26 May 2006

How Pomatias elegans comes out of its shell

I have had 2 live P. elegans for almost 2 years, but only recently started paying close attention to them. Here are a series of photographs of how they come out of their shells.

ABC

As explained in the previous post about this species, when the snail is completely withdrawn into its shell, its aperture is sealed with a hard, calcareous operculum that is permanently attached to the top of the snail's foot. In A, the operculum is starting to lift up. In B, the red arrow points at the tip of one of the tentacles that is "peeking" out. Unlike most other land snails, the eyes of P. elegans are not at the tips of its tentacles. However, there must be some chemical receptors on the tentacles, perhaps in the darker tips, that the snail seems to be using to check out the outside world before any other part of the body is brought out. Right below the tentacle, you may also notice the tip of the proboscis (it is easily visible in the original high resolution picture). In C, both tentacles and the proboscis are out. Notice that each tentacle is pointing in a different direction, which, again, seems to suggest that the snail is using them to sense the surroundings.

DEF

In B and C, the deeply furrowed part to the other side of which the operculum is attached is actually the sole of the foot. In D, the snail has twisted its foot around and is bringing the sole in contact with the ground. The head has also twisted around and is momentarily facing the ground. In E, most of the sole is resting on the ground, the left eye is visible at the base of the left tentacle, but the shell hasn't moved to its proper position yet. Finally, in G, the snail has twisted its shell around to its "cruising" position on top of the operculum. The proboscis in contact with the ground is visible in the front. The snail is ready to push the pedal to the metal!

This seemingly complex set of maneuvers, which is reversed when the snail is withdrawing into its shell, is necessary to protect the head, the most vital part of the snail's body that is exposed to the outside. When a snail senses danger, it withdraws its head into its shell first, followed by the rest of the foot. But, the problem is that before the snail can come back out, it needs to put its tentacles out to determine if it's safe to come out. The snail accomplishes that by doubling over itself within its shell. Another reason why the snail's head needs to be located near the aperture, even though it goes in before the foot, is that the lung and its opening are located in the head.

In this case, unintelligent evolution couldn't come up with a better solution other than turning the snails into contortionists.

Interestingly, the land snails that have lost their opercula during evolution go thru the same sequence when they are withdrawing or coming out of their shells.

8 comments:

Clare said...

Fascinating - and I always thought the snail was a simple creature. Great pictures. Really appreciated seeing this. Just wondering - is it thought that this is a more primitive form than the snails with two pairs of tentacles?

AYDIN ÖRSTAN said...

The pulmonate land snails with 2 pairs of tentacles evolved from marine ancestors that presumably had only one pair of tentacles. Pomatias elegans is not a pulmonate snail (even though it does have sort of a lung) and it has one pair of tentacles. So, in an evolutionary sense, it is more primitive than pulmonates. It tends to get complicated, because several groups of snails became terrestrial independently.

Clare said...

Thanks Aörstan. Nothing in biology is ever simple, is it.

www.nemoramjet.com said...

Mr. Örstan, can I use the last photo (F) for a non-porfit arts project where I am trying to draw all basic orders of life? I'd really like this cute snail to be the representative for the gastropoda.

AYDIN ÖRSTAN said...

Thanks for asking. Yes, you may use the photo, but I request that my name be cited as the photographer.

Genie said...

Thank you, Aydin Bey, for the wonderful photographs. I don't know a lot about snails but I have earrings with opercula and wondered just how and where exactly the operculum is attached to the snail's body and how the opening/closing mechanism works. Now, thanks to your pictures, I know.

J said...

Hi. I have a little freshwater snail and sometimes he will awkwardly protrude out of his shell and there is this tentacle coming out from behind his head that is usually not visible. I took a photo of it if someone thinks they know about snails I could email it to them.
S/he seems healthy but I cannot figure out what it is... I'm very curious!

Anonymous said...

To J:

Assuming that your freshwater snail has an operculum and thus also has separate sexes, it may well be a male and the extra "tentacle" that you see sometimes (when the snail stretches the front part of its body far out of its shell) is probably the penis. It is located not very far behind the head, on the animal's right-hand side.

Susan J. Hewitt