Worm Snail

Image: California Academy of Sciences

Ahhh! Sometimes I just have to lie back and bask in nature's boundless capacity to be completely stationary!

We've already seen ONE way in which molluscs take on the worm's lifestyle. Aplacophorans were molluscs who dropped their shell, became longer and thinner, and took to burrowing underground or wrapping themselves around corals.

Worm Snails are different. It turns out you don't actually have to get rid of your shell to live like a worm!

Image: Natural History Museum Rotterdam

You just need a really weird shell.

There are about 150 species of these marine snails around the world, all in a family called Vermetidae.

Vermes is Latin for "worm". Isn't it odd how it just looks like a German (or Indian) mispronouncing "worms"? Also it's where "vermin" comes from.

Image: Chad King (SIMoN / MBNMS)
A snail shell unlike any other

Worm Snails create rather shambolic shells that coil and meander all over the place, sometimes reaching several inches in length. It's clearly not the kind of thing a snail would want to carry on their back, they're slow enough as it is. But it's OK, because they're cemented onto rock. These snails aren't going anywhere!

Image: Ria Tan

Within this ridiculous shell is an almost perfectly ordinary snail. They have their little tentacles and on their slimy foot could be an operculum for sealing their shell shut when they don't want to be disturbed. They also have a mouth which hungers for food. What is a sedentary snail to do?

There are two ways for a Worm Snail to feed. One is to simply filter edibles out of the same water they take in to breathe. This is much like those other molluscs, bivalves like mussels and scallops. Others do things differently. It's our old friend the mucus net!

Video: Daragh Owens
Sea Butterflies and Fat Innkeeper Worms also feed with a mucus net. Nice to see it in action!

Mucus is an abundantly available, theoretically edible, bio-degradable resource, so Worm Snails make good use of their sneezes. They exude their mucus and let it spread out in the water above. Once a sufficient amount of tiny plankton has stuck to it, they eat it aaaaaaaall up!

As we all know, there's something deeply alluring about a massive web of mucus spewing forth from one's mouth and wafting delicately in the breeze. So it's inevitable that the mucus net would be used in reproduction.

Worm Snails are not hermaphrodite (it bears noting), so the female uses her net to catch the sperm packets released by the male. Her eggs are thereby fertilised, and eventually a whole bunch of tiny Worm Snails are released into the world. They look quite normal at first as they crawl around looking for a nice place to spend the rest of their lives. Only then will they cement themselves to the sea floor and grow their increasingly bizarre shell.

Image: Ryan Somma
Some youngsters don't go far, and giant colonies develop

Clearly, this is different from what we usually think of when we hear the word "worms". This is more in the style of tube worms, those annelids who secrete a tough tube around themselves to live in. The tubes of Worm Snails and tube worms are sometimes difficult to tell apart but if you take a look inside, you'll see that Worm Snail tubes have an inner lining of nacre. This is the smooth, iridescent mother of pearl found in the shells of various molluscs.

Just a bit of glitz and glamour to remind Worm Snails where they come from!