SNAIL REACTION TO WATERCRESS LEAF TISSUES - REINTERPRETATION OF A MUTUALISTIC ALARM HYPOTHESIS

1. Both emergent and submersed growth forms of the amphibious mustard watercress (Nasturtium officinale) contain appreciable concentrations of glucosinolate that deter feeding by a variety of aquatic grazers. G lucosinolate and myrosinase are stored separately in plant tissues, bu t undergo hydrolysis upon tissue damage to form characteristic isothio cyanates and nitriles. 2. Snails of the genus Physella readily consume yellowed, senescing leaves of watercress, yet are reluctant to attack fresh, green leaves. Laboratory behavioural studies confirm that thes e preferences are related to intrinsic properties of the leaves indepe ndent of periphyton concentrations. Snails are attracted to crushed ye llow leaves, yet are actively repulsed by isothiocyanates liberated by crushed, green leaves, despite the higher nitrogen content of the gre en tissue. 3. The aversion response to crushed green leaves confirms r eports by Haynes & Taylor (1984) and others that snails avoid crushed leaves of watercress. However, our interpretation of the interaction i s distinctly different from theirs. Rather than a damaged plant signal ling imminent danger to host periphyton grazers, the liberation of the compound 2-phenylethyl isothiocyanate from damaged tissues acts as a deterrent that protects the plant against opportunistic herbivores, wh ich include certain snail species.