This is the wonderful Hawaiian Bobtail Squid. And they really are full of wonder. Bobtail squid are one of nature's "bioluminescent" - light emitting - organisms. As if this is not enough, they are also a great example of that most inspiring and uplifting phenomenon of symbiosis; the bioluminescence is provided by a species of bioluminescent bacteria that live in special "light organs". Nature may be red in tooth and claw but sometimes creatures can get along. (Actually, all multicellular animals are really ecosystems of numerous bacteria, some of which are "friendly", but most don't have anything as cool as glow-in-the-dark Vibrio fischeri.)
The presence of the Vibrio allows the squid to alter its brightness to match the ambient light. It does this using an ink sac, which can essentially block out the light to differing extents. Simple but effective. What do the bacteria get? The squid feed them on amino acids and sugars, which are pumped into the light organ.
As with many marine symbioses, the bacteria are spread horizontally - adult squid vent bacteria daily into the ocean, which can then colonise new squid hatchlings. Although fantastic, this can present a headache for evolutionary biologists trying to predict responses to climate change, for example. Not only do you have to account for host adaptation but potential adaptation (or frailty) of the symbiont, and the interaction of the two; indeed, the paper that drew my attention to these guys was looking to do just that. (In it, they use one of my programs, which is nice (and always surprising) - not just for the citation but also because, abeit in a tiny and trivial way, I feel like I am contributing to the understanding of these amazing creatures.)