Bioluminescent countershading in midwater animals: evidence from living squid.

@article{Young1976BioluminescentCI,
  title={Bioluminescent countershading in midwater animals: evidence from living squid.},
  author={Richard Edward Young and Clyde F. E. Roper},
  journal={Science},
  year={1976},
  volume={191 4231},
  pages={
          1046-8
        }
}
Midwater squid respond to overhead illumination by turning on numerous downward-directed photophores; they turn off the photophores when overhead illumination is eliminated. The squid are invisible when the intensity of the photophores matches the intensity of the overhead illumination. These results strongly support the theory of ventral bioluminescent countershading. 
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