Chromatophores from Hawaiian Bobtail Squid

Hawaiian bobtail squid are nocturnal predators, remaining buried under the sand during the day and coming out to hunt for shrimp at night near coral reefs. The squid have a light organ (chromatophore) on their underside that houses a colony of glowing bacteria (Vibrio fischeri). The squid uses this bacterial bioluminescence in a form of camouflage called counter-illumination, masking its silhouette by matching moonlight and starlight; thus hiding from predators swimming below. The light organ is attached to the ink sac and it can use this ink like a type of shutter to control the amount of light. This likely helps the squid adjust to variable light conditions, for example cloudy nights or a full vs. new moon. In this image of a juvenile squid, the bi-lobed light organ and ink sac in the center of the squid's mantle cavity is clearly seen. Each chromatophore contains pigment granules surrounded by nerve and muscle fibers. When these muscles are contracted, the pigment sac expands, creating a larger surface area of color. When the muscles relax, the pigment sac can shrink to a small dot, 15 times smaller than their expanded size, hiding the color. Width of image is 0.7 millimeters.