The Smell of Fear Induces Deep-Bodied Individuals in Juvenile Gilthead Seabreams (Sparus aurata)

Predatory stimuli have been proved to induce morphometric and behavioural changes across taxa, including some freshwater fish species. However, the mechanisms, consistency, direction and adaptive value of such changes are controversial, especially in marine species. We tested, for the first time in a saltwater species, if predatory cues may induce changes in the body shape of the gilthead seabream (Sparus aurata) by exposing naïve juveniles to the olfactory stimulus of their natural predator in the seagrass meadows, the black scorpionfish (Scorpaena porcus), previously fed with seabream conspecifics. The experimental setup was based in a nested replicated before-after-control-impact design. Differences in the body shape were tested using geometrical morphometrics before the experiment, after one month with the stimulus and after another month without the stimulus to test the resilience of the potential morphological changes. The odor cue treatment induced significant changes in body shape, producing deep-bodied individuals, in the foreseen direction according to the theoretical  adaptive expectations, since deeper bodies enhances survival in front of predators. Moreover, after one month of stimulus withdrawal, the average direction of body shape changes was retained but was not longer significant. We discuss the mechanisms and consequences of these results for marine juvenile fish survival.