Euprymna scolopes

Euprymna scolopes, commonly known as the Hawaiian bobtail squid, is a small species of squid native to the shallow coastal waters of the Hawaiian archipelago. Adults generally reach a mantle length of only about 3 centimeters, making them one of the smallest cephalopods in the Pacific. They are famous for their unique relationship with bioluminescent bacteria.

This squid houses a colony of bioluminescent bacteria called Vibrio fischeri inside a specialized light organ on its underside. The squid uses the bacteria's blue-green glow as a form of counter-illumination, matching the moonlight from above to eliminate its shadow on the ocean floor. This camouflage hides the squid from predators swimming below.

The Hawaiian bobtail squid is not born with the Vibrio fischeri bacteria; it must acquire them from the surrounding seawater. The squid uses specialized ciliated appendages to catch the specific bacteria from the water column and funnel them into its light organ. Once the bacteria are established, the squid changes its surface structures to prevent other bacteria from entering.

Scientists extensively study Euprymna scolopes as a model organism to understand the molecular mechanisms of animal-bacterial symbiosis. Because the squid's light organ is directly linked to its immune system and daily rhythms, it provides a simple, easily observable system for studying how animals recognize beneficial bacteria. It also helps researchers understand how bacteria communicate via quorum sensing.