Monday, January 17, 2011

Sharks and Monochromacy: examining predator's limited color vision

It's a colorful world we live in. At least it appears that way. A red apple is not actually producing red light; it's absorbing other non-red light wavelengths and reflecting those wavelengths that we perceive as red. Our eyes have evolved to contain three types of color receptors that respond to certain light wavelengths - basically reds, greens, and blues - which puts us in the category of trichromats. Other animals are dichromats, having two color receptors, while many others have little or no color vision.

Why? Well, partly its evolution's adaptive radiation at work, where animals evolve certain traits or abilities to best respond to their environment and thereby enhance their chances for survival.

In the aquatic world, color can take on a different purpose as the water environment effects various wavelengths. Red wavelengths fade quickly - the underwater world is cast in a blue or green hue because these wavelengths - particularly blue - can travel further, deeper into the water column. So, if you are a predator, what works best in hunting prey? Having a full-range of color sensitivity or limited monochromatic vision that best matches your environment?

In a paper recently published in the natural science journal, Naturwissenschaften, researchers from the University of Queensland, Australia, (including my niece, Dr. Susan Theiss) studied the color vision capabilities of 17 different species of sharks, from small bottom-dwellers like the port jackson shark to open water species like tiger and bull sharks. In many cases, what they found was a monochromatic trend toward blue but with some interesting exceptions.

Juvenile lemon sharks have a greater sensitivity to red, perhaps because of the amount of time spent in shallow waters where a fuller color spectrum would exist. But as adults, they appear to lose that sensitivity as they move into deeper waters. Bull sharks, which can spend up to the first 5 years of life in shallow, brackish water areas, seemed to have a similar sensitivity to red but research has yet to show that they too become more monochromatic as they mature, although it is a distinct possibility.

Many skates and rays are trichromatic and, as these species are close cousins to sharks, it begs the question as to what evolutionary advantage monochromatic vision might provide sharks. Well, in a very unscientific observation, as a filmmaker I can concur with one of the prevailing theories: contrast. Many broadcast video cameras have viewfinders which can switch from color to black and white. With black and white, contrast is all you are left with, and many cameramen feel they are able to focus more accurately and even track the on-camera action better in black and white, as if the fullness of color becomes a visual distraction.

And perhaps that is what the shark needs: a greater sensitivity to high contrast derived from monochromatic vision which helps to discern prey from its surrounding background. Combined with their other electroreceptive capabilities like the Ampullae de Lorenzini and sensitive lateral line, sharks may have developed the most efficient form of vision for hunting, one that has evolved from its surroundings over millions of years.

Susan and her colleagues are continuing to study the color vision capabilities of various shark species. Its microscopic, exacting, and tedious work (sharks can't exactly cover one eye and read from an eye chart: EFPTOZ . . .), but with that knowledge we can gain a better understanding of shark behavior and how, as apex predators, they effectively contribute to a healthy marine ecosystem.

Read the entire paper on sharks' color vision in Naturwissenschaften.

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