Ultrastructure of snail grazing damage to calcicolous lichens

The identification of damaged lichens is often difficult due to changes in the morphology of regenerating specimens. We examined the ultrastructure of grazing damages to four species of calcicolous lichens (Aspicilia calcarea, Physcia adscendens, Tephromela atra and Xanthoria parietina) and free-living cyanobacteria (family Chroococcaceae) caused by individuals of four species of land snails (Chondrina clienta, Balea perversa, Clausilia bidentata and Helicigona lapicida). We also investigated the radular structure of the four lichen-feeding snails to examine whether differences in radular morphology result in species-specific grazing damages. Individuals of all four snail species removed the cyanobacteria layer covering the limestone or lichen surfaces. The four lichen species were grazed to a different extent by the different snail species. SEM-images showed that B, perversa left distinct depressions on the thalli of A. calcarea, whereas H. lapicida grazed off the thalli of this lichen rather evenly. Both snail species left visible radular traces on the lichen surface. In contrast, Ch. clients left shallow depressions without radular traces on the thalli of A. calcarea. In Tephromela atra, grazing damages were observed on both thallus and ascocarp. Ascocarps of T. atra were partly grazed by B. perversa. Helicigona lapicida grazing on T. atra removed more or less evenly the entire lichen tissue including the ascocarps. In foliose lichens, grazing by Ch. clienta, B. perversa and CI. bidentata resulted in depressions of different depths, while H. lapicida removed entire pieces of the thalli. In general, radular traces were less distinct in foliose lichens than in crustose lichens.