For decades, disruption of the bilateral symmetry of body structures has been related to underperformance and, hence, to fitness. In fish, this concept coupled with the claimed evidence for increased fluctuating asymmetry (FA) of pairs of hard structures, such as otoliths under conditions of stress, has led to the use of otolith FA (OFA) as a proxy for individual fitness and population success. Interpreting those significant differences in OFA in relation to stress-inducing environments and using them to identify suboptimal conditions for survival is especially appealing in the context of, for example, assessing climate change using historical otolith collections. Despite several works that give evidence to support the usefulness of OFA approach, we report on a number of unpublished and published datasets from a wide range of both, wild populations and experimental designs that show no correlation between OFA and any of the usual indicators of fitness or between OFA and expected stress gradients. Our results suggest that there may be a strong bias in the published literature towards positive relationships, despite the enormous importance of negative results for understanding the significance of OFA. These results might shed light on the interpretability of OFA in both laboratory and field studies.

Chemical cues from predators induce a range of predator-induced morphological defences (PIMDs) observed across fish taxa. However, the mechanisms, consistency, direction and adaptive value of PIMDs are still poorly studied. Here, we have tested if predatory cues can induce changes in the body shape of the juvenile marine fish Sparus aurata reared under controlled conditions without the presence of predators by exposing individuals to the olfactory stimulus of a fish predator. We tested our hypothesis using a nested replicated before-after-control-impact experiment, including recovery (potential reversibility) after the cessation of the predator stimulus. Differences in the size-independent body shape were explored using landmark-based geometric morphometrics and revealed that, on average, individuals exposed to a predatory cue presented deeper bodies and longer caudal regions, according to our adaptive theoretical predictions. These average plastic responses were reversible after withdrawal of the stimulus and individuals returned to average body shapes. We, therefore, provide evidence supporting innate reversible PIMDs in marine naive fish reared under controlled conditions. The effects at the individual level, including fitness and the associated applied implications, deserve further research.

We determined the efficacy of marking the otoliths of three small-sized progenetic gobies to validate their increment periodicity. These small gobies have high mortalities and rearing difficulties, making direct validation difficult. The otoliths were marked by immersing the fish in a bath of alizarin red S. The fishes were euthanatized and the number of increments in their otoliths laid down after the fluorescent mark were counted and compared with the number of elapsed days. The results validated the daily periodicity of Aphia minuta and Pseudaphya ferreri. The high mortality hindered the validation of Crystallogobius linearis.