Back in 2006, for a brief period I kept an individual of the native philomycid slug Philomycus carolinianus and an individual of the introduced slug Arion subfuscus in captivity. The slugs lived in the same container and as far as I could tell, their interactions were peaceful. I published my observations as a short note in the 2007 issue of Tentacle (p. 14).
About a week ago, I made a similar observation, this time in the wild. I was searching for Megapallifera mutabilis, another philomycid slug. I found one squeezed inside a tight crevice in a tree and there was an Arion subfuscus a few centimeters below it.
If the 2 species had antagonistic interactions, they probably wouldn't rest so close to each other. Or would they?