How Fish Disappear

Light is essential for many organisms. Plants can’t survive without it, of course, but animals also rely on light—and for more than just seeing what’s around them. Light helps regulate circadian rhythms, which are the fluctuation of biological processes, like hormones, over the course of a day. Even blind mice can regulate their circadian rhythms. We’re so accustomed to equating light with sight that the idea of light influencing organs apart from eyes takes a little mental adjustment.

Fish, for example, have photoreceptors—specialized cells that convert light to electrical signals—not only in their eyes, but also in their skin and on their pineal gland, which links vertebrates’ nervous systems with their endocrine systems. A recent Proceedings of the National Academy of Sciences paper highlights this link. The researchers initially expected to investigate changes in hunger patterns over the course of a day, but wound up studying how light can change fish skin color. The research team studied a member the agouti gene family, agrp2, found only in the pineal gland. Agouti genes encode for hormones secreted by the pineal gland, implicated in skin and hair color, as well as obesity.

But agrp2 is found only in fish, and some fish, unlike mammals, can change color quickly, a camouflage strategy called background adaptation. Melanosomes, pockets of the pigment melanin within the specialized cells, can aggregate or disperse, lightening or darkening the fish’s skin tone.

To get a good look at the melanosomes, researchers studied zebrafish: since zebrafish embryos are transparent, researchers can easily observe cellular movement in the developing organism. (In this video, watch the zebrafish’s circulation). Although sometimes scientists modify cells to see them (like making them glow green), the zebrafish’s melanosomes are easy-to-visualize dark spots along the embryo’s body.

When the researchers placed zebrafish embryos against white, grey, or black backgrounds, the melanosomes changed the skin color accordingly. The researchers then altered agrp2 expression and observed whether the melanosomes behaved differently. Light causes aggregation of zebrafish melanosomes in front of a white background, so the fish get lighter in color and match their background better. Reducing agrp2 expression hampered this, so the fish stayed dark.

Perhaps the most fascinating finding was that the pineal gland and agrp2 were all a fish needed to change its skin tone—blindfish embryos responded to white backgrounds just as effectively as fish that could see. Only embryos without pineal glands had trouble, which pointed to the pineal gland as the light-sensing organ integral to the response.

For anyone enthralled with fish’s lightning-quick color changes, the new study helps illuminate how those changes happen. Not only are the fish responding to light without using their eyes, the response does not seem to require the brain in any way. How cool is that?