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Aguado & Marin (2007) analysed the interaction between Cratena peregrina (Gmelin, 1791) and predatory fish in laboratory and field assays, using both live aeolids and artificial models. Penney’s comment (2009) on the article reopens the old question of the role of nematocyst-based defence in nudibranchs. Penney suggests that the defensive mechanism of C. peregrina is chemical because the nematocysts used in assays could be unarmed. Penney argues that (1) Aguado & Marin (2007) obtained nematocysts by macerating Eudendrium hydroids with a mortar and pestle, and this cannot be considered equivalent to kleptocnidae isolated from C. peregrina; (2) the method by which the authors attempted to incorporate nematocysts into the test food is unclear; and (3) regardless of how nematocysts were added to the artificial food models, it is possible that no functional nematocysts would remain by the time fish encountered them. The main purpose of our article was to demonstrate that fish learn to avoid the warning coloration pattern of C. peregrina, due to associating bad taste or unpleasant experience with the colour pattern. Nematocysts are contained in the defensive exudates and in the external part of the body of C. peregrina, which can only be regarded as circumstantial evidence that these defensive cells or the molecules that they contain represent a deterrent for predators. Of course, the article does not provide experimental evidences that the source of deterrence is the nematocysts, but there is no proof, either, that chemical defences play an active role. If we support the general idea that a positive result for deterrence alone cannot be taken as evidence for a nematocyst-based defence, the same argument must apply to chemical-based defence. We must classify nematocyst-based defences as a special form of chemical defences because nematocysts contain harmful molecules. From this point of view, C. peregrina transfers chemicals from food to the cnidosacs at the tip of the ceras. There is no evidence that C. peregrina contains secondary metabolites other than the nematocyst toxins. Chemical analysis supports the general hypothesis that the nematocysts are the main weapons. Cimino et al. (1980) studied three species of the hydroid Eudendrium (E. rameum, E. racemosum, E. ramosum) in the Bay of Naples (Italy), which are prey of C. peregrina. These authors found the same pathway of polyhydroxylated steroids (Cholest-4-en-4, 16b, 18, 22R-tetrol-3-one 16,18-diacetate) in the hydroids and in the nudibranchs. Unfortunately these steroids do not play a defensive role in hydroids or nudibranchs. Some aeolids contain pigments and secondary meta

Chemical or nematocyst-based defence in the nudibranch Cratena peregrina? - a reply to B.K. Penney