While in Florida 2 weeks ago, I spent quite some time collecting data on the intertidal snail Batillaria minima. In the 2 beaches where I was studying them, the distributions of these snails were often patchy: there would be areas 2-3 meters wide where I would see none of them and then there would be a cluster of 50 or 100 snails.
I kept some snails in captivity for a few days. I noticed that captive snails also had a tendency to aggregate. I carried out some simple, preliminary experiments by piling the snails up in the centers of containers and then monitoring their behavior over time.
Here are the photographic records of one such experiment. Initially, there were 40 snails around the center of a box whose bottom measured approximately 11x12 cm.
7 minutes later:
24 minutes later a large aggregate had formed against the top wall:
56 minutes later, the aggregate along the top wall was still there; in addition, there were 2 smaller clusters (or 1 diffuse cluster) along the left wall:
I think the clusters were fluid in that snails continuously moved in and out of them, but because the individual snails were not marked in these experiments, I don't have data to support that claim. I saved that experiment for a future trip to Florida.