The scent of fear makes sea urchins go ballistic

Este artículo contiene 12 páginas, 4 figuras, 1 tabla.Background: Classic ecological formulations of predator–prey interactions often assume that predators and prey interact randomly in an information-limited environment. In the feld, however, most prey can accurately assess predation risk by sensing predator chemical cues, which typically trigger some form of escape response to reduce the probability of capture. Here, we explore under laboratory-controlled conditions the long-term (minutes to hours) escaping response of the sea urchin Paracentrotus lividus, a key species in Mediterranean subtidal macrophyte communities. Methods: Behavioural experiments involved exposing a random sample of P. lividus to either one of two treatments: (i) control water (fltered seawater) or (ii) predator-conditioned water (with cues from the main P. lividus benthic predator—the gastropod Hexaplex trunculus). We analysed individual sea urchin trajectories, computed their heading angles, speed, path straightness, difusive properties, and directional entropy (as a measure of path unpredictability). To account for the full picture of escaping strategies, we followed not only the frst instants post-predator exposure, but also the entire escape trajectory. We then used linear models to compare the observed results from control and predators treatments. Results: The trajectories from sea urchins subjected to predator cues were, on average, straighter and faster than those coming from controls, which translated into diferences in the difusive properties and unpredictability of their movement patterns. Sea urchins in control trials showed complex difusive properties in an information-limited environment, with highly variable trajectories, ranging from Brownian motion to superdifusion, and even marginal ballistic motion. In predator cue treatments, variability reduced, and trajectories became more homogeneous and predictable at the edge of ballistic motion. Conclusions: Despite their old evolutionary origin, lack of cephalization, and homogenous external appearance, the trajectories that sea urchins displayed in information-limited environments were complex and ranged widely between individuals. Such variable behavioural repertoire appeared to be intrinsic to the species and emerged when the animals were left unconstrained. Our results highlight that fear from predators can be an important driver of sea urchin movement patterns. All in all, the observation of anomalous difusion, highly variable trajectories and the behavioural shift induced by predator cues, further highlight that the functional forms currently used in classical predator–prey models are far from realistic.The Spanish Ministry of Science and Innovation funded this research (project CTM2010-22273-C02, CTM2017-86695-C3-1-R, CTM2017-86695-C3-3-R, and CGL78156-C2-R1). JFP acknowledges fnancial support from the Spanish Ministry of Education (scholarship AP2008-01601) and the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 795315.Peer reviewe