Morphometric traits of shells determine external attack and internal utilization marks in the Roman snail in eastern Germany

Overexploitation, habitat destruction and a changing climate threaten populations of the Roman snail (Helix pomatia Linnaeus, 1758), which has led to a high protection status in Germany. Vertebrate and invertebrate predators, including parasites and facultative parasitoids, further cause pressure on populations. Given the conservation concern for H. pomatia and its rarity in the study region (Cottbus, Germany), we studied how predators and facultative parasitoids utilize H. pomatia shells with a focus on non-invasive field methods. As previous studies indicated that shell size may affect prey selection by predators, morphometric traits were measured in eight subpopulations. We identified the total number and percentage of H. pomatia shells that showed external attack marks by predators and internal utilization marks by Diptera pupae and related those utilization patterns to the morphometric traits of shells. A large proportion of the shells in local subpopulations showed signs of external attack and internal utilization, and both utilization forms were positively correlated. External attacks by predators were more frequent in larger shells and internal utilization by Diptera was more common in shells with higher body density. These results suggest a considerable pressure by predators and potential facultative parasitoids on H. pomatia populations in the study area. Future research should focus on the relationship between snails from the family Helicidae and flies from the genus Discomyza. Conservation programmes should consider abiotic habitat conditions together with potential trophic interactions to maximize the success of conservation strategies.