31 March 2008

Assiminea succinea and her operculum

Yesterday's post was about the tiny semi-terrestrial snail Assiminea succinea and a simple immersion test I had subjected them to in Florida last week. Tim Pearce, in his comment, questioned the identification of the snail. Because the snail in the pictures had eyes at the tips of its tentacles and because no operculum was visible in the pictures, Tim wondered if the pictured snail was instead a common land snail.

A little bit of background information may be necessary to clarify Tim's comments. The land snails that are familiar to most people have 2 pairs of tentacles with the longest pair carrying eyes at their tips. They also don't have opercula. Those are the pulmonate snails in the group Stylommatophora. Among their evolutionarily closest relatives are the snails in the Basommatophora, the adults of which also lack opercula, but have their eyes at the bases of their 1 pair of tentacles. Here is an example: Melampus bullaoides.

Now the plot starts to thicken. The species in the 3rd pulmonate group the Systellommatophora also lack opercula, but have eyes on the tips of their tentacles. In contrast, all other non-pulmonate land snails have opercula and their eyes are at the bases of their 1 pair of tentacles. Here is an example: Pomatias elegans.

To further thicken the plot, I should add that most marine snails, the polyphyletic prosobranchs, have their eyes at the bases of their 1 pair of tentacles and many have opercula.

So, if the snail that I identified as Assiminea succinea didn't have an operculum, it would likely to be a stylommatophoran, although, as Tim noted, it did not have 2 pairs of tentacles. The latter trait is, however, not a universal one among the stylommatophorans as Tim also noted. For example, the snails in the genus Vertigo have only 1 pair of tentacles. Here is an example: Vertigo gouldi.

Confusing? Tell me about it.

Fortunately, to settle the matter quickly I remembered that I had additional pictures of the subject snails and their opercula. Here is a set showing how the little Assiminea succinea comes out of its shell. In the 1st photo the aperture is mostly covered by the thin operculum, while in the 2nd photo, the snail's foot is pushing the operculum (arrow) out of the way. Those little elongated things visible thru its shell, and also outside the shell, are fecal particles, which is another characteristic that separates non-pulmonate snails from at least the stylommatophoran pulmonates; in the latter group, as far as I know, the feces are always in the form of strings and never expelled as individual particles.


I will leave the discussions of the evolutionary implications of all of this, which Tim's touches upon in its comment, to future posts.

Assiminea succinea is in the family Assimineidae. Here is a brief discussion of the characteristics of the family. A new Assiminea species, A. mesopotamica, was described last year by Glöer et. al (Mollusca 25:3, 2007). Their description notes both the presence of an operculum and the location of the eyes.


Snail said...

Don't forget that the Amphibolidae (or is Phallomedusidae? Who knows? I've lost track.) is a pulmonate family with an operculum. Just thought I'd contribute some confusion.

I don't think they occur in the US, though, do they?


Ah, yes, the Amphibolidae, the only pulmonate family in which the adults have opercula. They are aquatic though, aren't they? No, I don't think they are to be found on this side of the Equator.

At least some ellobiids hatch with opercula that get resorbed as they develop. These are all nice clues indicating that the pulmonates evolved from an aquatic ancestor with an operculum.

Anonymous said...

Well it is hard to say if Amphibola is "aquatic" or not. Sort of half and half?

Apparently they are found near high tide level in mangrove areas. During low tide they graze around on the film of micro-organisms on top of the mud, mostly bacteria I think. The snail gets covered with water about an hour a day.

Does that count as aquatic? Maybe so.

Susan H.

Snail said...

If we get a nice sunny day (and a low tide at a reasonable time), I'll pop down to a site that I know is normally crawling with Salinator (um ... Phallomedusa) and take some pics.

snail mom said...

hi there, well, i am not at all into the snailing world. however, my 3 year old found a land snail and i think he's cute. i'd like to keep him. would you be willing to take a look at a picture of him and possibly id him for me? i've been keeping him in a moist environment with soil, sticks, leaves, rocks, and some fruit. he's growing and pooping so i guess i'm doing alright. he's about 1 cm right now and we live where we found him, in central IL. you can leave a comment on his picture on myspace to respond.


thanks! snail mom

ps. the only way to get that link to work on here is to copy it line by line into the browser address box. sorry.


snail mom: please go to www.tinyurl.com, get a shorter address for the picture of your snail & post it here. Thanks.