Glandular quinoline-derivates protect crustacean woodlice from spider predation

In evolutionary time, aquatic crustaceans colonized land and faced new terrestrial predators such as spiders and ants. We tested the hypothesis that the crustacean terrestrial woodlouse <i>Porcellio scaber</i> produces defensive metabolites that provide protection against terrestrial predators. When attacked by a predator, <i>P. scaber</i> expels proteinaceous secretions from its tegumental glands. Analyses of gland secretion extracts by gas chromatography-mass spectrometry and by liquid chromatography-mass spectrometry revealed four metabolites: methyl 8-hydroxy-quinoline-2-carboxylate, methyl 8-hydroxy-4-methoxy-quinoline-2-carboxylate, methyl 8-(sulfooxy)quinoline-2-carboxylate and methyl 4-methoxy-8-(sulfooxy)quinoline-2-carboxylate, the latter three being natural products not previously known. In behavioural experiments, <i>Steatoda grossa</i> spiders readily preyed on <i>Tenebrio molitor</i> beetles but avoided chemically well-defended <i>P. scaber</i>. When beetles were rendered chemically well-defended by topical applications of either <i>P. scaber</i> gland secretion extract or synthetic metabolites identified in these extracts, spiders rejected the beetles as prey. Our data support the hypothesis that <i>P. scaber</i> produces defensive metabolites against terrestrial predators. We show that the crustacean <i>P. scaber</i>, like many insects, is chemically defended against predators.