Finding a mate is at the top of just about every to-do list in the animal kingdom. This might involve following the smell of pheromones or triangulating the source of a mating call; in the snail Littorina saxatilis, it turns out to require tracking your beloved by the trail of her slime [$-a].
That's according to a paper in the latest issue of Evolution, in which Kerstin Johannesson and coauthors took video of male and female snails to catch slime trail-following in action. And it occurred to them that slime-following could be a component of speciation in L. saxatilis. This particular snail comes in two forms, or "ecotypes": a small one that lives in the crevices of exposed rock faces and a larger one that lives in quieter, sheltered pools. When Johannesson et al. presented male snails with slime trails from each ecotype, the males preferred to follow trails made by females of their own ecotype.
This is what's called assortative mating - preferentially mating with similar individuals - and it's usually thought of as a first step towards speciation. Whether L. saxatilis ever eventually evolves into two species is another question, though. The world is full of experiments in speciation, where adaptation to local conditions or difficulty moving between populations can cause a species to begin diverging. But it's just as likely that the forces pushing a species apart will change or disappear, and diverging groups re-merge into a single interbreeding population. Part of the fun of studying the natural world is finding things like snail's slime trail discrimination, and trying to figure out what will happen next.
K. Johannesson, J.N. Havenhand, P.R. Jonsson, M. Lindegarth, A. Sundin, J. Hollander (2008). Male discrimintation of female mucous trails permits assortative mating in a marine snail species Evolution, 62 (12), 3178-84 DOI: 10.1111/j.1558-5646.2008.00510.x