New findings show that the lantern shark employs luminescence on its belly to camouflage itself in the water column. The deep water shark can control the intensity and wavelength of the light it emits ventrally so that it matches the downwelling light from the above. This phenomenon known as counter-illumination has been described from a few other species such as shrimps and squids but not previously from large vertebrae predators.

Most light in the aquatic environment comes directly from the sun, i.e. from above and as anybody who has dived can testify to then this mean that everything moves above you in the water column has a very distinct silhouette against the downwelling sunlight.  This makes it very difficult to remain hidden. Some luminescent shrimps and squids have therefore developed a method of directing their light emissions  downward to act as counter-illumination to hide their silhouettes from predators. However some sharks hunts from above and therefore would also benefit from counter-illumination.  

Belgian and Norwegian biologists from the Catholic University of Louvain and the University of Bergen have now found such counter-illumination in the luminescent lantern shark, Etmopterus spinax, a deepwater shark from the Norwegian fiords. The scientists captured sharks and measured the intensity and wavelength of the light it emitted in the laboratory when illuminated with different lighting conditions from above. They furthermore measured ambient light conditions at 200 m depth in the Norwegian fiord where they captured the sharks.

The experiments showed that sharks in the laboratory spontaneously emitted light similar to the properties of light found in the fiords.  Thus the biologists can deduce that sharks can use counter-illumination to camouflage themselves from prey when attacking from above, although more direct experiments are needed to show if they use this strategy in the wild.

Source:
Claes, J. M., Aksnes, D. L. and Mallefet, J. (2010) Phantom hunter of the fjords: Camouflage by counterillumination in a shark (Etmopterus spinax). Journal of Marine Biology and Ecology 388: 28-32.

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