Snails avoid the medulla of Lobaria pulmonaria and L. scrobiculata due to presence of secondary compounds

Lichens are frequently grazed by various invertebrates, such as snails and slugs. However, these gastropods discriminate between the various layers of the lichen thallus. Likewise, carbon based secondary compounds (CBSCs), some of which are known to deter lichenivores, are unevenly distributed between the various layers. In this study, the degree of rejection of medullary CBSCs by gastropods is investigated. The snail Cochlodina laminata was offered the lichens Lobaria pulmonaria and L. scrobiculata with and without CBSCs. The secondary compounds were removed by rinsing dry thalli in acetone. The snails completely avoided the medulla of thalli with natural levels of CBSCs. However, they grazed through all layers perpendicularly after these compounds had been removed. Hence, the medullary compounds restrict feeding by gastropods to the cortical and sometimes also the photobiont layer. (C) 2011 Elsevier Ltd and The British Mycological Society. All rights reserved.