01 September 2010

Snail homing

Do land snails and slugs home? In other words, do they use the same feeding or resting spots repeatedly, day after day?

The idea that terrestrial gastropods, especially the more familiar larger ones, such as the snail Helix aspersa or the slug Limax maximus, home has been around for more than a century. These early claims of snail homing were almost invariably based on anecdotal or circumstantial evidence. For example, Taylor included the following statement in his Monograph of the land & freshwater Mollusca of the British Isles (1894-1900, volume 1, p. 312).

Helix aspersa is particularly noticeable for its love of home and for the exertions it will make to regain its shelter, having been observed to traverse with great labour broad dusty roads or climb rough walls to reach some favourite food, and when satiated not retiring at daybreak to the shelter of any convenient crevice, as might be supposed, but almost invariably retracing its often toilsome and arduous way to reach its favourite shelter, a peculiarity that has been verified by many observers on numerous occasions.
Note that Taylor didn't bother to give citations for the independent verification by the readers of the "numerous" demonstrations of snail homing.

Back in October 2008 when I was in Turkey, one of the urban parks near where I was staying had plenty of Eobania vermiculata. On a particular day, I noticed that many of those snails had become dormant on the walls surrounding the park. So I decided to carry out a little experiment to determine if Eobania vermiculata had a homing instinct. I numbered the shells of several snails and their locations on the wall. The idea was to see if they would return to the same spot after foraging for food, presumably during the night. The next day when I returned, I found some of the snails missing and some still dormant although not at the exact spots where they had been marked, but near them. Initially, I thought this demonstrated that some snails were indeed homing. Then I realized that my procedure was flawed, because I had no way of knowing if the snails had left their resting spots, foraged and then returned to the same spots or if they had simply moved a few centimeters away before becoming dormant again. One day I will carry out a better planned experiment.

Homing or not homing? The previous afternoon when I marked this snail, it was within the black circle; the next morning it was outside the circle. But how far had it travelled before becoming dormant again?

A few days ago it came to my attention* that a public experiment to test snail homing had been initiated in the U.K. and it was in fact about to end. I'm eager to learn their results.

*I thank my regular reader Joan Jass for sending me the link to this news item.

1 comment:

Kathryn said...

Hi Aydin. I have been convinced for a long time that some snails home. Observing big Euglandina's in the Southern US you often see the same individual cruising the same feeding ground and seeming to hide in the same area of a porch or yard. I have also observed in Northern Mexico where you have mountains with just Humboldtiana and Euglandina eating them that I could often find Euglandinas by looking for piles of predated Humboldtiana shells and flipping over the nearest rock for the predator. I still don't know if many snails use the same refuge or the same individual repeatedly forages and returns but I would love to find out. I have no problem believing snails are smart enough to home.