Direct, but not indirect, predation-risk cues deter larval settlement in a scyphozoan jellyfish.

Settlement represents a key transition from dispersal to recruitment, during which predation can strongly influence population dynamics through both consumptive and non-consumptive effects. Predation generates chemical cues that convey information about the presence of predators, attacks on prey, ingestion of prey and post-attack conditions. Predation-risk cues can be direct, when emitted by the predator itself, or indirect, when released by prey, yet how larvae respond to these cues during settlement remains poorly understood in many systems. Here, we assessed how larvae of the scyphozoan jellyfish Aurelia coerulea respond to different predation-risk cues by conducting four experiments manipulating predator kairomones, conspecific alarm cues, conspecific post-attack cues and predator diet cues. We hypothesised that settlement responses would depend on the information provided by cues, with an avoidance response expected for those indicating direct predation risk. Settlement was deterred in response to direct cues (predator kairomones) and the combination of direct and indirect cues (predator kairomones with conspecific alarm cues). In contrast, settlement was unaffected by indirect cues alone (conspecific post-attack cues) and cues not associated with predation (mechanical damage cues). In addition, predator diet cues had no further effect beyond that of predator kairomones alone. These results suggest that larval behaviour is shaped by predation risk, with information about the presence of predators playing a key role in eliciting antipredator responses during settlement. Our findings provide mechanistic evidence for understanding how predation-risk cues mediate non-consumptive effects during habitat selection in organisms with complex life cycles, highlighting how antipredator responses during a critical life-history transition can influence recruitment success.