The swell shark is, well, just swell!
This shy catshark (yep, that's a real family of fish) grows to about one metre (39 inches) in length. It loves to hang out in deep water reefs and rocks where it feeds on small fish and crustaceans. If a human approaches one, it will lay motionless, hoping to not be noticed. If that doesn't work, it will then swell to twice its normal size to appear more threatening. (Which is where its name actually comes from.)
Considering how much this character wants to be left alone, you'd think that it would be the last animal to, say, glow in the dark. But you'd be wrong!
Indeed, the swell shark uses biofluorescence to glow. It has pigments in its skin that capture blue light and reemit it as green. The effect is unmistakable ... if you're another shark, that is.
And that is the key. Because only animals with the particular eyes of another swell shark is able to notice this colouration. To humans, for example, the swell shark is a fairly drab fish (although its spots are quite lovely).
But thanks to a new 'shark-vision' camera, scientists have been able to replicate the way in which these animals see their world. Not to mention, their shark cousins. So far, researchers don't know for sure why a swell shark wants to stick out in this way, though they suspect that it's for the same reason as a lot of bright markings in nature: finding a mate!
You can witness the green beauty of these wary fish for yourself in this video.