The carabid beetle Pterostichus melanarius (Coleoptera: Carabidae) is a predator of slugs. A paper1 I read today provides preliminary evidence that the European slug Tandonia budapestensis may have evolved a toxic skin or slime to defense itself against the beetles.
Symondson gave starved individuals of P. melanarius either a dead slug of one of 3 species or blowfly maggots and monitored the beetles' mortality afterwards. The slugs used as food were T. budapestensis, Deroceras reticulatum and Arion distinctus.
Fifty percent2 of the beetles fed on T. budapestensis died within two days of feeding upon slugs and 30% of the remainder after the second feeding, which took place a week later. In contrast, none of the beetles fed on other species of slugs and maggots died during the first 2 days after the first feeding.
The author speculates that the skin of T. budapestensis may contain a toxin and that this toxin may also be a defense against the avian predators of this slug. The author further speculates that the orange median stripe of this slug (see drawing below) may function as a warning signal against birds and other visual predators.
I will offer one criticism of this study. The slugs had been killed by freezing and before they were given to the beetles, they were cut open. If I were doing a study like this, I would use live slugs. It is unlikely, but nevertheless possible that the killing and cutting of the slugs changes the composition and toxicity of their slime as a result of some enzymatic reactions.
Nevertheless, the results are intriguing. As the paper notes, slugs depend primarily on their mucus to defend themselves against their enemies. It is, therefore, not too surprising that some species have also evolved the ability to produce toxins in their skin or mucus to strengthen their defenses.
Tandonia budapestensis (from Kerney & Cameron. A Field Guide to the Land Snails of Britain and North-west Europe. 1979).
1. Symondson, W.O.C. 1997. Does Tandonia budapestenis [sic] (Mollusca: Pulmonata) contain toxins? Evidence from feeding trials with the slug predator Pterostichus melanarius (Coleoptera: Carabidae). J. Moll. Stud. 63:541-545.
2. The paper says 50%, but the actual number is 11 dead out of 20 beetles, or 55%.