Worm with a many butts

A sponge’s hallows and networks of passages provide living spaces for many small creatures. There’s not much the sponge can do to stop them. Some become permanent residents and others are transient.

At the top is a single worm painstakingly extracted from its sponge. Below is a close-up showing its head. Ponz-Segrelles & Glasb, Göttingen University.

Two of the more bizarre permanent residents are similar worms. They live in a sponge’s intake passageways, but as these are narrow, a worm can’t grow fat or it will block the flow of water, so it has to grow long. The unique thing about it is that, as it gets longer, its tail end branches to extend down different canals. And it does this over and over until its body is spread out, winding through the sponge. It has one head, but many trunks and can get so long that when extracted from its sponge it looks like a large tangled mess. While plants and fungi branch, it’s extremely unusual for an animal to do it.

These worms are pretty much stuck in their sedentary lives, since they can’t really crawl away, and a worm and its segments quickly disintegrate outside of their sponge.

So, if you haven’t thought of this already, this unusual body plan raises the question of waste disposal. It solves this problem by having a butt on the end of each of its branches. Oddly, they have intestines, but no one has yet detected food in them. Plus they only have one mouth. There’s no indication they’re eating their host sponge, so they might be absorbing food through their skin.

Nearly all animals get by just fine with only one butt, but this worm can have hundreds. A Spanish biologist said her group counted more than five hundred on one worm, but thought others could easily have more than a thousand.

Its head is deep inside the sponge, but its tail ends extend to or hang out of many of the sponge’s openings. Or several back ends can hang out of one of the larger holes.

Ponz-Segrelles & Glasb, Göttingen University.

Now, this is where it gets really weird. When it comes time to reproduce, the butts start growing simple heads with eyes and a brain. Then one evening, the wormlets all detach and take off, swimming around until they find a butt head of the opposite sex. Then they spawn and die.

There are other worms that do this, but they have to live their lives with only one butt.

 

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