Cognitive plasticity in teleost fish

Phenotypic plasticity is a mechanism that permits to obtain different phenotypes from a single genotype and it is widespread in the animal kingdom due to its adaptive value. Researchers have mostly focussed on plasticity of behavioural and morphological traits, yet evidence suggests that also cognitive traits, i.e., those traits involved in perceiving, storing, and elaborating information from the environment, might display phenotypic plasticity. Here I presented an original work aimed to investigate cognitive plasticity in the vertebrate group with greater brain neurogenesis, and therefore greater theoretical potential for cognitive plasticity, the teleost fish. I exploited three species choose based on their biology to investigate plasticity in response to different factors, Poecilia reticulata, Oryzias latipes and Danio rerio. I exposed the subjects to different treatments in which I manipulated biotic and/or abiotic factors of the environment and then I administered batteries of cognitive tasks to the subjects to understand whether and how their cognition was affected. The factors investigated across five studies were: 1) the level of habitat complexity or environmental enrichment; 2) the level of resource predictability in the environment; 3) the social environment in terms of group size and group complexity; 4) the season; 5) a form of human-generated pollution. The results obtained from the first four studies provided sufficient evidence to support to presence of phenotypic plasticity in fish cognition. For instance, in guppies, high habitat complexity determined a phenotype with greater learning abilities. Moreover, being in a stable and smaller social group improved inhibitory control. Several evidence points towards the idea that this plasticity might be adaptive: for instance, in predictable environments fish showed greater learning abilities, which are arguably beneficial to adapt behaviour to predictable conditions; conversely, in unpredictable environments, the fish developed enhanced inhibitory control and cognitive flexibility, cognitive functions that permit to adapt behaviour to changing situations. In medaka, I found that photoperiod is used as a proxy of season to determine increased inhibitory and learning abilities in winter and I detected seasonal changes in the production of hormones that might be an underlying mechanism for the observed cognitive plasticity. In the last study of the thesis, I exposed groups of zebrafish subjects to microplastic pollution and then I tested them in a learning and a cognitive flexibility task. I found no evidence of cognitive plasticity when looking at this phenotypes. However, a physiological analysis of brain tissues in a subsample of subjects indicated significant alterations due to the pollutant. I therefore conclude that the possible of non-adaptive plasticity in fish cognition in response to human-induced pollution cannot be completely ruled out. In conclusion, this thesis increased the knowledge on cognitive phenotypic plasticity in teleost fish suggesting that it is widespread, it can be adaptive, and can be triggered by different environmental factors. Future studies of fish cognition, especially those interested in interindividual variation, should consider the role of cognitive plasticity.