Acorn Worm

Image: David Shale

It's a dangerous business trying to handle an Acorn Worm. Not because it can harm YOU in any way, but because they're so soft and fragile that they're liable to fall apart in your hands.

And yet this delicate worm is just a few wrong turnings on the evolutionary tree from having its very own backbone!

Image: Moorea Biocode

There are currently less than 100 known species of Acorn Worm in the class Enteropneusta. Most of them reach about 10 to 45 cm (4 in to 1.5 ft) long, but one species can reach a least 1.5 metres (5 ft) in length.

An Acorn Worm is split into three parts: a proboscis, a collar, and then a long trunk which makes up the bulk of their length. The proboscis is attached to the collar by a fleshy stalk, but you usually can't see it because of the shape of the collar. In other words, the collar acts like a collar and covers the stalk (or neck). The proboscis ends up looking like a little acorn in its cup, hence the name.

Unlike real acorns, you almost never see a fully grown Acorn Worm. They spend almost all their time hidden within mucus-lined, U-shaped burrows constructed in soft sand near the sea shore. Some species have mucus and cilia all over their proboscis so that they can take in plankton from the water, but most will simply swallow lots and lots of sand to digest tiny bits of edible detritus therein.

Image: Doug Greenberg

With their gut full of sand, the Acorn Worm creeps backwards in its burrow until its rear end pokes out of one of the entrances. Except that this is now very much the EXIT, because the Acorn Worm promptly excretes a long cylinder of biologically cleansed sand.

These piles of faeces are the only thing most people ever see of an Acorn Worm. They produce them at low tide, as if they don't want to get the water too dirty.

Image: budak

If you're lucky, you might catch them in the act and see a bit of worm tail. Wouldn't that be nice!

Acorn Worms have almost no sensory organs at all. Their mouth is situated on the collar, and food is directed towards it by a groove lined with cilia. That groove might allow them to taste. Aside from that, there are a bunch of nerve endings on their body. And that's it! It seems the only thing an Acorn Worm knows about the world outside is whether it consists of a) a burrow, i.e. good or b) something other than a burrow, i.e. bad.

One thing Acorn Worms have in their favour - their chief glory, perhaps - are gill slits. There can be hundreds of them in rows just behind the collar. They're used for breathing. Water comes in through the mouth and is passed out through the gills, with oxygen absorbed along the way.

Image: Spengel, Johann Wilhelm

Two things. All the stuff that went in through the mouth but didn't go out through the gills moves on to the intestines. The gills are right there on the gut. This is why the name of their class is Enteropneusta, or "gut breather".

Secondly... gill slits! These are wormy things with gill slits! They even use them much like a fish. Or indeed that ancient, living fillet-of-fish known as the Lancelet. Gill slits are one of the big chordate characteristics, even humans, lizards and all the other vertebrates have them at some point in their embryonic development. As do all those weird chordates like salps and sea squirts.

For a long time Acorn Worms were placed within the phylum Chordata. Not only because of their gill slits, but also for their dorsal nerve cord and a little, hollow structure called the stomochord which was once thought to be homologous to the notochord. These days they're placed within a phylum of their own called Hemichordata, which means "half chordate".

Image: R. Lutz

Acorn Worms seem to be one half chordate, one half nothing at all. Or maybe one half mucus.

They're sorely lacking in muscle. Food is drawn into the mouth by the waving of cilia, water is passed through the gills by cilia, even the intestines push their burden along using cilia, not muscles. The entire trunk is all rather flimsy and weak. It's like they needed the intestine to be a certain length, but then they just sketched in the surrounding body with a pencil and lost interest. Earthworms are powerful body builders in comparison!

Image: owe CJ, Terasaki M, Wu M, Freeman RM Jr, Runft L, et al

It's a little different when you look at the proboscis and the collar. There are actual muscles here for dragging the rest of the body along when burrowing. The trunk is covered in yet more cilia to help out, but it's slow going for an Acorn Worm. There's also a kind of heart in the proboscis, but it's just a pulsating sack of fluid with no arteries or any kind of opening to the rest of the body. Acorn Worms have no real blood vessels at all. Also their blood is colourless. And they have no brain.

It seems to be a dark, uneventful life on the beach for our poor Acorn Worm. Hidden in his burrow, dragging his body up and down the U-bend. Waiting for low tide so he can use the bathroom. Using the bathroom is probably the highlight of his day.

But wait! Didn't I hear somewhere that the abyssal depths of the deep sea are a land of opportunity for all benighted underlings trodden underfoot by overmighty topsiders?

Maybe I just overheard myself thinking?

Video: Inner Space Center

Journeys to the deep in more recent times have revealed numerous new species of Acorn Worm that have all been placed in their own family: Torquaratoridae. The name means "neck plough" after the peculiar ornaments on their head. It would be difficult to burrow underground with those things and indeed, they DON'T!

Here in the abyss, Acorn Worms have a chance to really express themselves and get up to all sorts of antics. Many of them have acquired some really nice colours of purple, blue or orange. They've completely given up on burrowing and instead crawl along the seabed at blistering speeds of 3 entire inches per hour. I don't know if someone sat in a submarine watching that, but deep sea Acorn Worms get around using cilia as they still don't have much in the way of muscle. But they're in the deep sea now, that kind of thing is normal here.

They consume the soft muck of the sea floor as they go, digesting all the bits and pieces that have fallen from the upper zones. They don't have to wait until low tide to use the bathroom any more. Instead, a constant stream of cylindrical faeces follows them everywhere they go. Bliss! This is what they call liberation! By this trail it can be seen that some species crawl in a neat spiral as they eat, while others roam around in an apparently random, higgledy-piggledy stroll.

Eventually, they have enough of this particular spot and start to consider some place new. Then, something extraordinary happens...

Video: The Royal Society

They completely evacuate their gut and start to rise from the seabed! This worm, who on the beach was so secretive and secluded, so alone and so hidden in her dank lair, emerges in splendour to drift slowly to pastures new. She does so with the aid of a mucus balloon, because Acorn Worm.

There's one final, interesting point about deep sea Acorn Worms. Or at least one of them. It's to do with reproduction.

Acorn Worms are all either male or female. The female produces a gelatinous mass full of eggs which the male fertilizes before the ocean water breaks up the mass and disperses the eggs. In a few species, the eggs hatch directly into miniature adults. In the rest, they develop into planktonic larvae. The larvae might then grow into a miniature adult, but some species must pass through a second larval stage first. It's called the tornaria.

Image: Moorea Biocode
Tornaria larva

This tornaria is a weird, little thing covered in bizarrely convoluted bands of cilia. They're not quite unique, though. They look just like the bipinnaria larvae of starfish! It seems that hemichordates may in fact be more closely related to echinoderms than they are to chordates.

That might seem entirely ridiculous, but all three groups are in fact united by an important characteristic. During the early stages of embryonic development, the very first opening which develops on the blastula is called the blastopore. In chordates, hemichordates and echinoderms this blastopore eventually becomes the anus. The mouth comes later. For this reason, they all belong to an enormous group of animals called deuterostomia, or "second mouth".

The other big group of animals is protostomia, "first mouth", where the blastopore becomes the mouth and the anus has to develop later. This group is super-duper huge and includes arthropods, molluscs, annelids, flatworms, nematodes and loads more.

So while you may look at a starfish and think the two of you could scarcely be any more different, you actually have a lot more in common than you might think. Do you like to eat mussels and oysters? Enjoy long (very long) walks on the beach? Hey, give it a chance...

But back to our deep sea Acorn Worm! One species has been found with a dozen embryos of baby Acorn Worms embedded in her back! This is the only Acorn Worm ever seen brooding her young. Just because you're a proboscis and a bunch of gill slits held together with mucus, it doesn't mean you can't love!

The deep sea is like an enormous mental hospital; it's full of strange characters and unsettling facial expressions, and you never know what you'll have to do to deal with the madness. It's wonderful to see how Acorn Worms have flourished in the abyss! Sometimes you need eternal darkness to truly blossom.