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.