called "stolons" in botany; strawberry cuttings and other horizontal plant stems are also called stolons) are equipped with paddles, driving directions, libido, etc.

In short, the autonomous sex glands made by Ramisyllis are located in the dim middle ground between the detachable penis and the college freshman. The group to which these worms belong-sylids-may be unique among bilaterally symmetrical animals in this strange reproductive strategy, known as "twin divisions." Of course, certain insects do similar things because they produce short-lived adults whose sole purpose is to strike extremely tiny and extremely urgent boots, but they usually live much longer as larvae. They will not germinate from existing insects. This is a very mycological way of doing things.

In fact, the image of the Ranunculus in the branch of its reproductive worm is surprisingly similar to the photo of the fungus Fusarium with its unique boat-shaped spores. The stolons of other unbranched soft plants can also form bunches or chains, just like fungal spores.

It may be this reproductive habit that allows sylid worms to grow a variety of body options. The ability to make branches with sexually seeking cloning may be just some mutations, rather than replacing regular bits.

Nevertheless, some things about this story still trouble me. If their entire body can absorb dissolved food, why emphasize reaching the countless backs of the sponge surface? In a specimen dissected by scientists, it was found that bunches of worm buttocks were stuffed into a sponge cul-de-sac. Scientists interpret this as the backside's attempt to reach the surface is blocked. The tail also contains a bright white pigment of unknown function, regardless of whether they reach the top or not.

Why is it so important for the tail to find an exit? Is the organic matter dissolved outside the sponge really so delicious? Why do they apply paint equivalent to highway reflective paint? Is it just for sun protection? Or are there other uses?

Although Ramisilis was obviously doing what I would do with a thousand loot-shaking them-what exactly it did with them was still a mystery.

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

The American Association for the Advancement of Science Science Journalism Award-winning science writer Jennifer Fraser wrote an ingenious amoeba blog for Scientific American. She has degrees in biology, plant pathology, and scientific writing. Follow Jennifer Frazer on Twitter: Nick Higgins

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