These fish may feel pleasure while being groomed by other fish

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