A neat little video of a neat little squid that you all got a chance to see in lab:
Note the burying behavior — during the day, these guys bury themselves with sand to escape predation. Generally, they only emerge at night to hunt. On their underside is a light organ that is colonized by bioluminescent bacteria — Vibrio fischeri — which they use for countershading by matching the aperture of the organ to the luminosity of the moonlight coming down.
More on the symbiosis here: