Prey groups influence the efficacy of individual defensive responses to chemical cues from predators

Background/Question/Methods

Predator chemical cues (scents) have been widely demonstrated to influence developmental and behavioral plasticity of aquatic prey.  For example, crustacean amphipods may attain smaller body size and associate with structure to reduce predation by visual fish predators.  However, to date, few studies have examined how groups of prey exposed to a predatory scent may influence each other’s anti-predator response.  The present study investigated growth of individual and groups of juveniles of the freshwater amphipod Hyalella azteca chronically exposed to chemical cues from a sympatric fish predator (the longear sunfish Lepomis megalotis).  Amphipods were placed into containers both as isolated individuals (n = 20 per solitary treatment) and as groups of three individuals (n = 20 per group treatment) and half the replicates in each treatment (n = 10) exposed to water containing chemical cue of sunfish, and the remaining half (n = 10) exposed to untreated water (control) for a period of 30 days.  Body length was calculated for all amphipods on days 10, 20, and 30 using image analysis.  Following the 30 day experiment, all amphipods were further tested for behavioral responses to a structural refuge following acute exposure to sunfish chemical cue.

Results/Conclusions

Individuals held in groups chronically exposed to sunfish chemical cue attained body sizes that were significantly (P < 0.05) smaller than amphipods in groups chronically exposed to ‘no chemical cue’ controls.  Solitary individuals chronically exposed to either sunfish chemical cue or ‘no chemical cue’ controls showed no significant difference in body size.  Behavioral trials revealed that amphipods held in groups spent significantly (P < 0.05) more time associated with structure than indiviudal amphipods.  These results demonstrate for the first time a group effect on adaptive phenotypic plasticity in individuals exposed to a predator scent.  The mechanism of this group effect is unknown but could be related to a stress cue that is released by amphipods in the presence of predator chemical cues.