Bobbing and fin-flicking in a small benthic fish

Most anti-predator strategies increase survival of individuals by signalling to predators, by reducing the chances of being recognised as prey, or by bewildering a predator's perception. In fish, bobbing and fin-flicking are commonly considered as pursuit‑deterrent behaviours that signal a predator that it has been detected and thus lost its surprise-attack advantage. Yet, very few studies assessed whether such behavioural traits are restricted to the visual presence of a predator. In this study, we used the yellow black-headed triplefin Tripterygion delaisi to investigate the association between these behaviours and the visual exposure to (1) a black scorpionfish predator (Scorpaena porcus), (2) a stone of a size similar to that of S. porcus, (3) a conspecific, and 4) a harmless heterospecific combtooth blenny (Parablennius sanguinolentus). We used a laboratory-controlled experiment with freshly caught fish designed to test for differences in visual cues only. Distance kept by the focal fish to each stimulus, frequency of bobbing and fin-flicking were recorded. Triplefins kept greater distance from the stimulus compartment when a scorpionfish predator was visible. Bobbing was more frequent in the visual presence of a scorpionfish, but also shown towards the other stimuli. However, fin flicks were equally abundant across all stimuli. Both behaviours decreased in frequency over time suggesting that triplefin become gradually comfortable in a non-changing new environment. We discuss why bobbing and fin-flicking are not exclusive pursuit‑deterrent behaviours in this species, and propose additional non-exclusive functions such as enhancing depth perception by parallax motion (bobbing) or signalling vigilance (fin‑flicking).