Journal archives for August 2019

08 August, 2019

How to tell a Brown-lipped Snail from a White-lipped Snail

Two very pretty and very similar-looking species of land snails in the genus Cepaea are native to most of Western Europe. And both of them are introduced in some parts of North America. These two species are: the Brown-lipped Snail, aka the Grove Snail, Cepaea nemoralis (adults almost always have a dark out-turned and thickened lip on the aperture of the shell) and the White-lipped Snail Cepaea hortensis (adults almost always have a white out-turned lip on the aperture of the shell).

Both species live in colonies. They both have a fairly large globose shell, which can be yellow, red, or any pale or mixed shade of those colors. The shell can be plain in color or banded. When the shell is banded, it can have from one to five dark bands. Those bands can be narrow, or they can be so wide that two or more bands merge together.

One important thing to know is that you cannot put a species ID on a live one of these snails, or an empty shell, unless it is adult. The ID of juveniles needs to be left at the genus level. Even dissection cannot separate the juveniles into species.

How can you tell if the snail is an adult? In snails of this genus (and in many other land-snail genera), once the snail reaches adulthood/sexual maturity, the shell stops growing any larger, and instead it grows thicker. In particular, the lip (the very edge of the opening of the shell) in adults becomes greatly strengthened, strongly reinforced, and also somewhat out-turned, a bit flared-out. So, in adults the lip of the shell is thick and strong, and it is out-turned to a certain degree.

With a lot of experience, you can tell an adult from a juvenile quite easily just by looking, but until then, before trying to ID a live Cepaea to the species level when you are in the field, there is to a way check to see if the lip is mature. If you press gently on the side of the lip and it is still soft and flexible, then the individual is a juvenile. And in general if there is no sign of thickening and no out-turned appearance to the lip, the snail is a juvenile, and you will have to leave the ID as "Cepaea".

EVEN MORE IMPORTANT: A live juvenile or subadult Cepaea snail that is active will always appear to have a white lip on the shell. But what you are seeing is usually the live mantle tissue which is wrapped over the edge of the shell, actively laying down more shell material. That is how the shell increases in size. And any brand-new shell material will also appear whitish, yellowish, or even transparent. This apparent pale lip is not an indication that the shell is mature.

If an individual snail is starting to become sexually mature, you may see that the thicker lip of the adult is partially formed. You have to look closely then, because even if the adult will end up with a dark lip on the shell, the thickening often starts out pale; the dark pigment seems to be laid down a bit later, when the construction of the thickened lip is almost finished.

Almost all of the time with adult snails it is true that an adult White-lipped Snail has a white lip on the shell, and an adult Brown-lipped Snail aka Grove Snail has a dark lip to the shell. Very rarely there are exceptions to this rule, but it is best to stick with it 99.9% of the time.

Here on iNaturalist we currently have numerous observations of Cepaea that have been misidentified, and many well-meaning people have subsequently "agreed" with those IDs, causing them to become Research Grade.

Research Grade observations are fed to the AI, our Computer Vision tool. Large numbers of misidentifications cause the AI to learn the species incorrectly, and then it offers incorrect suggestions. And that helps perpetuate the mistakes!

If anyone who reads this post would like to help me sort out some of this confusion, please drop me a line. Although many (not all) of the IDs of the Dark-lipped Snail are correct, there are still dozens of observations of "White-lipped"Cepaea which will need to have the ID adjusted to the genus level, and adding a comment with "Please see" and a link to this post.

Thanks for any help you can give in sorting this out. Not only do we need the humans to learn this correctly, but, perhaps even more importantly, we want the AI/Computer Vision to learn it correctly too.

Posted on 08 August, 2019 13:13 by susanhewitt susanhewitt | 8 observations | 13 comments | Leave a comment