Counterillumination
This website was created as a project for and undergraduate Biology Program at Davidson College.
Last Updated 10.23.2005

Ventral Photophores and Counterillumination

Counterillumination or countershading is a unique antipredatory behavior common to midwater cephalopods, decapod crustaceans and fishes. Counterillumination camouflages an animal being viewed from below by mimicking the downwelling light and reducing their dark silhouette. All squid that use this method have photophores on the ventral surface of their mantle. This technique is dependant upon bioluminescence for it to be effective; a transparent body often is not sufficient. It is important to note that counterillumination is only feasible in areas where the bioluminescent pattern is believable. Too close to the surface, and the altered silhouette will be too dark. Too deep into the bathypelagic zone, and illumination will be too bright and could potentially attract predators (Jones 2004).

The importance of closely matching ambient conditions has caused squid to become specialized in monitoring the quality of downwelling light as well as emitting appropriate versions of it. Euprymna scolopes uses an extraocular photosensitive vesicle to monitor changes in light as it migrates from the ocean floor vertically through the water column to an area about 400m below the surface where it remains for the night. Abralia sp. and Abraliopsis, in contrast, monitors water temperature as a guide of the emission color necessary to mimic at differing depths.
Figure modified from homepage