How do octopuses defend themselves?

In addition to having adaptable and ingenious camofluage, octopuses (as well as most cephalopods) can squirt an infamous "ink" from a special glandular pouch to enable their escape. The ink is actually melanin, the same pigment that determines how dark or light a human's skin tone is. Some species of deep-ocean octopus actually produce a bio-luminescent ink.