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Are sharks color blind?
Date: January 19, 2011 Source: Springer Science+Business Media Summary: Sharks are unable to distinguish colors, even though their close relatives rays and chimaeras have some color vision, according to new research by scientists in Australia. Their study shows that although the eyes of sharks function over a wide range of light levels, they only have a single long-wavelength-sensitive cone type in the retina and therefore are potentially totally color blind. Share:
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Sharks are unable to distinguish colors, even though their close relatives rays and chimaeras have some color vision, according to new research by Dr. Nathan Scott Hart and colleagues from the University of Western Australia and the University of Queensland in Australia.
Their study shows that although the eyes of sharks function over a wide range of light levels, they only have a single long-wavelength-sensitive cone* type in the retina and therefore are potentially totally color blind. Hart and team’s findings are published online in Springer’s journal Naturwissenschaften.
“This new research on how sharks see may help to prevent attacks on humans and assist in the development of fishing gear that may reduce shark bycatch in long-line fisheries. Our study shows that contrast against the background, rather than colour per se, may be more important for object detection by sharks. This may help us to design long-line fishing lures that are less attractive to sharks as well as to design swimming attire and surf craft that have a lower visual contrast to sharks and, therefore, are less ‘attractive’ to them,” said Prof. Hart.
Sharks are efficient predators and their evolutionary success is thought to be due in part to an impressive range of sensory systems, including vision. To date, it is unclear whether sharks have color vision, despite well-developed eyes and a large sensory brain area dedicated to the processing of visual information. In an attempt to demonstrate whether or not sharks have color vision, Hart and colleagues used a different technique — microspectrophotometry — to identify cone visual pigments in shark retinas and measure their spectral absorbance.
They looked at the retinas of 17 shark species caught in a variety of waters in both Queensland and Western Australia. Rod cells were the most common type of photoreceptor in all species. In ten of the 17 species, no cone cells were observed. However, cones were found in the retinae of 7 species of shark from three different families and in each case only a single type of long-wavelength-sensitive cone photoreceptor was present. Hart and team’s results provide strong evidence that sharks possess only a single cone type, suggesting that sharks may be cone monochromats, and therefore potentially totally color blind.
The authors conclude: “While cone monochromacy on land is rare, it may be a common strategy in the marine environment. Many aquatic mammals − whales, dolphins and seals − also possess only a single, green-sensitive cone type. It appears that both sharks and marine mammals may have arrived at the same visual design by convergent evolution, in other words, they acquired the same biological trait in unrelated lineages.”
*Note: There are two main types of photoreceptor cells in the retina of the eye. Rod cells are very sensitive to light and allow night vision. Cone cells also react to light but are less sensitive to it. Eyes with different spectral types of cone cells can distinguish different colors. Rod cells cannot tell colors apart.
What Kind of Color and Markings Do Sharks Have?
Coloring in any animal depends upon whether the animal is a predator or prey species, whether it’s venomous, poisonous or essentially harmless. Because sharks tend to be top-of-the-food-chain predators, their colors are not usually flashy. For the most part, sharks depend upon counter-shading to blend with darker, murky colors of deep water when viewed from above, and lighter colors of sky and shallow water when viewed from below, to help catch their prey by surprise.
Elegantly Understated
Just as we tend to wear neutral colors when we don’t want to stand out, so sharks depend upon understated colors to keep them as unobtrusive as possible. Many more recognizable sharks around the world fit this category. The great white shark (Carcharodon carcharias), bull shark (Carcharhinus leucas) and bronze whalers (Carcharhinus brachyurus) are examples of sharks that lean toward neutral grays with creamy underbellies — employing counter-shading as insurance against detection from above or below. All three species inhabit coastal waters and prefer hunting near shorelines. The bull shark is also sometimes found in freshwater — traveling far up rivers from ocean outlets.
Some sharks jazz up the basic color schemes — gray, brown, tan or white — with added stripes, spots or other patterns. Not that these embellishments are particularly flashy — most are practically unnoticeable as only slightly darker or lighter shades of the overall body coloring — but they serve as distinguishing marks for identification. They also break up the shark’s body lines to mimic nearby substrate, which helps the shark blend into the background. Tiger sharks (Galeocerdo cuvier) and broadnose seven-gill sharks (Notorynchus cepedianus) are good examples of this sort of faint, same-color patterning. The broadnose, in particular, is interesting because its varied-sized speckles and spots look like the sandy bottoms and rocky shorelines that it frequents while hunting.
Gaudy Get-Ups
Not many sharks go all out for colors and patterns, but among those that do, some are real show-stoppers. For general elaborateness of ornament it would be hard to beat the wobbegongs of the Australian coral reefs. Their assorted green, yellow or brown blotches, lines, squiggles, spots and O-shaped rings are decorative enough, but with the oddly-shaped fleshy protuberances around their bodies, they look like something between a fringed rug and an entire coral reef. The ornate wobbegong (Orectolobus ornatus) and the tassled wobbegong (Eucrossorhinus dasypogonare) are especially distinctive members of the unusual shark family Orectolobidae.
An inconspicuous tan-colored wallflower by day, the cookie-cutter shark (Isistius brasiliensis) pulls out all stops at night. One of the rare sharks to display bioluminescence, it uses its built-in light show to lure much larger animals close in where it can take a bite out of them. It does this by lighting up its belly in a greenish glow using special light-emitting organs called photophores. The entire underside glows except for one small fish-shaped area. When a predator comes close to eat the “fish” — highlighted from behind by what appears to be sky — the sneaky cookie-cutter shark bites, twists and dashes away with its meaty prize — leaving a deep, circular, cookie-cutter-shaped wound on its victim. Many whales, porpoises, sharks and other marine mammals bear distinctive circular scars from the work of this clever, hit-and-run shark.
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