Chiton

Image: Nuytsia@Tas

Chitons! They look like... like... nothing else! I'm sure this style was in vogue a few hundred million years ago, but these days they're really going out on a limb. Not that they have a limb.

Chitons are almost 1,000 species of decidedly retro mollusc who reach anywhere between less than 1 cm (0.4 in) and over 30 cm (1 foot) long. They belong to a class called Polyplacophora, which means "bearer of many plates", or "look! It has a suit of armour!"

Image: dracophylla

Actually they bear precisely eight plates, also known as valves.

Image: igor_nz

The valves are made of a tough mineral called aragonite, the same stuff in most mollusc shells, and in most circumstances, that's all you'll see of a Chiton. They're not like snails, who have to poke their head out of their shell to get anywhere, let alone those fool-hardy slugs who throw all caution to the wind with their unabashed nudity. Chitons can stay completely covered at all times. They are masters of defence.

The key is in the flexibility of their shell. Each valve is a separate piece that slightly overlaps the next and there is articulation between each one. It means the Chiton can bend its body up and down as it wanders over uneven terrain and the platemail will follow suit.

Image: Malcolm Storey

They can even roll up into a ball!

Video: ewilliamsish

This flexibility is important because most Chitons live in rocky, coastal waters and intertidal zones. There are a few who live in the deepest of deep seas, but most use their armour and squished, floor-hugging body shape to bear the crashing and thrashing of waves. None live in freshwater or land.

Obviously it would be a bit rubbish if the valves were really, REALLY separate, because then they'd be falling off all over the place. This isn't great for normal clothes let alone armour. So all the parts are kept together with the girdle, which wraps around the edge of the animal and is itself strengthened with yet more aragonite.

Image: Bill & Mark Bell
Acanthopleura spinosa

Some Chitons express themselves with their girdles. This one seems to be saying stuff like "go away", "leave me alone" and "I will hurt you".

Image: Nuytsia@Tas
Snake-skin Chiton, Chiton pelliserpentis

Others appear to be wearing a snazzy, snakeskin girdle.

Image: C Wood
Black Katy Chiton, Katherina tunicata

Some have a huge girdle which covers most of their armour...

Image: Malcolm Carlaw
Lined Chiton, Tonicella lineata

While others want to show the world their amazing valves. This one seems to be communicating ideas like "party" and "disco".

Image: jkirkhart35
Gum Boot Chiton, Cryptochiton stelleri

And some oddities hide their valves beneath a girdle which covers their entire body. It's like wearing your suit of armour under some jogging bottoms.

Image: jkirkhart35
Underside, showing the foot. Gills can be seen in the mantle cavity

If you ever manage to pry a Chiton off the rocks, you'll get to see why you found it so difficult. They have a long, sticky foot, like a snail, which helps them clamp down and resist the waves. And the curiosity of others. You might also see the gills in the mantle cavity between the foot and the girdle. Chitons take in water through a hole near their mouth, pass it over their gills and release it via another hole toward the back end.

Image: WoRMS for SMEBD
Acanthochitona fascicularis

Within the mouth is their radula, a tough organ covered in teeth and used to rasp algae off the rocks. Unless it happens to be one of the few Chitons who smother unsuspecting prey with their enlarged girdle and let the slaughter commence. It's as they say, the best defence is to kill everyone.

Some Chitons have their teeth covered in a mineral called magnetite, which is an oxide of iron, the most magnetic mineral in the natural world and real, even though it sounds like a comic book invented it in the 50s.

Image: matt knoth
Lined Chiton, Tonicella lineata

Near the radula is another organ which basically tastes the floor to find food. Also there is an assortment of tiny eyes dotted about the shell that can only really tell light from dark and then nerve endings in their foot can at least tell sand from stone. And that's about it. They don't even have tentacles like a snail.

It's a simple world for the Chiton. Which works well because they don't have a brain. They scarcely have anything you could really call a head. I guess it would provide too much of a target for attackers; brains are a sign of weakness for the Chiton.

Video: Dofleini1
Chitons do it standing up

Reproduction is a simple affair, too. There are separate males and females and most will simply release their gametes into the sea, where larvae will hatch to drift and swim for a while before settling. In some species the female will keep the eggs in her mantle until they hatch, so at least some Chitons extend their defensive instincts to their progeny.

Image: Ryan Wick

In any case, Chitons have managed to cling to life as tenaciously as they cling to the shoreline. There's is an ancient lineage that dates back some 400 million years and never acquired such newfangled innovations as a brain, tentacles, a proper head or good vision. They just got their shell and ran with it.

They are, in fact, the complete opposite of an octopus. But then that's why molluscs are so cool!