The possible role of volatile secretions as intra- and interspecific alarm signals in Gyrinus species

Gyrinid beetles have paired pygidial glands producing a secretion consisting of both a high molecular weight fraction, rendering them toxic for predatory fish, and usually also volatile fractions (8 of 10 north European Gyrinus species), which act as alarm signals. The presence of multi-species aggregations on the water surface raises the question whether the pygidial secretion of one species can elicit an escape response in other species. There were no differences in escape reaction to signals between two strongly scented species (G. aeratus and G. substriatus) and one species without obvious scent (G. opacus ). Furthermore, signals were received across the species borders, although only the two species emitting volatile components were able to alarm other individuals. The two scented species (G. aeratus and G, substriatus) often aggregate in large groups on open water, whereas the unscented species (G. opacus and G. minutus) rarely do so, but preferably occur singly or in loose groups among the emergent marginal vegetation of various standing waters. This suggests that the presence of alarm signals is correlated with behaviour and habitat choice. Furthermore, the putatively weak competitor G. opacus spends significantly more time below the water surface than G. substriatus, perhaps as a result of it lacking an alarm function. The occasional presence of individuals of the two unscented species in large aggregates of G. aeratus on open surface, may indicate that they may take advantage of the alarm signals produced by individuals of another species and as result escape approaching danger. It is not known whether G. opacus and G. minutus have lost the ability to produce volatile alarm substances or whether they never have had it. Comparisons with the ca 11000-year-old subfossil localities of G. opacus, however, indicate that at least from that date this species has inhabited localities similar to the present with respect to predation risks and other properties, i.e. mainly small, predatory fish-free waters.