Scientists find fish that ‘sees’ with its skin ‘even after it dies’

‘The animals can literally take a photo of their own skin from the inside’

Reef-dwelling hogfish monitor changes to their own skin colour based on the environment in which they swim, sensing their surroundings using special light-sensing cells on their skin “even after they die”, a new study reveals.

The research, published in the journal Nature this week, improves our understanding of the behaviour and evolution of these fish, and how certain animals are able to monitor their own skin colour changes and quickly adapt.

“They appear to be watching their own colour change,” study co-author Lori Schweikert said.

“In a way they can tell the animal what its skin looks like, since it can’t really bend over to look,” explained Sonke Johnsen, another author of the study.

Many fish, including squid, amphibians, reptiles and fish, have the natural ability to rapidly change colour, with the trait evolving multiple times in several different animal species.

Creatures find this trait useful for adapting to environmental temperature changes, attracting mates and providing camouflage, said researchers, including those from University of North Carolina, Wilmington in the US.

Cells on their bodies called chromatophores, which contain pigments, crystals or tiny reflective plates, enable these animals to rapidly change their colours within minutes or less.

Hogfish, for example, change their colour to camouflage and escape predators, or for social signalling.

The reef fish is common in the western Atlantic Ocean from North Carolina to Brazil and is known for its colour-changing skin.

It is known to morph from white to mottled to reddish-brown in a matter of milliseconds to blend in with corals, sand or rocks.

They do this by moving pigment around within their body’s chromatophore cells to expose or cover the white tissue underneath.

However, it remains unclear how hogfish regulate and perceive these colour changes.

What particularly surprised scientists in the study is that the fish continued its camouflage even though it was no longer alive.

In the new research, they used microscopy to examine hogfish skin in detail by measuring the impact of light on different parts of the fish.

Researchers found that light receptors, called SWS1, that are underneath the chromatophore may be involved in the process.

They said these cells are sensitive to the light shining through colours expressed by chromatophores, specifically the wavelength of light that is present in their coral reef habitat.

These receptors, according to scientists, provide feedback to the fish on where and how colour changes are occurring in different parts of their skin.

“By examining the morphology, physiology, and optics of dermal photoreception in hogfish (Lachnolaimus maximus), we describe a cellular mechanism in which chromatophore pigment activity (ie, dispersion and aggregation) alters the transmitted light striking SWS1 receptors in the skin,” scientists wrote in the study.

This feature enables the reef-dwelling fish is to monitor the chromatophores and sense information about their own colour change performance.

“The animals can literally take a photo of their own skin from the inside. In a way they can tell the animal what it’s skin looks like, since it can’t really bend over to look,” Dr Johnsen explained.

“Just to be clear, we’re not arguing that hogfish skin functions like an eye,” Dr Schweikert said, adding that eyes are capable of more than just detecting light.