The Skeptical Fishkeeper: Pigmentation

Author: Laura Muha

While visiting a friend the winter before last, I heard a familiar burbling sound coming from a dark corner of their family room. It was an aquarium containing a goldfish their children had won a few years earlier at the county fair.

The poor thing was several inches long and swimming in cramped circles in a barren 10-gallon tank. But what shocked me even more than the fish’s living conditions was its color—or rather, lack thereof. Instead of being the vibrant orange of the goldfish I keep in my backyard pond, this one was a sickly bleached-out color that reminded me of jaundice.

“It used to be orange,” my friend said when I asked her about it. “But we don’t have a light for that tank, and if you don’t keep a light on fish, they lose their color.”

Color Changes That Took My Breath Away

Fast-forward to this past summer. I am spending the afternoon with master aquarist Rosario LaCorte, and I’m standing in his fishroom going gaga over a tank of boesemani rainbowfish Melanotaenia boesemani. I’ve always thought my own quintet of boesemanis were beautiful, with their shimmering powder-blue heads and glowing golden tails, but the sunset-orange tails of LaCorte’s fish put mine to shame.

“Wow!” I gasped. “I’ve never seen boesemanis with color that deep!”

“They’re a particular strain you don’t see very often,” LaCorte told me, adding that mine were almost certainly the more common gold-tailed fish. “But diet is also a big factor.”

While I’ll obviously take the word of an internationally known aquarist like LaCorte over that of my non-hobbyist friend, the two encounters did get me thinking about the subject of coloration in fish. Because let’s face it, don’t we all secretly hope our fish could be so gorgeous that people’s jaws drop when they see them? And aren’t dazzling colors usually the key to that? If they weren’t, why would anyone bother to dye fish or painstakingly breed strains with eye-popping colors?

Still, I couldn’t help wondering how much influence we aquarists have over a fish’s coloration, assuming that we aren’t selectively breeding them for that (nor, I hope, injecting them with dye, a practice that most experienced aquarists agree is despicable). Is it possible, as my friend insisted, that keeping a fish in the dark will cause it to lose its color? And why do the colors of so many fish fade as they age? Is it a factor of the aging process itself or the cumulative result of something we’re doing—or not doing—as fishkeepers? What about the commercial foods that promise to crank up coloration? Do they help? Are there alternatives?

A Fish of a Different Color

Before delving into those questions, I decided it would be helpful to understand where a fish’s color comes from, so I placed a call to Dr. Hank Bart, an ichthyologist and director of Tulane University’s Museum of Natural History. He explained that a fish has two layers of skin: a thin transparent layer called the epidermis, and under it, a thicker layer called the dermis. The latter contains irregularly shaped cells called chromatophores, most of which consist of a core with many microtubules branching off of it.

Some of these chromatophores produce melanin, which is a brown or black pigment; others store carotenoids, which are red, orange, and yellow pigments that fish obtain mostly from their diet; and still others, called iridophores, don’t have any pigment at all. Rather, they contain crystalline deposits most often composed of metabolic wastes such as purine. These structures act like little mirrors or prisms, reflecting and refracting light and giving the illusion of silver, white, blue, or green pigmentation. “So those colors are actually an optical illusion,” Bart said.

Nature or Nurture?

But the coloration of many species isn’t static, something that will come as no surprise to any of us who’ve carefully observed the inhabitants of our own tanks. Environmental triggers such as changes in light or water temperature or the approach of a predator can cause rapid changes in the color of many fish; so can their overall stress levels or mood (my male boesemanis, for instance, turn dark as ink when they’re courting or feuding).

Chemically Mediated Changes

These rapid changes are generally the result of chemically mediated changes within the fish—a surge of adrenaline, for instance, at the sight of a predator—that cause pigment to migrate within its chromatophores, Bart said. When the pigments spread out into the microtubules of the cell, a fish’s color grows more vivid, and when they migrate toward its core, the fish appears paler—a useful evolutionary mechanism, since if you can make yourself harder to see when a predator approaches, you’re less likely to be eaten. (Alternatively, if you’re a predator and can make yourself less visible, you’re more likely to be able to sneak up on your prey.) It’s a phenomenon I witness daily in my yellow tang, which becomes pale and mottled each evening when the tank lights go off, and brightens to a sunny yellow each morning when they go back on.

Growth Changes

However, a fish’s color can also change more gradually as a result of age and growth stage—how many juvenile fish have the same coloration as their parents?—not to mention diet and environment. For instance, my Neolamprologus multifasciatus shell dwellers from Lake Tanganyika were all the same pearly beige with tan stripes when I put them into my tank two years ago. But those that took up residence in the heap of granite at the rear of the tank (which also has a black backdrop) gradually turned a dark gray and their stripes became dark brown. Those living in light-colored snail shells on the sandy substrate have remained the same light beige.

Chromatophore Changes

Such morphological changes occur not from pigment migrating in and out of microtubules, but from an increase or decrease in the number of chromatophores a fish has, or the amount of pigment they contain. Not surprisingly, these changes in coloration tend to be more permanent.

Age

Some loss of color may happen naturally as a fish ages. Although the subject of aging in fish hasn’t been studied extensively, Bart said that the phenomenon is probably similar to the loss of hair color in humans as we age. “It’s really a consequence of a discontinuing production of melanin,” he said. “Since fish have the same pigment—melanin—and since they senesce [reach old age], just like any other animal, it’s probably true that their colors senesce and their patterns senesce.” If you look at a tank of fish in a public aquarium, he said, it’s easy to pick out the oldest ones, and not just because they tend to be the biggest or the ones with the most scars. “The [duller] color is the first thing you notice,” he said.

Let There Be Light

There’s one other factor that has a profound influence on a fish’s coloration, and that is light (although not necessarily in the way that my friend with the ailing goldfish meant).

While exposure to sunlight does stimulate melanin production in humans, it’s not quite so cause-and-effect in fish. “I don’t think that fish tan,” joked Dr. Stan Weitzman, a world-renowned ichthyologist and a research scientist emeritus at the Smithsonian Institution’s National Museum of Natural History.

Effects of Light Wavelengths

Different wavelengths of light do, however, bring out different colors in fish, he said. Take, for instance,the characin Tyttocharax. “In the wild when you see them, males are a brilliant blue,” said Weitzman. “But that’s a structural color dependent on how sunlight hits them. Put them in an aquarium and they’re just pale fish.” A similar phenomenon can be seen in some species of pupfish. “You look at them from above and they have a brilliant blue area on the back; you take them out of the water, you don’t see it at all,” Weitzman said, adding that bringing out a fish’s best colors in aquaria often means experimenting with different types of lighting.

That’s not to say, however, that the color of my friend’s fish was an entirely light-dependent optical illusion, Weitzman continued. Certainly, light does play some role in fish coloration; seasonal changes in daylight, for instance, trigger many species in the wild to come into their intensely colored breeding dress. And the scientific literature includes many reports of fish that faded in color after becoming blind (something that would seem to have some implications for fish kept in the dark). I also encountered a couple of references to early studies which found that the melanin-producing cells of goldfish kept in utter darkness were contracted and that their pigment was “finally reduced.”

Enlightenment

However, my friend’s fish wasn’t kept in utter darkness; it had room lighting available to it. And while I know it’s always dangerous to make assumptions, I’d be willing to bet—and Weitzman and Bart agreed—that its pale coloration was more a reflection of its overall stress level and state of health than it was the light level of its tank. After all, someone who crams a 3-inch goldfish into a 10-gallon tank and keeps it in the dark is unlikely to be paying much attention to its diet, or even the quality of the water it’s living in. (In fact, fading coloration in fish can also be a sign of illness—and sadly, my friend’s goldfish died soon after I saw it.)

That does, however, bring me to one area in which we fishkeepers can have a significant impact on a fish’s coloration: what we feed it.

They Are What They Eat

What a fish eats, obviously, is critical to its overall good health, just as what we humans eat is critical to ours. However, diet also contributes directly to a fish’s coloration in the form of carotenoids, fat-soluble pigments that fish get from food. “If you’ve ever boiled seafood, it’s what gives it that red color,” Bart explained.

When fish eat carotenoid-rich foods such as plants, algae, shellfish, and insects, they use some of these carotenoids for metabolic processes. “But they can also sequester it and use it as pigment,” said Bart. In other words, even though my friend’s fish was kept in dim lighting, if it had been eating a carotenoid-rich diet, some of that pigment would have been laid down in its cells and its color almost certainly would have been brighter.

That’s the principle behind commercial foods that promise to enhance a fish’s colors; they’re loaded with carotenoids, said Weitzman. And, he added, “They do work, to a certain extent.”

However, he said, you can’t just feed color-enhancing commercial foods and expect fish to show their best colors, for the same reason we humans can’t just pop vitamins to supplement a fast-food diet and expect to thrive. Natural foods contain many substances scientists haven’t even identified yet, but which probably contribute in some way to the vibrant colors we see in fish in their natural habitat (and to optimal health in humans).

“It’s very complex,” Weitzman said. “A lot of fish that we keep in aquaria, if you were to see them in the wild, you would just go wild because their color is so much brighter.”

Take, for instance, the Von Rio tetra Hyphessobrycon flammeus, which is also sometimes called the red or flame tetra. It’s a lovely reddish color in aquaria—at least when it’s not stressed—but in the fishes’ native habitat in Brazil, it’s another matter entirely. “I had the privilege of seeing one of the few populations in the wild, and there, the fish was blood red,” said Weitzman. “I had never seen that before.”

Recreating Natural Diet

Although it’s impossible to totally reproduce a fish’s natural diet in captivity, fishkeepers such as LaCorte, who feed lots and lots of live foods, come as close as is possible to mimicking it—and in the process, bring out remarkable coloration in their fish.

LaCorte says the key is not just feeding live foods, but feeding a wide variety of them, including daphnia, like those from his backyard pond built specially in order to cultivate them. That can be supplemented by many kinds of shrimp: newly hatched brine shrimp, mysis shrimp, even cocktail shrimp. “When I go to a Chinese restaurant, I take a bag home, not for me but for them,” LaCorte said with a chuckle, gesturing at his fish.

He stopped in front of a tank of Colombian red and blue tetras Hyphessobrycon columbianus. “When you feed them [correctly] you see the colors they get?” he said, pointing to the fishes’ glowing scarlet fins.

Staring at the fish in admiration, I could only agree.

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