For the first time, a species of fish has demonstrated evidence of possible self-awareness by passing the famous mirror test.

In a series of experimental observations conducted by Japanese researchers, bluestreak cleaner wrasse – a petite coral reef fish – were able to recognize that their reflection represented their own body.

Until now, the only non-human animals that have passed the test – regarded among behaviorists as the benchmark for higher level cognitive capacity – have been a handful of mammals (several apes, elephants, orcas, and dolphins) and birds in the crow family. (A controversial study suggests that ants may have passed too)

However, several recent studies have "reveal[ed] that the perceptual and cognitive abilities of fish often match or exceed those of other vertebrates, and suggest the possibility that the cognitive skills of fish could more closely approach those found in humans and apes," the authors wrote in the pre-publication version of their study, currently posted on bioRxiv.

During a mirror test experiment, the animal in question is placed in front of a mirror after a colored mark, typically a sticker, is placed on a prominent part of its body. If the animal then tries to examine and/or remove the mark from that part of itself, but does not attempt to do so in the absence of a mirror or in response to an invisible mark, the creature is considered to understand the nature of reflections, and is thus capable of understanding a concept of self.  

Though cleaner wrasse – selected by the team due to past evidence of complex cognition – do not have hands or beaks with which to investigate and try to remove a mark with, they do show a specific behavior wherein they scrape parts of their body against rocks and other materials in order remove irritants and parasites from their scales. This was used as the pass/fail cue for 10 cleaner wrasse subjects, all housed in separate tanks.

In a preliminary experiment, the researchers first observed the animals’ behavior in front of a mirror before adding marks into the mix. Seven of the wrasses passed through all three stages of a behavior pattern that past research has identified in self-recognizing animals. First, the fish reacted to their reflection as if it was another individual, next they performed unusual behaviors not typically seen without the mirror, and third, the fish began to gaze at and closely examine their reflection.

Moving on to the true mirror test, the team injected a colored gel right under the surface of the skin of eight of the wrasses, creating a mark resembling an external parasite. Three of four wrasses marked on their throats showed repeated scraping in that area yet exhibited no response to a non-marking injection and did not scrape when the mirror was absent.

Three of four marked on the head engaged in significant head scraping when the mirror was present, but the authors did not base their conclusions off these individuals as they were seen head scraping at other times too.

“The results we present here will by their nature lead to controversy and dispute, and we welcome this discussion,” the authors wrote. “We do not consider that the successful behavioural responses to all phases of the mark test should be taken as evidence of self-awareness in the cleaner wrasse, but rather that these fish come to understand that the mirror reflection represents their own body.”

They conclude by noting that their findings could lead to two distinct paradigm shifts in the field: Either we start to entertain the possibility that fish, previously believed to be unintelligent animals, are self-aware because they passed the same test as ‘higher’ animals, or we conclude that these behaviors are based on a cognitive process other than self-recognition.

“If the former, what does this mean for our understanding of animal intelligence? If the latter, what does this mean for our application and interpretation of the mark test as a metric for animal cognitive abilities?”