Developed in the 1970s, the mirror test is considered the gold standard for determining self-awareness and involves marking an animal with an odorless dye and then observing whether the animal becomes aware of the mark (i.e. turns to get a better look at it, pokes at the spot on its body or in the reflection, etc.) when faced with a mirror. While the cleaner wrasse may not have the appendages to indicate awareness as animatedly as animals like chimpanzees and elephants, they do appear to recognize their own reflections.
"Appear" is the operative word here. The study's researchers from the Max Planck Institute for Ornithology (MPIO) in Germany and Osaka City University (OCU)in Japan are clear that the results are open to interpretation.
"The behaviours we observe leave little doubt that this fish behaviourally fulfils all criteria of the mirror test as originally laid out," senior author Alex Jordan said in a statement. "What is less clear is whether these behaviours should be considered as evidence that fish are self-aware — even though in the past these same behaviours have been interpreted as self-awareness in so many other animals."
So how exactly did the researchers get fish to take part in a test of any kind? Probably with a lot of patience. The cleaner wrasse may have been good contenders for the experiment because they have a habit of scouring other fish for parasites, so they seem to have a capacity for observation. To test how that attentiveness might translate to their own bodies, researchers placed colored marks on the fish in areas the subjects could only see in reflection. If a fish touched or investigated the mark in any way, it "passed" the test and researchers considered it a demonstration that the animal perceived its own reflected image.
If you're wondering how in the world a fish could touch or investigate anything with a complete lack of limbs, consider the fact that those terms were used rather loosely in the study. If the fish scraped their bodies on hard surfaces after viewing themselves in the mirror, the scientists considered those scrapes attempts to remove the markings.
And that's not an unreasonable consideration given that the fish never tried to scrape off transparent marks when a mirror was present, and they never appeared to get rid of the colored marks when no mirror was present, which suggests their self-scraping was in response to the visual cue of their reflected image. To further back that theory up: Unmarked fish that interacted with marked fish across a clear divider didn't try to remove anything from their own bodies, and they didn't try to remove marks placed on the mirror itself (which you might expect, given their natural inclination to seek out environmental parasites).
