Terrestrial slugs (Mollusca: Gastropoda) share common anti-predator defence mechanisms but their expression differs among species

Predation is a key selective force in the evolution and maintenance of multiple traits in prey animals. Terrestrial slugs (Mollusca: Gastropoda) are an interesting group in which to study anti-predator behaviour because few predators are mollusc-specialists. Ground beetles (Coleoptera: Carabidae), however, are important slug predators and slugs appear to possess general defences when carabids are encountered. Slugs may avoid areas recently visited by carabids (primary defence) or, when attacked, exude highly viscous mucus (secondary defence). Slugs are, however, a polyphyletic group which are present in diverse habitats with varying predation risk. We examined whether avoidance of cues from Pterostichus niger, a generalist carabid predator, is a primary defence shared by a number of slug species from different taxonomic families representing varying degrees of ecological specialization and found, for the first time, that multiple slug species appear to exhibit a general avoidance of carabid scents. Additionally, we examined whether mucus chemistry is altered in response to predation attempts, and whether this is a secondary defence common to multiple species. We found that calcium, a vital element for molluscs, is secreted in greater amounts when slugs are attacked. Choice experiments with P. niger demonstrated that calcium did not affect beetle feeding behaviour but that high-viscosity gel deterred feeding. We discuss how calcium secretion may contribute to mucus viscosity and represent an effective secondary defence mechanism common to slugs. Results from no-choice feeding trials suggest that carabid beetles may prevent slugs from exuding defensive mucus by attacking the mantle region. Overall, we show that slugs share common primary and secondary defences but that their expression varies among species, possibly reflecting the natural risk of predation experienced by each slug species. Additionally, some species-specific anti-predator traits were observed which cannot be explained as responses to carabids alone, highlighting the importance of considering multiple predators when interpreting anti-predator behaviour in prey animals.