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Everything You Want To Know About Flatfish Chromatophores
2 years ago
Salmontations,
Flatfish possess chromatophores. They are special cells that can change color. They use these to adapt their scale pattern to blend in with the substrate they're on. Amazing stuff, and really important, too. Flatfish are demersal ambush predators and prey to larger fish. Hiding is essential to survival. But have you wondered how they are able to change color? Is it a conscious or unconscious process? How do all of those cells coordinate? I may have some answers!
I dug into a research paper by Derek Burton [1] which examined the physiological mechanisms of flatfish chromatophores in great detail. It is the source of all I'm about to share.
Let's begin with taxonomy! What is a flatfish? A flatfish is a teleost (a subphylum which means bony fishes, further, they are under the class of actinopterygii, the ray-finned fishes) in the order of Pleuronectiformes. They are characterized by their asymmetrical body plan as adults, where one eye migrates to the other side of the head during metamorphosis. They lie flat on the sea floor. The blind side is characteristically a pale white, whereas the dominant side is covered with color-changing scales. The dorsal and anal fins run the length of the body on the dorsal and ventral sides. The fins have highly articulate spines that allow them to prop themselves up, crawl, and throw substrate over themselves to become buried.
That's neat! Now, not all chromatophores are alike. Some are responsible for specific pigments and colors, some react to neurotransmitters more than the others. So how many types are there on a flatfish? Five! There are epidermal melanophores, dermal melanophores, xanthophores, iridiphores, and erythrophores. Clusters of these different cells are present in different proportions on a flatfish's body, and give each one a distinct base pattern; between species, and between individuals. Here's what these grouping look like: https://64.media.tumblr.com/27fc881.....fb28c41143.pnj
And here are the proportions of cells that make up all of these distinct clusters: https://64.media.tumblr.com/c1e1675.....ebc9e1de00.pnj
The cell type that stands out the most in this graph is the great concentration of iridiphores in the white spots. As a matter of fact, the blind side of a flatfish is mostly made up of iridiphores! Why are they so bright? Well, these cells contain elongate crystals of guanin; a copper-gold alloy. Shiny! These crystals can become covered by melanosomes in order to reduce their prominence. Notice how the density of iridiphores decreases from white to dark.
Iridiphores are relatively inflexible when it comes to color change. But there are other cells that can take up that mantle: those present in the general scaling.
Before I start talking about hormones and neurotransmitters, I'd like to mention that flatfish are not the only fish who possess chromatophores. Salmon and tilapia have these cells, as well as others. Control of their pigment is largely hormonal, though. Given the importance of a flatfish's camouflage, they have adapted greater control over their colorful dermal cells. Their changes have some hormonal factor, but it is largely neurotransmitters that manage this complex active camouflage system.
I spent hours making this schematic specifically for this purpose, so behold: https://64.media.tumblr.com/8aa3104.....3193d2ab17.pnj
It's a lot to take in. It was a lot to sort. I had to make a truth table of cells and stimuli, and sort out which effect one had on the other. This is not entirely comprehensive, either! Depending on which group these cells are in, they react more quickly or more slowly to these stimuli.
For example: Melanosome concentration in melanophores in the dark band group reacted quickly when the background the flatfish was on changed from white to black. White-spot melanophores respond more rapidly to noradrenaline than dark band melanophores. If you wanna know how each cell responds to each stimuli, check out the paper!
Following my colorful graphic, we can see that the chain begins with visual stimuli. A flatfish settles in a new area of sea floor. The sun is rising and the colors of the substrate become more visible. This signal is processed by the autonomic nervous system, specifically the sympathetic nervous system. Unconsciously, the work of adapting the scales’ pigment starts. Melanin hormones are released from the pituitary gland as necessary to achieve the desired change, and neurotransmitters are sent such that the right scales become the right hue and brightness. This could take dozens of minutes or even hours, but it happens. The flatfish is now well adapted to their new environment.
If that isn't the most fascinating thing to you, I don't know what could top it! There many more questions to be answered, and this is just a simplified overview of the general physiology at play. In my future research, I plan to set out to determine which substrate colors and patterns are imitated by flatfish by using complex images featuring various shapes and colors.
[1] DEREK BURTON, 2002; The Physiology of Flatfish Chromatophores (https://sci-hub.se/10.1002/jemt.10166)
Correction: The melanin hormones from the pituitary gland don't necessarily instigate change. That's the job of the neurotransmitters. The hormones do, however, keep things from changing. The neurotransmitters set melanosome aggregation or dispersion, and the hormone concentrations maintain them.
Caveat: Some chromatophore activation was done outside of the animal, so it's ambiguous how big of a role some of these mechanisms play in the flatfish.
I dug into a research paper by Derek Burton [1] which examined the physiological mechanisms of flatfish chromatophores in great detail. It is the source of all I'm about to share.
Let's begin with taxonomy! What is a flatfish? A flatfish is a teleost (a subphylum which means bony fishes, further, they are under the class of actinopterygii, the ray-finned fishes) in the order of Pleuronectiformes. They are characterized by their asymmetrical body plan as adults, where one eye migrates to the other side of the head during metamorphosis. They lie flat on the sea floor. The blind side is characteristically a pale white, whereas the dominant side is covered with color-changing scales. The dorsal and anal fins run the length of the body on the dorsal and ventral sides. The fins have highly articulate spines that allow them to prop themselves up, crawl, and throw substrate over themselves to become buried.
That's neat! Now, not all chromatophores are alike. Some are responsible for specific pigments and colors, some react to neurotransmitters more than the others. So how many types are there on a flatfish? Five! There are epidermal melanophores, dermal melanophores, xanthophores, iridiphores, and erythrophores. Clusters of these different cells are present in different proportions on a flatfish's body, and give each one a distinct base pattern; between species, and between individuals. Here's what these grouping look like: https://64.media.tumblr.com/27fc881.....fb28c41143.pnj
And here are the proportions of cells that make up all of these distinct clusters: https://64.media.tumblr.com/c1e1675.....ebc9e1de00.pnj
The cell type that stands out the most in this graph is the great concentration of iridiphores in the white spots. As a matter of fact, the blind side of a flatfish is mostly made up of iridiphores! Why are they so bright? Well, these cells contain elongate crystals of guanin; a copper-gold alloy. Shiny! These crystals can become covered by melanosomes in order to reduce their prominence. Notice how the density of iridiphores decreases from white to dark.
Iridiphores are relatively inflexible when it comes to color change. But there are other cells that can take up that mantle: those present in the general scaling.
Before I start talking about hormones and neurotransmitters, I'd like to mention that flatfish are not the only fish who possess chromatophores. Salmon and tilapia have these cells, as well as others. Control of their pigment is largely hormonal, though. Given the importance of a flatfish's camouflage, they have adapted greater control over their colorful dermal cells. Their changes have some hormonal factor, but it is largely neurotransmitters that manage this complex active camouflage system.
I spent hours making this schematic specifically for this purpose, so behold: https://64.media.tumblr.com/8aa3104.....3193d2ab17.pnj
It's a lot to take in. It was a lot to sort. I had to make a truth table of cells and stimuli, and sort out which effect one had on the other. This is not entirely comprehensive, either! Depending on which group these cells are in, they react more quickly or more slowly to these stimuli.
For example: Melanosome concentration in melanophores in the dark band group reacted quickly when the background the flatfish was on changed from white to black. White-spot melanophores respond more rapidly to noradrenaline than dark band melanophores. If you wanna know how each cell responds to each stimuli, check out the paper!
Following my colorful graphic, we can see that the chain begins with visual stimuli. A flatfish settles in a new area of sea floor. The sun is rising and the colors of the substrate become more visible. This signal is processed by the autonomic nervous system, specifically the sympathetic nervous system. Unconsciously, the work of adapting the scales’ pigment starts. Melanin hormones are released from the pituitary gland as necessary to achieve the desired change, and neurotransmitters are sent such that the right scales become the right hue and brightness. This could take dozens of minutes or even hours, but it happens. The flatfish is now well adapted to their new environment.
If that isn't the most fascinating thing to you, I don't know what could top it! There many more questions to be answered, and this is just a simplified overview of the general physiology at play. In my future research, I plan to set out to determine which substrate colors and patterns are imitated by flatfish by using complex images featuring various shapes and colors.
[1] DEREK BURTON, 2002; The Physiology of Flatfish Chromatophores (https://sci-hub.se/10.1002/jemt.10166)
Correction: The melanin hormones from the pituitary gland don't necessarily instigate change. That's the job of the neurotransmitters. The hormones do, however, keep things from changing. The neurotransmitters set melanosome aggregation or dispersion, and the hormone concentrations maintain them.
Caveat: Some chromatophore activation was done outside of the animal, so it's ambiguous how big of a role some of these mechanisms play in the flatfish.
Best fishes,
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fletcher~
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fletcher~ 