Guest post by Emily Argo
Countershading, originally described in the late 1800s, is when one side of an animal is dark and the other is light, serving as a form of camouflage. In fish, such as the Atlantic Bluefin Tuna (Thunnus thynnus) pictured, this typically means the ventral side (bottom) is light and the dorsal side (top) is dark. This is useful for fish because the dark dorsal side helps them blend in the with substrate or deeper water below if they are being viewed from above. Then, the lighter dorsal side helps fish blend in with the water (and light backdrop of the sky) above them if you are looking at the from below. Countershading is seen in fish species in coastal and open ocean habitats.
Atlantic Bluefin Tuna are one of many pelagic species that exhibit countershading.
However, some species, such as the Nile catfish (Synodontis batensoda), exhibit reverse countershading where the ventral side is dark and the dorsal side is light. The Nile catfish feeds while swimming upside down in the water column and the reverse countershading helps it camouflage.
While evidence of the mechanisms that drive countershading are lacking, studies suggest that there is an adaptive advantage to countershading in aquatic habitats where the scattering of light through the water column remains relatively uniform throughout the day compared to terrestrial environments (Ruxton et al. 2004).
Can you think of other animals that exhibit countershading?
Reference:
Ruxton, Graeme D., Michael P. Speed, David J. Kelly, What, if anything, is the adaptive function of countershading?, Animal Behaviour, Volume 68, Issue 3, September 2004, Pages 445-451, ISSN 0003-3472, http://dx.doi.org/10.1016/j.anbehav.2003.12.009.
(http://www.sciencedirect.com/science/article/pii/S0003347204001794)