Zebrafish suffering from chronic stress as a result of a genetic mutation showed signs of depression in behavioural tests.

Meanwhile, a doctoral thesis on stress response and frustration in fish by research fellow Marco Antonio Vindas at Norway's Environmental and Life Sciences University has also established that salmon have emotional responses, defined as more or less unconscious reactions in the brain triggered by consuming or positive situations.

In the German investigation, zebrafish suffering from chronic stress as a result of a genetic mutation showed signs of depression in behavioural tests. An analysis of the "lethargic" zebrafish showed so a highly elevated concentration of the stress hormones cortisol, CRH and ACTH stress hormones.

Fish on Prozac

The scientists, therefore, postulated that these fish were suffering from chronic stress and were exhibiting certain aspects of depressive or perhaps hyper-anxious behaviour. To put this assumption to the test, the researchers added the antidepressant fluoxetine (a.k.a. Prozac to some) to the water. Shortly afterwards, the fish's behaviour returned to normal.

Frustrated Salmon

In the Norwegian experiment, salmons were taught to associate light with a reward in the form of food. Omitting an expected reward triggered frustration. Levels of dopamine - a neurotransmitter which is produced when new situations are encountered dopamine and help increase attention - not only increased but also to a pretty high level. It was as if encountering ongoing unpredictability required the fish to be more attentive, helps leading to a constant dopamine production.

An adverse effect of sustained heightened dopamin levels is diminished memory and reduced capacity to react normally.

As dopamine is also associated with the sensation of pleasure and plays a role in addiction, decreasing levels can create withdrawal with all the sensations of feeling blue that come with it.

Crabs too

As reported a couple of years ago in this publication, a study conducted by researchers from Queen's University demonstrated that crabs suffer pain and dopamine retain a memory of it.