A small animal swimming in moonlit shallow water casts a shadow to anything looking up. Counterillumination removes it: the animal emits light downward at an intensity matching the light coming down from the surface, so its silhouette disappears against the background.
The bobtail squid does not make that light. Within hours of hatching, its light organ is colonised from seawater by the bioluminescent bacterium Vibrio fischeri, filtered from thousands of other bacterial species present. The organ has ink-derived reflectors and a muscle-derived lens to shape the beam, and the squid adjusts output by shuttering the organ. Each dawn it expels most of the bacterial population and the colony regrows during the day.
This partnership became a foundational model system for studying how animals select specific microbial partners, largely through work by Margaret McFall-Ngai and Edward Ruby. What is still being worked out is exactly how the squid's immune system distinguishes the correct symbiont from the many similar bacteria it encounters.