Role of Serotonin in crayfish anxiety and anti-predatory behaviour

Encountering threatening situations is a common occurrence in the living world. When exposed to danger for extended periods, behavioural response manifests as stress-induced sustained alertness, which is known as anxiety. It is essential to understand that anxiety can have long-lasting effects on an organism’s normal behaviour and interactions. This thesis aims to understand the role of serotonin in stress responses among crayfish. Crayfish are a keystone species in an aquatic ecosystem. Changes in their behaviour can negatively affect their inter- and intra-specific interactions. This can disrupt their role in the normal functioning of the ecosystem. The research involves three studies targeting stress under different conditions: exposure to low pH, the presence of fluoxetine, and injection of exogenous serotonin, dopamine, and their blockers. The first study focuses on the impact of freshwater acidification on crayfish response to alarm order (blood from conspecifics), and if this response is mediated via serotonin. The study reports that animals exposed to pH6 exhibit a significantly reduced behavioural response to alarm odour compared to the control (pH8) group. In addition, the animals do not show an elevated level of haemolymph glucose. The latter is mediated via the Crustacean Hyperglycaemic Hormone (CHH) which has been linked to serotonin changes and is an indicator of stress. The study shows that freshwater acidification can impair crayfish olfaction, leading to reduced anti-predatory responses and the behavioural response may be influenced by serotonin. The second study investigates the impact of fluoxetine, a pharmaceutically active compound (PhAC), on anxiety response in crayfish. Fluoxetine is an SSRI (Selective Serotonin Reuptake Inhibitor) which elevates serotonin levels by preventing its reuptake in the synapse. Animals were exposed to three different levels of fluoxetine (high, low, and intermediate doses) and exhibited an anxious response by spending more time with slower active speeds in the dark zones of the plus-maze behaviour setup when compared to controls. The crayfish that were exposed to Fluoxetine had elevated haemolymph glucose levels which suggested that serotonin is involved in the anxiogenic response. The study highlights the potentially harmful effects of pharmaceuticals in freshwater ecosystems, including changes in behaviour that can impact survival sustainability, and species interaction. The final study took a more direct approach regarding the impact of serotonin on crayfish anxiety-like behaviour (ALB). Animal groups were injected with physiological saline, serotonin, and dopamine at 2 doses each (1 μM and 10 μM) alone and in combination with their respective blockers (50 μM maleate salt and 10 μM methergine), in the pericardial sinus. Dopamine at 10 μM dose while serotonin at both tested concentrations, caused ALB among crayfish. The anxiogenic effect was successfully blocked when administered with either of the blockers. However, 1 μM dose of Dopamine did not affect behaviour but it appeared to have an anxiogenic effect when administered with methergine. Overall, our results emphasize the involvement of a complex functional interplay among biogenic amines in ALB in crayfish with blockers showing effects on more than one biogenic amine. It also leaves an open-ended question on the role of dopamine in anxiety responses which needs to be further investigated.