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 that they had an extremely elevated concentration of the stress hormones cortisol, CRH and ACTH.

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 was taught to associate light with a reward in the form of food. Omitting an expected reward triggered frustration. Levels of dopamin - a neurotransmitter which is produced when new situations are encountered to and helps increase attention - not only increased but did to a quite high level. It was is if encountering ongoing unpredictabity required the fish to be more attentive which led to an constant production of dopamin.

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

As dopamin 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 comes with it.

Crabs too

As reported a couple of years ago in this publication a study conducted by reseachers from Queen's University demonstrated that crabs not only suffer pain too but also retain a memory of it.