Carinariid

Image: Roger R. Seapy
Carinaria japonica

This is what happens when you give a Sea Elephant a cute, little hat.

So we saw the Sea Elephants, those strange snails that swim through the (seven) seas on a single fin. Now it's time to take a look at their close relatives, the Carinariidae family.

They're a tiny bit more conventional than our old friends, the charming and lovely, eccentric and flamboyant Sea Elephants. Thing is, their attempt at "conventional" is charming, lovely, eccentric and flamboyant.

Image: Roger R. Seapy

First of all, the similarities. All of them have a long, transparent body with their foot squished at the sides so it becomes a fin for swimming. The body can be divided into three parts:

the long, flexible proboscis that contains their mouthparts at the end,

the trunk, which is the main part of the body,

and the tail, which can provide a burst of speed when the swimming fin isn't enough.

They also have the visceral nucleus, where their guts, gonads, heart and gills are all packed into an impossibly tiny space so that passing predators don't see right through their skin and gaze at a tempting hamper of juicy offal. The interesting thing is what covers that visceral nucleus...

Image: Roger R. Seapy

It's their shell! Like Sea Elephants, larval Carinariids look a lot more like normal snails with their comparatively large, spiralled shell. But Carinariids don't get rid of it when they metamorphose into an adult. Instead, they retain a teensy, transparent shell that looks less like a protective home and more like a decoration. Their guts are packed into this thing and it's all held up on a stalk. It's like a huge plant in a tiny plant pot!

But some Carinariids are different; even as an adult, their shell is microscopic and all that viscera grows up around it. They end up with a shell embedded in their guts!

It's like this...

Image: Roger R. Seapy
Larval Carinariid shell

Carinariids start out utterly puny, with a conventional shell less than a millimetre across. Sea Elephants lose this shell when they become adults. Carinariids in the Cardiopoda genus retain it forever, embedded in their digestive gland...

Image: Roger R. Seapy
Juvenile Carinariid shell

For other Carinariids, the shell begins to grow. More and more rings are added, plus a keel to help with swimming.

Image: WoRMS for SMEBD
Adult Carinariid shell

Eventually it acquires the adult, cap shape, and right at the tip will forever be the original larval shell, known as a protoconch.

Like the Sea Elephants, Carinariids have great eyes and a demonic radula for crushing and crunching their prey.

Image: Roger R. Seapy
Pterosoma planum

And like Sea Elephants, they're really, really weird.