Fin‐flicking behaviour as a means of cryptic olfactory sampling under threat of predation

Risk of predation is a ubiquitous component of behavioural decision-making (Lima & Dill 1990). For small littoral fishes, chemical cues released from predators and/or injured conspecifics, comprise important sources of information about the presence and nature of predation risk (see Kelley 2008; Ferrari et al. 2010 for recent reviews). Behavioural responses to these chemical cues include area avoidance, increased shoal cohesion, reduction in activity, movement out of the water column, use of shelter and finflicking (Lawrence & Smith 1989; Ferrari et al. 2010). Collectively, these behavioural responses reduce the probability of predation (Mathis & Smith 1993). Fin-flicking is a behaviour in which fish quickly sweep their pectoral fins anteriorly, matched with concomitant counterbalancing forward thrusts of the anal and caudal fin, resulting in no net movement. The function of fin-flicking has not received much attention in the context of antipredator behaviour. Fin-flicking by glowlight tetras (Hemigrammus erythrozonus, Durbin 1909) in response to conspecific chemical alarm cues induced greater shoal cohesion in conspecifics and also deterred attack behaviour by a potential predator, the Jack Dempsey cichlid (Rocio octofasciata, Regan 1903; Brown et al. 1999). Be that as it may, the precursor function of fin-flicking signals may be more utilitarian in nature and only later became co-opted into a visual signalling system. This brief communication presents a test of the hypothesis that fin-flicking by alarmed fish serves the selfish benefit of allowing individuals to sample chemical cues in the surrounding waters whilst remaining motionless to avoid attracting attention from predators. Receptors that detect chemical alarm cues and predator odours are arranged in rosettes in the nares, which are blind pouches in the rostrum (Doving & Lastein 2009). Advection (mass water movement) of water to the nares is generated by ram ventilation as fish move through the water column. When risk is detected, fish are faced with a trade-off because the physical swimming movements needed to assess predation risk are conspicuous to predators. Fin-flicking behaviour may be a solution to this trade-off if it allows individuals to discreetly create microcurrents of water past the nares resulting in continuous sampling of chemical information of the surrounding water without the accompanying movements that would compromise crypsis. Java moss (Taxiphyllum barbieri) was chopped into 2-mm pieces to create small neutrally buoyant particles that would allow us to visualise water currents generated by fin-flicking. In one recording session, an adult fathead minnow (Pimephales promelas, Rafinesque 1820) was placed in a shallow white plastic trough that afforded good visual contrast with the moss fragments. In a second recording session, an adult fathead minnow was placed into a narrow aquarium with a small amount of clay to visualise water currents. Skin extract was prepared by killing an adult fathead minnow by cervical dislocation and then lightly scoring its flanks with a clean razor blade. One minnow was killed for each session of filming. The minnow carcasses were then soaked in 100 ml of deionised water for 10 min. The test dose of alarm cue was 10 ml of minnow skin extract administered to the side of the container to induce fin-flicking behaviour. All procedures were approved by the Minnesota State University Moorhead Institutional Animal Care