Manta Rays react to a mirror in an ape-like way, by circling in front of it, wiggling their fins, and blowing bubbles. See the study in Journal of Ethology here.
The mirror test is a classic test of self-awareness, premised on the theory that recognizing your own reflection in a mirror means that you must have a sense of "self." And when researchers placed a large mirror inside a tank with two manta rays, the manta rays behaved in quite an unusual manner-- with actions that strongly imply self-awareness.
Often, animals confronted with a mirror will attack it, or behave socially, as if the mirror is another individual. The manta rays showed no sign of social behaviour; instead, they performed significantly more repetitive behaviors in front of the mirror and repeatedly circled in front of it (video here) In apes and dolphins, similar behaviour has been taken as evidence of self-awareness.
An important next step would be to test the manta rays with the Mark Test, Researchers can mark the rays with a spot that can only be seen with the aid of a mirror-- like what was done with magpies, dolphins, elephants and more (each link takes you to a video of the animal reacting to a marking on its body by looking in a mirror). I particularly recommend the dolphin video; here's another adorable dolphin mirror video. After marking the mantas, would they spend more time looking specifically at the mark, twisting their body to catch a glimpse in the mirror?
It is important to note that the mirror-self-recognition test really only demonstrates one thing: that animals can understand when they're looking at themselves in a mirror. But for now, it's our best way of testing an animal's sense of "self."
One thing is clear: if I were a manta ray with a mirror, I would spend 100% of my free time marveling at how majestic I look while swimming around.
Looking good. Giant manta rays have been filmed checking out their reflections in a way that suggests they are self-aware. Only a small number of animals, mostly primates, have passed the mirror test, widely used as a tentative test of self-awareness. ... But not everyone is convinced that the new study proves conclusively that manta rays, which have the largest brains of any fish, can do this. ... Csilla Ari, of the University of South Florida in Tampa, filmed two giant manta rays in a tank, with and without a mirror inside.The fish changed their behaviour in a way that suggested that they recognised the reflections as themselves as opposed to another manta ray.