Science News - How hogfish see with their skin

There are a number of animals in the ocean that can flash colors right before your very eyes. When threatened, a common cuttlefish may get a black stripe, like a masked bandit, over its eyes as its tentacles flair angrily. An octopus will change its color and texture to match its surroundings, making it practically invisible in the landscape. 

How do these animals know when to change color? Are they expertly taking in their surroundings with their eyes, or is there something else at work? In an effort to get to the bottom of this phenomenon, researchers Schweikert, Fitak, and Johnsen from Duke University in Durham, North Carolina have decided to take a look at another one of nature's magicians, the hogfish.

Taken by Albert Kok - Wikimedia Commons

Hogfish are brilliantly colored reef fish famous for their mating behaviors. All hogfish are born female, but after a female matures and reaches a certain level of social dominance, the female changes her sex and becomes a male. While hogfish live in groups, there is only one male, guarding his own personal harem. If something should happen to him, a dominant female will rise up, change sexes, and replace him. 

Hogfish are just as adept at changing their color as they are at changing their sex, shown below in a video uploaded by Mark Karl in 2014.

The key to these changes lie in pigmented skin cells called chromatophores. Chromatophores rapidly rearranged pigments in the skin to change the color, morphing the appearance of either a small area on the animal or even the entire animal itself. However, scientists are still trying to figure out what tells chromatophores to change color. Rather than the hogfish seeing with their eyes, researchers believe hogfish are, in a sense, seeing with their skin, a process called dermal photoreception.

To test out this theory, the researchers at Duke University looked at the genes hogfish have in both their skin and in their eyes. Examining genes is one of the best ways to look at what is happening throughout the body because they serve as the blueprint for proteins in the cells. Proteins are essentially the workers of each cell, so by looking at genes, the researchers were looking at the job descriptions of proteins in the eyes and skin of hogfish. After these proteins do their job, however, the rest of the workers in the cell need to know what to do. Messages can be spread through the cell by other proteins in a signal pathway, a phenomenon similar to a game of telephone. Scientists can also see the code for this kind of communication in the genes. Essentially, the scientist looked at the job description of various workers in each cell as well as how these workers communicate to describe what was going on. 

Photo by Bernard Dupont - Wikimedia Commons

What the researchers found is that hogfish can see with their skin, although not in the same way they can see with their eyes. Hogfish have a protein in their skin called SWS1 that can sense ultraviolet light; in contrast, hogfish eyes have five separate proteins that can be used to see. In addition, the signal pathway that SWS1 uses to communicate, called cAMP, is different from the signal pathway used by the proteins in the eyes, or cGMP. This means that while hogfish use both their eyes and their skin to see, the eyes and the skin don't have the same proteins, and these proteins don't communicate in the same way. While both the eyes and the skin of hogfish can be used to sense the light around them, both parts of the body are doing it in a completely different way.

This discovery is groundbreaking; it is the first time that scientists have been able to show that color-changing fish can sense visual signal, light, with something other than their eyes. There is a lot more work to be done to figure out how much the color of the fish is impacted by the eyes versus the skin and how this factors into how other animals change color. Still, this is an exciting first step in exploring one of nature's coolest phenomenon.

Article Citation:

Lorian E. Schweikert, Robert R. Fitak, Sönke Johnsen. De novo transcriptomics reveal distinct phototransduction signaling components in the retina and skin of a color-changing vertebrate, the hogfish (Lachnolaimus maximus). Journal of Comparative Physiology A, 2018; DOI: 10.1007/s00359-018-1254-4