We know that animals with larger brains, such as monkeys and other mammals, respond to what happens to them based on their "mental state" and previous emotions. It was thought that this was a characteristic only of the most complex animals, but a search by a group of Portuguese scientists, published in Scientific Report (here the article, in English), showed how the fish brain also functions in the same way.
The researchers subjected some bream (Sparus auratus) to positive stimuli, such as food, or negatives - such as immobilization. In this way they induced particular "mental states" (satisfaction or fear), together with other conditions, such as the predictability or otherwise of a stimulus, which modified their importance.
The physiological responses of animals to stimuli and conditions were then measured, in the form of a group of genes, the level of cortisol in the blood (the stress hormone) and behavior along with other fish.
Scientists have found that the combination of stimuli and the importance of the same for animals triggers different behavioral responses: escape or approach to the stimulus. The fact that the same stimulus presented in a predictable or unpredictable manner involves different responses, demonstrates that even animals such as fish respond according to their emotions, that is "how they feel" at that moment.
Ancient origin. This experiment also confirms experimentally other previous observations, which had suggested that the basic emotions (satisfaction, fear, tranquility, etc.) were also present in animals with a less complex brain than that of mammals. And that therefore it is an ancient evolutionary mechanism, also present in animals that have separated from mammals hundreds of millions of years ago.