In the time I've spent watching the interactions between slugs and snails, I've had slugs and snails painted in tow very different ways in my mind. The slugs were like the meat-headed bullies who did nasty things to others just because they could, even if their victim was much larger than themselves. They seemed to attack the snails (and other slug species) almost on principle.
The snails, on the other hand, were the quiet, timid victims in the whole thing, always taking shit but never flinging any back. They seemed content to just slime away from the aggressor as quickly as possible.
However, I no longer look at them this way. As I sat in bed one night playing video games, I took the time occasionally to stare at the bin of slugs and snails sitting not three feet away from me. What I saw confused me at first, then shocked me. On the side of said bin, right in plain view, there was a peacefully sleeping slug. Beside him, there was a small snail, only a fraction of the slug's size. The snail seemed to be inching closer and closer to the slug. By that time, my face was plastered to the side of the bin in my fascination.
The snail started biting the slug! It didn't look terribly aggressive, more like he was trying to eat him, but it was biting! As the snail bit, the slug woke up and scooted away by a centimeter or two. But the snail followed and continued his reign of nibbly terror. Whenever the slug would move, the snail would follow. He would not leave that slug alone for anything.
At first, I though this behavior was a sign that they needed more protein, so I gave them a generous helping of both salt-free peanut butter and hard-boiled egg. Despite the smorgasbord, I continued to see this behavior the more I looked for it. It almost appears to be some sort of a game for the snails, because the slugs (it's always just slugs) on the receiving end are always asleep. It would seem that my dear snails are not so innocent and meek after all. They appear to have a passive-aggressive streak a mile wide!
I find myself wondering, with no small amount of humor, if the snails have finally become tired of the slugs' abusive actions toward them, and are now giving the slugs a taste of their own proverbial medicine.
On more than one occasion, I've also seen a snail who was being harassed by a slug fling his shell back and forth with startling violence, as if he were trying to beat off the offender with it. However, this only seems to occur in larger, adult snails.
Or perhaps the reason is slightly more mundane. Maybe this behavior has gone on all along and, because I pegged the snails as unfailingly innocent, was blind to the fact that they'd do such a thing. As always, any thoughts or theories are welcome.