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Skin Vision - The Fish Senses Light differently from Eyes

Colouration in animals is produced by reflection and scattering of light by cells and tissues, and by absorption of light by chemical pigments within cells of the skin.

In ectotherms, cells containing pigments are called chromatophores and are largely responsible for generating skin and eye colour.

When most people think of colour change, they think of octopuses or chameleons - but the ability to rapidly change colour is surprisingly widespread.

Many species of crustaceans, insects, cephalopods (squid, cuttlefish, octopuses and their relatives), frogs, lizards and fish can change colour.

They all have one thing in common: they are ectotherms (animals that cannot generate their own body heat in the same way as mammals and birds) and only ectotherms have the specialised cells that enable colour change.

The ocean covers the major part of our planet, accommodating a quarter of all known species, influencing our climate and offering a coastal habitat for half of the world’s population – and despite this we are destroying it. Fish aquarium may be the part of decoration, but it speaks lot about the ocean, where 70 percent earth is occupied by the water and the sea food is also liked by many in the world.

There is an interesting fact detected by the Duke University that a particular species of fish changes color of the skin as per the surroundings.

The hogfish is a pointy-snouted reef fish that can go from pearly white to mottled brown to reddish in a matter of milliseconds as it adjusts to shifting conditions on the ocean floor. Genetic analysis reveals new evidence to explain how the hogfish uses its skin to “see.” The hogfish can change from white to spotted brown to reddish depending on its surroundings.

In a new study, researchers show that hogfish skin senses light differently from eyes. The results suggest that light-sensing evolved separately in the two tissues, says Lori Schweikert, a postdoctoral scholar with Sönke Johnsen, biology professor at Duke University.

Scientists have long suspected that animals with quick-changing colors don’t just rely on their eyes to tune their appearance to their surroundings—they also sense light with their skin. But exactly how “skin vision” works remains a mystery.

With “dermal photoreception,” as it is called, the skin doesn’t enable animals to perceive details like they do with their eyes, Schweikert says. But it may be sensitive to changes in brightness or wavelength, such as moving shadows cast by approaching predators, or light fluctuations associated with different times of day.

Schweikert, Johnsen, and Duke postdoctoral associate Bob Fitak focused on the hogfish, or Lachnolaimus maximus, which spends its time in shallow waters and coral reefs in the western Atlantic Ocean, from Nova Scotia to northern South America. It can make its skin whitish to blend in with the sandy bottom of the ocean floor and hide from predators or ambush prey. Or it can take on a bright, contrasting pattern to look threatening or attract a mate.

The key to these makeovers are special pigment-containing cells called chromatophores, which, when activated by light, can spread their pigments out or bunch them up to change the skin’s overall color or pattern.

The researchers took pieces of skin and retina from a single female hogfish caught off the Florida Keys and analyzed all of its gene readouts, or RNA transcripts, to see which genes were switched on in each tissue.

Previous studies of other color-changing animals including cuttlefish and octopuses suggest the same molecular pathway that detects light in eyes may have been co-opted to sense light in the skin.

But Schweikert and colleagues found that hogfish skin works differently. Almost none of the genes involved in light detection in the hogfish’s eyes were activated in the skin. Instead, the data suggest that hogfish skin relies on an alternative molecular pathway to sense light, a chain reaction involving a molecule called cyclic AMP.

Just how the hogfish’s “skin vision” supplements input from the eyes to monitor light in their surroundings and bring about a color change remains unclear, Schweikert says. Light-sensing skin could provide information about conditions beyond the animal’s field of view, or outside the range of wavelengths that the eye can pick up.

Together with previous studies, “the results suggest that fish have found a new way to ‘see’ with their skin and change color quickly,” Schweikert says.

 

Chander Mohan

Krishi jagran/New Delhi



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