Acoela

Image: Samuel Chow

How to live with almost no body at all!

Acoels are several hundred species of small, flattened worms, usually under 1.5 cm (0.5 in) long. They're found across much of the world in marine environments; some use their cilia to crawl between grains of sand on the sea floor, others infest the surface of corals and feast on whatever goodies they find on the mucus there, some live in the brackish waters of mangroves and there are even a few who travel through the tiny channels found in polar ice, seeking out algae to feed on.

Image: Bernard DUPONT
Lots of Acoels hanging out on a Bubble Coral

They were initially thought to be flatworms, which is reasonable enough given that they're worms who are flat. They're named after the fact that they lack a coelem, the body cavity found in most other animals. Both flatworms and Acoels are solid flesh from skin to gut.

However, molecular studies show that Acoels aren't flatworms at all. They're rather more simple than that. Yup. They're MORE SIMPLE than flatworms!

Image: Stevie Smith
Symsagittifera roscoffensis are packed full of symbiotic green algae

Acoels are already named after their lack of an empty space, which is kind of weird when you think about it, but they also lack another empty space: the gut. I must admit, I have a hard time picturing this! Acoels have a mouth for food to go in but it seems this food then goes straight into digestive cells rather than hanging out in a gut first.

They also lack what's known as a protonephridium. Humans and other vertebrates have kidneys which get rid of waste products from the blood. Invertebrates have a completely different organ for the same purpose, called the nephridium. The protonephridium is just right for flatworms and several other simple, wormy creatures. Apparently Acoels eschew such internal housekeeping.

Video: xenoturbella
Some Acoels are colourful

Speaking of bad housekeeping, while Acoels are hermaphrodite, they don't have gonads. The cells that produce eggs and sperm are simply scattered about the body. It's like going to someone's house and finding a sock on the kitchen counter and a plant pot full of gardening tools in the bathroom. What a mess!

Another strange thing about Acoels are their nerve cords. Animals can be divided into two enormous groups; deuterostomes, like us humans, other chordates and echinoderms, with our dorsal nerves cords. That means we have that thick bundle of nerves running down our backs in the spinal column. The other lot are the protostomes, they include annelids, arthropods, molluscs, AND flatworms, and their nerve cord runs along the belly.

Video: Adolfo Outeiro
That little, annoying thing better watch himself

Acoels? They have nerve cords along the belly, along the back and several more in between. This sets them apart from pretty well every other animal in the world!

When Acoels were considered to be a bunch of unusual flatworms it was quite easy to ignore all these peculiar characteristics, or at least brush over them. Now that Acoela is recognised as a distinct group of its own, these peculiarities have taken on a whole new significance. These humble worms may bear characteristics dating back to early animal life, when even flatworms were a mere twinkle in the eyespot.

Image: Christopher Laumer
Bright white statocyst on the right

Oh, and some Acoels have eyespots. There can be up to 4 of them, each one composed of just two cells, making them the simplest of any animal. Pretty much all of them also have something called statocyst, which is an organ that lets them tell up from down. This must surely be their pride and joy. A real life organ!

That isn't the only organ they have, though. You might think a simple, marine worm like this would dump their eggs and sperm around the place and leave them to it, but no. While there are some who reproduce asexually by simply splitting in two, Acoels usually engage in real copulation and internal fertilisation.

Image: Wikipedia

Some species have an opening into which a soft, muscular copulatory organ can slip in and dump a load of sperm which can then be stored in an organ called the bursa until required. Others don't have such an opening, and they just stab each other with needles. The sperm cells then migrate to the bursa. Also the sperm cells have two flagella rather than one, which is just strange.

So after all that simplicity and missing body parts, things suddenly get very complicated when it comes to sex. At least we can see where their interests lie. Acoels clearly play their music on a one-stringed guitar.