Acoustic mobbing behaviour: vocal fish responses to predation risk through sound communication

In both terrestrial and marine ecosystems, prey species exhibit various reactions to predators, including avoidance, fighting and freezing. Acoustic communication associated with mobbing has been depicted in numerous tetrapod species. This sonic behaviour remains however largely unknown in fish although mobbing has been shown in different coral reef fish as a response to predation risk. This study focused on the communication strategy of prey species in the presence of predators. We tested the reaction of the holocentrid Sargocentron caudimaculatum when facing a moray eel and compared the sounds they produced against the predator with sounds produced when the fish are hand-held, recorded in standardized conditions. When introduced in mesocosms to a moray eel, S. caudimaculatum produced distinct distress sounds, resembling staccato calls, and swam towards the predator with their heads pointed in its direction and their dorsal fin erected. This observation supports a mobbing behaviour with specimens shifting from an escape behaviour to an aggressive response in presence of predator. Moreover, these sounds are different from those emitted during manual handling. This difference in acoustic signal suggests the coexistence of multiple warning/alarm sounds in this species. Holocentrids can produce different kinds of sounds witnessing their ability to provide graded information based on the perceived predation risk. We suggest that species of this family would be a suitable and promising group with which to test the hypothesis of acoustic coordination where individuals could inform their conspecifics about an increase in the immediate predation risk through appropriate alarm calls and trigger an escape response of the group if needed.