Jaw Worm

Image: Matthew Hooge

Molluscs range from mussels who sit around in their shell all day doing nothing, to active, wide-eyed, predatory squid. Arthropods can have 6 legs, 8 legs or several hundred legs. Chordates can have lungs and a backbone, no lungs and a backbone or no lungs and no backbone either.

These are among the most famous and diverse of the animal phyla; splendidly bountiful branches of the tree of life. But let's not allow their mind-boggling variety cause us to overlook the more stark branches with their tiny animal life that barely anyone has ever heard of.

And so it's time for Gnathostomulida, an entire phylum of tiny, marine worms sometimes called Jaw Worms. They're truly minute, most ranging between 0.5 and 1 millimetre in length, and live buried in sand and mud beneath shallow, coastal waters. Some eke out a living in oxygen-deprived environments beneath the seabed.

Image: Martin V. Sørensen
Haplognathia is an outlier, reaching an enormous 4 mm long!

Something over 100 species are known so far, but with their tiny size and under-the-ground-under-the-water habitat there's no telling how many more are left to be discovered.

Jaw Worms make their way between the grains of sand and mud by beating the cilia that cover their body, much like a tiny, free-living flatworm. One difference is that while flatworms have several cilia sprouting from each skin cell, Jaw Worms have just one.

Image: Martin V. Sørensen

Other cilia and bristles provide them with all their senses, eyes being completely useless in the utter darkness underground.

Another difference between flatworms and Jaw Worms is... jaws!

Image: Martin V. Sørensen
JAWS! Aren't you glad to be 100,000 times bigger than these things?

Jaw Worms have a fearsome set of gnashers which wouldn't look out of place on a blood-curdling shark. Thankfully they use them merely to scrape bacteria, fungi and protists from the surface of individual grains of sand.

Once inside, a poor bacterium will see some more similarities with flatworms. Gnathostomulids have no respiratory or circulatory systems since they're small enough to simply absorb whatever minuscule amount of oxygen they need. They also have a gut like a sack, so all the waste products have to be regurgitated from the mouth. There's some evidence that at least a few species can develop a temporary anal pore to excrete waste, which gives whole new meaning to the phrase "bursting for a pee".

Image: Martin V. Sørensen
More jaws

Jaw Worms are all hermaphrodite, either male and female at the same time or starting out male and becoming female as they get older. A good half of their body is dedicated to sex organs, but they only produce one egg at a time. It's released when the mother's body wall ruptures to let it out. It sounds awful, but she soon patches herself up and goes on with her life.

Things are VERY different when you're a millimetre long!