Origin and specificity of predatory fish cues detected by Baetis larvae (Ephemeroptera; Insecta)

• We evaluated antipredator responses of mayfly larvae (Baetis) to familiar and novel predators.
• Cutaneous mucus cues from familiar (but not novel) predators reduced foraging activity in Baetis.
• Antipredator response of Baetis was independent of predator feeding status.
• Magnitude of response did not differ with relative risk of predation.
• Results elucidate the origin and specificity of chemical signalling in freshwater invertebrates.

Some prey respond to predator cues by altering their behaviour, but the response may depend on predator diet and identity. There are few data on how predator feeding status and diet affect the behaviour of aquatic insects and on the specificity of the response of these prey to different predators. We present the results of four bioassays conducted in microcosms to evaluate the relationship between predator fish diet, predator identity and the predator avoidance behaviour of mayfly prey of the genus Baetis to elucidate the origin and specificity of the antipredator response. Results from the first bioassay indicated that the response of Baetis to predatory trout is independent of diet, which enabled us to rule out the following potential origins of the signal: alarm cues emitted by wounded Baetis conspecifics, salivary enzymes released by feeding fish and excretion products that may contain Baetis and conspecific odours. Thus, we focused further experiments on fish cutaneous mucus as the potential origin of cues that cause Baetis to reduce its activity. In two bioassays, we observed that North American Baetis bicaudatus did not distinguish between five freshwater fish species that co-inhabit the same drainage, despite differences in their relative risk of predation (four predatory salmonid species and one omnivorous sucker species). However, in another bioassay, we observed that European Baetis rhodani larvae did not respond to cutaneous mucus of novel predators (carp, seawater turbot or frog), indicating that Baetis do not show a general response to all mucus donor organisms. Our bioassays identified mucus as the potential origin of the cue eliciting antipredator behaviour in Baetis, providing much needed insight into the specificity of fish-associated chemical cues that cause some prey to respond. Experimental approaches similar to the one used in this study may increase our understanding of the role of chemical cues in aquatic ecosystems.