Study Reveals Fish Can Experience Pleasure and Actively Seek It Out, Challenging Common Perception of Their Intelligence and Capabilities

Fish, often perceived as simple-minded creatures, may possess a more complex emotional landscape than previously thought, according to a new study published in the journal Proceedings of the Royal Society B.

Neuroscientist Caio Maximino from the Federal University of the South and Southeast Pará in Brazil explains that fish exhibit rich behavior driven by internal emotional-like states. This view challenges the common perception that fish are mindless organisms.

Previous research has focused on negative experiences such as fear, anxiety, and discomfort in fish. However, recent findings suggest that fish may also experience positive emotions, including pleasure.

Researchers turned to two coral reef species for their investigation: the bluestreak cleaner wrasse and the threadfin butterflyfish. The former consumes parasites on other fish, fostering a cooperative relationship among various species on the reef.

Threadfin butterflyfish were observed visiting cleaner wrasses more frequently than necessary, leading researchers to wonder whether they sought out cleaning for reasons beyond mere health benefits. Indeed, previous studies have shown that cortisol levels in other reef fish drop during a cleaning session, indicating a potential pleasurable sensation.

To test this hypothesis, the team conducted experiments involving opioid mimics and naloxone – a drug used to reverse overdoses by blocking opioid receptors. Injecting butterflyfish with an opioid mimic increased their preference for areas where they had previously been cleaned, suggesting that the pleasure experienced during cleanings was mediated by natural opioids in their brains. Conversely, naloxone reduced their interest in these locations.

The researchers also explored whether fish differentiated between liking and wanting the cleaning experience. By placing barriers in the tank to make it harder for the butterflyfish to reach the cleaner wrasse, they discovered that both the opioid mimic and naloxone had no effect on the fish’s motivation to navigate these obstacles and receive the pleasurable stimulation.

Maximino proposes that while opioids may influence how much a fish likes something, dopamine might govern the desire to seek out pleasurable experiences – essentially, a ‘fish wish’.

The study’s findings have significant implications for the treatment of fish in aquariums and aquaculture. Susana Peciña, a biopsychologist at the University of Michigan-Dearborn, finds the results exciting and suggests that we may need to reconsider how we care for these creatures to ensure they have more positive experiences and better lives.

Maximino argues that the capacity for fish to experience both negative and positive emotions is an ancient function in animal evolution. Peciña agrees, stating that if the results hold up in additional studies, they may reveal something deeper about what it means to be alive on Earth.