Overcoming Deterrent Metabolites by Gaining Essential Nutrients: A Lichen/Snail Case Study

Lichen secondary metabolites are generally considered as repellent compounds for lichen feeders. Nevertheless, if the only food available consists in lichens rich in secondary metabolites, lichenophages such as Notodiscus hookeri, a gastropod native from the Possession Island, seem able to implement strategies to overcome the toxicity of these metabolites. Thus, the balance between phagostimulant nutrients and deterrent metabolites appears to play a key role in their feeding preferences. To further understand lichen-gastropod interactions, we studied the feeding behavior of Notodiscus hookeri fed exclusively with Usnea taylori, a lichen rich in usnic acid and arabitol. Snail feeding choice experiments with intact lichens vs acetone-rinsed lichens were carried out to study the influence of secondary metabolites. Simultaneously, usnic acid and arabitol were quantified and localized within the lichen thallus using HPLC-DAD-MS and in situ imaging by mass spectrometry to assess whether their spatial distribution induce preferential snail grazing. Then, no-choice feeding experiments were devised using usnic acid and arabitol embedded in artificial diet, separately or together.  This case study demonstrated that the nutritional activity of N. hookerii was governed by the chemical quality of the food and primarily by nutrient availability (arabitol), despite the presence of deterrent metabolites (usnic acid).