Snail trails on Birch

Snail trail on Himalayan Birch (Betula utilis var. jacquemontii)

Last winter I was at the Arboretum at the university and I took a photograph of some Snail feeding sign on a Birch tree. I had seen a photograph before of the feeding sign in the book “Tracks and Sign of Insects and Other Invertebrates”, so I thought I knew what I was looking at. Turns out, I was correct. It is some sort of terrestrial Snail who made their way up the tree and was feeding on the algae growing on the bark.

From what I have learned about Birch trees and their bark, this feeding from snails can be beneficial. Paper Birch, like many other Birches, has white bark so that light and associated heat radiation is reflected off of the tree, which may seem counter intuitive for a tree which grows high in the north. Really though, it's not too helpful to be attracting that light and heat energy. A quote from Tom Wessels:

“Let's say the daytime high is 10°... If that sun hit dark bark for a few hours, and heat that bark up, and what happens to solids when they heat up? They expand. So the bark is expanding, the wood is expanding, and then the sun sets and it's instantly -20°: That bark is gonna contract at a faster rate than the wood underneath and it's gonna split right open in what is called “frost cracking.” It can also cause cambial tissue damage if the weather keeps fluctuating above and below [0°] degrees. Paper Birch has evolved to have white bark to reflect sunlight and keep itself cold so it doesn't get cambial tissue damage or “frost cracking.”

So, if that algae continues to grow, multiply, spread, die and decay to form a dark matte of decomposing vegetative material on the tree, this could potentially cause the Birch trees to heat up beyond what they are built for. In northern climes where temperatures can suddenly drop to below freezing in the winter, there could be some severe damage. This is where the Snails come in. They move along the tree eating all the algae and clearing up the Birch bark so it can retain the snowy albus glow.

Another way the Snails may aid trees by cleaning up the algae is by helping the trees photosynthesize. The leaves of trees photosynthesize (turn sunlight into sugars) for the trees, but some trees can also photosynthesize with their bark aswell. Trees like Trembling Aspen, American Beech, Paper Birch, and most others with smooth thin bark can photosynthesize with their bark when their leaves are missing in the winter. As the Snails consume the curtain of algae proliferating on the bark, the sun can shine through anew!

The Snails clean up the bark by eating the algae, scraping the algae off the bark with their “radula” and then stuffing it into their mouths. Radula comes from the latin “radere” meaning “to scrape”, which is just what they do. They open their mouths and stick out their radula, which could be imagined as a spikey conveyor belt which the Snails use to lick/scrape off the algae from the bark, or whatever other surface they are on while feeding. Those spikes are called “denticles” (which means “small teeth”) and these denticles are made of chitin (pronounced “kite-n”), the same stuff which makes up the exoskeleton of arthopods. These teeth break off every once in while, but then new ones grow back in their place.

Snails and Birches, what a team!

Now, my questions grew as the answers came about. I began wondering if there was a way to determine the direction of travel, and it turns out there is. I think of holding a peace sign with your fingers, and then curling those two fingers back into your hand. Imagine your fingers scraping away at algae as you curl them into your hand. This is sort of how the radula moves. Can you imagine your fingers creating a triangular shape from this scraping motion? Can you see the small triangles in the image? The broad end of the triangle is the direction of travel and the pointy end will face towards the rear of the Snail.

The Snail who left this trail was travelling from the lower right of the image towards the middle left side of the image.

There is also the next question, why are there many rows beside/above and below each other? It took watching a video of Snails on the internet to figure this out, but I did... I think. As many Snails feed they wave their heads back and forth and while they do so, they are taking mouthfuls of algae. For each mouthful there is a “clean” triangle free of algae, and the “dirty” spots in between where the Snail has missed the algae while moving their head.

In studying this Snail sign, I hope to encounter it again, and see what other patterns are coming up, and hopefully some new questions which may teach me something cool as I strive to know what's going on out on the land.