Strange Worm

Strange by name, strange by nature...

Meet Xenoturbella bocki, a worm-shaped enigma which reaches up to 4 cm (1.6 in) long and spends its time burrowing down to 100 metres (330 ft) beneath the bottom of the North Sea.

It was discovered in 1915 by Swedish zoologist Sixten Bock. For some reason, he never got around to writing a scientific description of the peculiar fleshling. Perhaps he just forgot? He'd been in Japan the previous year and was busy in 1917 travelling to Fiji and Australia via Petrograd in revolutionary Russia. He clearly had a lot on his plate.

Indeed, no description would be published during his entire lifetime. He died in 1946 and it was a further three years before another zoologist, Einar Westblad, finally wrote the paper. He named the creature after Sixten Bock, which was nice of him, but old Westblad was not to be left out. He got a piece of the pie in 1999 when another species was discovered and named Xenoturbella Westbladi. Justice!

But what does Xenoturbella mean?

It pretty much means strange or foreign flatworm. A lot of flatworms are parasitic of course, but the ones that aren't belong to a class called Turbellaria. The Strange Worm looks a lot like a free-living flatworm with a mouth situated halfway along its underside. There's no anus, though. They must regurgitate whatever they can't digest.

But what's so Xeno about them, you ask. Well, Einer Westblad was surprised at the tremendous lack of organs. Organs that even the flattest of flatworms would be expected to possess.

Strange Worms lack not only a brain, but even ganglia or major nerve cords. They have more of a nerve net akin to jellyfish. They don't have gonads either. The eggs and sperm of these hermaphrodites are produced near the gut and have to break into the stomach to be released via the mouth. I hope you remember to be thankful for your abundance of orifices.

Video: xenoturbella

So far it all sounds a lot like the Acoela, another group simple animals that were long thought to be weird flatworms. They even share one, treasured organ: a statocyst for telling up from down.

Indeed, there was a time when Strange Worms were thought to be a kind of Acoelomorph. But there are many differences between them. Differences to do with anatomical structure, membranes and cilia. Small differences that are easy to ignore but when you're a worm who's composed of almost nothing other than membranes and cilia, it turns out to be quite significant! So significant that an entire phylum has been erected containing just those two species of Xenoturbella: Xenoturbellida.

But they could easily lose that phylum. For a while, molecular studies showed that Strange Worms were in fact extremely strange bivalve molluscs. They were clams that lost their shells and ceased to look anything like a clam. Then it turned out that the samples were contaminated. The Strange Worm had clam eggs within their body. Apparently they'd eaten them! That's the kind of confusion that can happen when you know approximately NOTHING about the thing you're trying to study.

Some say Strange Worms and Acoela are so closely related they should be united into a new, shared phylum called Xenacoelomorpha. Others say they're not closely related at all, and only look similar because that's what you look like when you have no body parts.

This may be the case. Acoela are usually regarded as very basal Bilateral animals, meaning their branch of the tree splits off after sponges and jellyfish but before everything else, including real flatworms. DNA evidence shows that Strange Worms are quite different. They're Deuterostomes, most closely related to echinoderms and Acorn Worms. That makes them more closely related to humans than they are to actual flatworms!

Research is still ongoing, naturally enough. Just recently some scientists managed to watch the development of Xenoturbella eggs for the first time. The larvae that hatched never grew mouths so it's clear they couldn't eat. That's in common with Acoela, but not with echinoderms and Acorn Worms. So maybe there should be a phylum called Xenacoelomorpha after all?

Dear oh dear, what a palava! The more we learn about Strange Worms the stranger they get! Mr. Bock had NO idea what he'd stumbled on. And apparently, neither does any one else!