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Background matching: Researchers traced how zebrafish larvae blend into bright surroundings through background adaptation, identifying which cells in the eye and brain control it.
Tracing connections: Specialized cells in the retina signal the brain to control two hormones – raising levels of one that lightens the fish while lowering levels of one that darkens it.
Pinpointing the pathway: This work pinpoints cell types and connections, revealing how vision controls body-wide color changes – a fundamental response across some fish, amphibians, and reptiles – and opens up key mechanistic and evolutionary questions.
The ability of some animals to dynamically change color to match the brightness of their surroundings is one of nature's great survival tools, allowing flatfish to blend into sandy seabeds, frogs to adjust to the bottom of ponds, and chameleons to adapt to the tone of the foliage they sit in – abilities known as background adaptation. Researchers in the Max Planck Institute for Biological Intelligence's Genes – Circuits – Behavior department have now pinpointed the cells and connections underlying this response in zebrafish larvae, tracing the process from light detection in the eye to color changes in the skin.
When swimming across bright surroundings, these tiny fish get pale over tens of minutes, helping them avoid unwanted attention from predators. Previous work had discovered that this color change in the zebrafish involved cells in the retina that detect ambient light and hormones driven by the fish’s brain that lighten or darken the skin, but the specific types of cells and all the intermediate steps leading to hormone secretion had not been worked out. A new study, published in Current Biology, has provided important new insights into these puzzles.
Tracing the pathway
In zebrafish larvae, color change happens in specialized skin cells called melanophores, where melanin particles – the same pigment found in our skin and hair – either clump together to make the fish paler, or spread out to darken it. The new study identified the kinds of cells that detect the light in the retina and control the hormone shifts in the brain, driving these changes when the zebrafish swim over bright backgrounds.
"Retinal ganglion cells in the eye detect light and transmit this signal in the form of neuronal activity to neurons in the brain's hypothalamus. These, in turn, produce a hormone and secrete it into the bloodstream, causing the fish to lighten. At the same time, again acting on retinal ganglion cells, light suppresses a different hormone in a different population of neurons that darkens the skin," says Krasimir Slanchev, a senior scientist at the Max Planck Institute for Biological Intelligence and the paper's first author. He adds: “ The system works like a dimmer switch with two controls working in opposite directions.”
From detection to camouflage
Retinal cells initiating the color change sense the intensity of ambient light directly through a photosensitive protein called melanopsin, rather than relying on the eye's conventional photoreceptors. They control one of two melanin-concentrating hormones in fish – one regulates camouflage, while the other controls hunger and energy balance. Mammals lost the color-change hormone but kept its counterpart, and understanding this pathway can help scientists explore fundamental principles about how brains in vertebrates translate visual information into hormonal signals.
The team used genetic engineering and fluorescent labeling to visualize connections, eliminate specific cells, and trace individual neurons from the eye through the brain. Zebrafish larvae proved ideal for this research: many aspects of their brain structure resemble those found in humans, their genome can be modified to test gene function, and their transparency allows detailed examination under a microscope in living animals.
"Color change is critical for how some animals evade predators, find mates, hunt, and navigate their lives," says Herwig Baier, Director of the Genes – Circuits – Behavior department at the Max Planck Institute for Biological Intelligence. "Yet which cells control this and how they are wired still holds fascinating puzzles. This work opens broader questions about how sensory information transforms into hormonal signals and how this ancient vertebrate response was lost when mammals evolved – giving us a window into a survival mechanism our ancestors left behind. These comparative insights could illuminate fundamental principles of how brains translate what we sense into what we do."
Prof. Dr. Herwig Baier
Director
MPI for Biological Intelligence
herwig.baier@bi.mpg.de
Krasimir Slanchev, Eva Laurell, Irene Arnold-Ammer, Enrico Kuehn, Uyyashrinila Pandiarajan, Herwig Baier;
Molecular delineation of a retina-dependent photoneuroendocrine pathway in zebrafish
Current Biology, online 5 November 2025
https://doi.org/10.1016/j.cub.2025.10.025
https://www.bi.mpg.de/baier - Department website
When swimming across bright surroundings, zebrafish get pale over tens of minutes. Researchers now i ...
Copyright: © MPI for Biological Intelligence / Krasimir Slanchev
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