Behavioural and neuromolecular responses of aquatic organisms to environmental changes

Abstract

Global changes alter aquatic environments through increases in temperature and carbon dioxide, provoking a variety of responses at the organism level. Behavioural impairments in aquatic animals due to climate change imply that cellular alterations occur in their nervous system, but underlying mechanisms are difficult to pinpoint due to the complexity of neuro systems. Using neurological model species, I examine gene expression responses to near-future predicted temperature and CO2 conditions as animals perform specific behaviours, to assess what molecular changes occur during behavioural impairments and identify which mechanisms may drive such alterations of the behaviour. In zebrafish (Danio rerio), the exposure of larvae to warming during development increased locomotory activity when responding to a novel odour alongside upregulation of cell signalling, neuronal development and functioning genes and downregulation of genes involved in protein translation and ATP metabolism. Therefore, novel odour recognition in elevated temperature reveals an energetic trade-off between expensive baseline processes and responsive functions. Brain activity in zebrafish larvae, measured through calcium imaging, revealed that parental exposure to elevated temperature decreased the time till maximum brain activity when responding to an alarm cue. Brain activity also tended to last longer if either parents or offspring were exposed to elevated temperature, but its duration was similar to control when both generations experienced elevated temperature, revealing overall that parental thermal history can alter their developing offspring’s neuronal activity and help them shape their response to olfactory stimuli in future predicted conditions. Another future environmental change, acidification, provoked no differential expression but co-expression of genes involved in immune response and oxidoreduction in adult gills indicating that zebrafish are adapted for acid-base regulation in acidified conditions. However, as acidification reduced anxiety-like behaviour and provoked differential expression of genes involved in cytoskeletal organization, cellular transport, immunity and the visual neural system in the brain, zebrafish show sensitivity to acidification despite successful acid-base regulation. Finally, by leveraging the detailed knowledge on a reflex behaviour in California sea hare (Aplysia californica), I aimed to make a link between the behaviour, the neuro system and ocean acidification. Under acidified conditions, Aplysia relax tails faster after withdrawal and expression of sensorin-A, a mechanosensory neurons inhibitor, is increased. Habituation training under acidification produced longer tail relaxation and affected vesicle transport and expression of stress response genes. Finally, gabazine, a GABAA antagonist, did not restore normal behaviour with acidification, highlighting instead the possibility of other potential molecular drivers of behavioural impairments. This thesis shows that global changes alter molecular processes in the neurosystem of aquatic animals and such changes have consequences on behavioural outputs, which are therefore expected to be altered in many regards in our fast-changing environment.