- Between 200 and 1000meters deep from the surface lies the mesopelagic zone which is also called as twilight zone for the scanty light that it receives.
- Many animals living in this region are bioluminescent besides having visual systems specialized to optimize light capture.
- 5 species of mesopelagic bioluminescent sharks (lanternsharks and kitefin sharks) were found to have adaptations such as:
- Using hormones to control bioluminescence, but they rarely glow.
- Having the highest number of light-emitting organs (at times over 5000 in a single shark).
- A novel translucent tissue in the upper eye orbit of lanternsharks – believed to be used to ensure efficient counter-illumination.
- This is a camouflage technique used by mid-water animals which produces a ventral light similar to the down dwelling sunlight so as to camouflage their appearance from predators or prey situated below them.
- A hitherto undiscovered gap between the lens and iris, believed to maximize the light falling on retina.
- Higher density of photoreceptor cells (rods – which assist in vision in low light) than the bioluminescent sharks, probably to help detect moving objects better.
- The adaptations also contribute towards bioluminescent communication and helps confirm the usefulness of bioluminescence in sharks as a communicating tool.
- Read at: http://www.thehindu.com/todays-paper/tp-features/tp-sci-tech-and-agri/how-deepwater-sharks-see/article6289407.ece