Atlantid

Image: Roger R. Seapy

If you were expecting to see a technologically advanced human, mermaid or alien, I can only apologise. Atlantids are snails. Microscopic, swimming snails. So... I guess I don't apologise at all. You can see mermaids and aliens any time you want and when can't you see humans? Nah. Consider yourself lucky to see a microscopic, swimming snail for once.

We're looking at Heteropods again! Yaaaaay!

Image: Alvaro E. Migotto

It started with Sea Elephants, the weird, transparent, pelagic snails with their complete lack of a shell. Then there were Carinariids, the weird, transparent, pelagic snails with their tiny shells. Now it's time for Atlantids, weird, transparent, pelagic snails with large shells.

Image: Alvaro E. Migotto

After this, we will have seen all the Heteropods, also known as Pterotracheoids, also known as weird, pelagic snails with or without a shell of some description. As it happens, more than 60% of all known Heteropods are Atlantids, and there are only 20 or so species of Atlantid! Not like those mermaids and aliens... dime a dozen, right?

Image: Alvaro E. Migotto

As you would expect, Atlantids have much in common with the other Heteropods; they swim through the sea by waving a single fin to and fro, the fin being a modification of the sticky foot snails usually use to glide around and leave artistic trails of mucus all over the place.

Image: Roger R. Seapy

They have the large, lensed eyes for spotting prey, although this time they occupy a gigantic amount of space on their tiny head. Then they have a tentacle on either side of a proboscis which encloses the radula. The radula is like a tongue covered in teeth and the proboscis is like a snout.

So think of an anteater with teeth on its tongue, giant eyes and one flipper instead of four legs. Don't tell anyone about it, though. Unless you get paid to think up really weird monsters. Even then, it's probably best to go with mermaids. Meranteater?

In common with a few other Heteropods, Atlantids have a sucker on their fin but like the eyes, it's proportionally much larger than in other Heteropods. It's used to keep hold of prey as they tear strips out of soft flesh with their radula. Turns out their favourite food is Sea Butterfly, weird, pelagic snails who swim around on TWO fins. It's the Weird Pelagic Snail War.

Image: Roger R. Seapy

Also on the foot, next to the fin, is the opercular lobe with the operculum attached. "Operculum" is fancy speak for "little lid" and I guess "opercular lobe" means "thing with a little lid on". And this brings us to the shell.

Image: Roger R. Seapy
This shell is less than 2 mm across

The shell is what really distinguishes Atlantids from all the other Heteropods. It's huge! Absolutely gigantic! We're talking like, an ENTIRE centimetre! Sometimes. Only one or two species have a shell as large as 1 cm (0.2 in) across. Others are scarcely more than 1 mm (0.02 in), most are somewhere in between.

Image: Roger R. Seapy
Atlantid completely hidden in shell

The important thing for the tiny Atlantid is that her shell is big enough to contain her entire body if she wants it to. She can hide inside it, and then shut the door behind her with her fancy operculum.

She can't do that for too long, though. While Sea Elephants were neutrally buoyant and thus able to hover in the water by doing nothing at all, Atlantids do indeed sink due to that gargantuan shell. They have to swim all day, or else swim a little, allow themselves to sink a little, and then swim some more.

Image: Roger R. Seapy

This means they can't even retire into their shell at night when it's time to sleep. What will our tiny Atlantid do? It's time for technology. And yet again, nature shows us the incredible power of well-utilised mucus.

For us humans, extruding long strands of mucus commonly signals a day off work and a relaxing time in bed marred only by headaches and the extrusion of long strands of mucus. It's similar for Atlantids, but with an altogether higher class of mucus.

Come the night, our sleepy Atlantid releases long strands of mucus from her foot and goes to sleep. The strands can be up to 50 cm (20 in) long! So it's just... mucus. With a tiny snail hanging from it. The mucus is much more buoyant than the snail, so they need only remain attached to be sure they won't sink to the sea floor. It's like that sick nursery rhyme where a baby falls out of a tree. Hopefully mucus will prove more resilient than boughs.

Image: Roger R. Seapy
Tiny larval protoconch

Speaking of babies, larval Atlantids are ridiculously minute. They start off with a puny shell called a protoconch.

Image: Roger R. Seapy
Adult teleoconch, 4 mm across. Old protoconch in blue.

It grows and grows until it becomes the adult's teleoconch. Even then, you can still see the tiny protoconch in the middle, and a large keel develops to help with swimming.

Image: Alvaro E. Migotto

With a bit of food, and a lot of luck, our minute, larval Antlantid will grow into a weird, slightly less minute, adult Antlantid. And with their cosy shell, fancy door and floaty mucus you won't find them sinking to the bottom of the sea like those silly folks at Atlantis.