Krill (Euphausia superba - Antarctic krill)
"...If you were to see the whole of me you may only see my big black eyes, greeny gut, and glowing red tail. As I swim, my pleopods and feeding basket bits move so fast you barely see. But you may feel the water turbulence I make.
Sometimes, in the dark, I fluoresce. I love that you see my greeny-yellow stomach full of phytoplankton I have eaten. That gorgeous colour is chlorophyll A, a dye in the phytoplankton that enables photosynthesis. Quite magical, really, when you think that light is the source of most of what we know as life, and just how many krill there are, and how many whales."
Lisa Roberts 2019