We tend to credit animals like cats and dogs with a certain degree of mental complexity. But fish aren't usually afforded that kind of praise.
"They do not talk, they do not bark," says Caio Maximino, a neuroscientist at the Federal University of the South and Southeast Pará in Brazil. "We usually think, 'Well, these are very simple-minded animals. They are like little robots that do not do much.'"
But personally, Maximino doesn't believe that. "Those animals, they have very rich behavior that is mediated by these internal, emotional-like states," he says.
Previous research has largely focused on the negative experiences of fish that are driven by fear, anxiety and discomfort. "It has been demonstrated that they feel pain, for example," says Marta Soares, a behavioral physiologist at the University of Porto in Portugal. "And that was a huge step, actually."
But Soares and Maximino wondered whether fish could feel good, too. In a study published in the journal Proceedings of the Royal Society B, they and their colleagues conclude that fish can feel pleasure and that they actively seek it out.
"Fish like things, they want things," says Soares. "Basically, it would be nice to change, a bit, the view of people regarding fish."
Very cooperative fish
To determine what fish might feel, the researchers turned to two coral reef species. The first was the bluestreak cleaner wrasse. This silvery blue little fish with a jet black stripe eats the bloodsucking parasites on other fish, including predatory species that might otherwise eat them. The whole system, says Maximino, is "a model for cooperation."
"What they do is just clean, clean, clean from 6 a.m. to 6 in the night," says Soares. In the wild, "you have all sorts of different species" that stop by the wrasse's territory for a cleanup before moving on.
This includes the second species the researchers studied — the threadfin butterflyfish. The scientists wondered if these striking yellow, black and white fish might be visiting the cleaners for more than just the health benefits. Especially because in the lab, the butterflyfish didn't need to be cleaned — they came parasite-free.
Soares had previously shown that another type of reef fish's levels of cortisol, a stress hormone, dropped during a cleaning. "So we thought maybe there's something else" going on, says Maximino. "Maybe there's some pleasurable sensation that is being produced by this massage."
To find out, they ran a series of experiments. First, Maximino observed that butterflyfish preferred spending time in the part of the tank where they had previously interacted with a cleanerfish.
"Not only [did the fish have] a memory of being cleaned there, but he wanted to go there," says Maximino. "Like, 'This was a very nice place where I received a wonderful massage from this fish and well, I want this again.'"
Liking and wanting
Maximino and Soares knew that fish have an opioid system (just like us), which regulates both pain and pleasure. And they thought perhaps that system is at least partly responsible for the butterflyfish's interest in seeking out a cleaning.
To test this idea, the researchers injected the butterflyfish with a low dose of an opioid mimic, a drug similar to morphine that boosted opioid activation slightly.
The result was that "they spent much more time looking for this place where they experienced cleaning before," says Maximino. "So it increased their preference."
But when they injected the butterflyfish with naloxone — a drug that blocks opioid receptors and is used in people to reverse an overdose — they lost interest in the spot where the cleaners had been. This suggested that there may well be pleasure involved with the massage "and that this is mediated by those natural opioids in their brains," says Maximino.
The scientists' final question was whether there was a difference between liking the cleaning and wanting the cleaning. So they put a series of barriers in the tank that made it harder for the butterflyfish to reach the cleaner. This time, the same drugs — both the opioid mimic and the naloxone — had no effect. The butterflyfish continued to navigate the barriers to reach the cleaner, revealing that their motivation for the reward was not impacted.
This meant that this other sensation of desire to receive the pleasurable stimulation — think of it as a fish wish — may be governed by a different chemical like dopamine.
Maximino explains it this way: "The opioid system's like, 'This is pleasurable,' and the dopamine system's like, 'Go after what was pleasurable before.' Opioids change how much you like something, but they do not change how much you want something."
"The main takeaway is that fish experience some type of pleasure," he says, "and they work very hard to get this pleasure again."
Some scientists, however, may need a little more convincing.
"Can we unequivocally say that this is pleasure in fish?" asks Susana Peciña, a biopsychologist at the University of Michigan-Dearborn who wasn't involved in the research. "I'm not sure that I would say it in those words. Having said that, it's very difficult to measure pleasure in animals, period, let alone fish."
Still, she finds the results exciting. To her, they suggest we may need to rethink how fish are treated in aquariums and aquaculture. "Can we think of ways they can have more positive experiences, better lives?" she says.
Ultimately, what this and other experiments are revealing, says Maximino, is that fish have rich behaviors that may be controlled at least partly by feelings — both negative and positive.
"Pleasure and wanting and desire and all these positive emotions that we feel — it's not only humans and chimps and cats and dogs," he says. "Fish, they can also feel it. So this is a very ancient function."
It's a capacity that Maximino argues was likely quite important in animal evolution.
Peciña agrees. If the results hold up in additional studies, she says, they may suggest "something deeper about what it means to be alive on Earth."
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