Why Aquarium Fish are Different Colors

Tropical and marine fish colors brighten up our tanks — and they also serve specific purposes in our fish’s wild habitats.

Whether a beginning aquarist or highly educated expert, some of the first things we notice about a fish are color and pattern. Beginners often select fish solely on the “pretty factor.” Is the fish attractive? Will it look nice in my tank and with the other fish? In this article, you will learn the purposes and uses of colors and patterns in fish.

When we think of colorful fish, we probably think of the many colorful shallow-water species, such as the copperband butterflyfish (Chelmon rostratus) or the royal gramma (Gramma loreto) of the saltwater reefs, but some of the freshwater species can be just as colorful. The lemon cichlid (Neolamprologus leleupi) blazes brilliant yellow-orange; the hump-head cichlid (Cyrtocara moorii) of Lake Malawi is a variable blue. The male redtail notho killifish (Nothobranchius guentheri), with its pale blue scales edged in red, its yellow and red fins, and a brilliant red tail is almost gaudy (unfortunately, the species’ females can best be described as “dull”).

Defensive Coloration
Fish colors and patterns are more than just “pretty.” They serve important functions in intra- and interspecies communication. Color and pattern in fish may announce or warn, identify species or gender, and serve active roles in both offense and defense. The survival of an individual fish depends on its ability to find sufficient food and avoid predators. Survival of the species depends on the individual’s ability to find a mate and to successfully reproduce.

The physical and behavioral characteristics of an individual fish are the result of a long sequence of minor random changes (mutations) inherited from its ancestors. Small changes that lead to greater hardiness and survivability for the species often are passed along to succeeding generations. Detrimental mutations are often weeded out early by natural selection or predation.

Camouflage. One of a fish’s best defenses is the use of color and pattern that make the fish more difficult to see or to make a predator mistake it for something else. Camouflage can take many forms, but perhaps the most commonly observed pattern is countershading. (For more on this form of piscine camouflage, see the “Countershading” sidebar.)

Many bottom-dwelling fish display demersal coloration, or patterns of mottled spots and splotches that mimic or resemble the bottom substrate, such as stones, shells and algae. Consider the many variations of dots and splotches among the Synodontis catfish species or the muddy colors of North America’s Ameiurus bullheads. Banjo catfishes (Bunocephalus spp.), with their wide, lumpy bodies and slender tails, go a step further; one could easily be mistaken for a clump of stones or a chunk of rotting wood.

Vegetal coloration helps a fish blend into the light and dark patterns of aquatic vegetation and shadows. The walleye (Sander vitreus), beloved of sports anglers, provides a good example of this, with its variable green-patterned back and yellowish belly. All-silver coloration, as displayed by the silver dollar (Metynnis mola) of South America, reflects the fish’s surroundings — are you seeing the fish itself, the plants or reflections? And the Amazon leaf-fish (Monocirrhus polyacanthus) perfectly imitates a single dead, floating leaf. In its native kelp beds, the leafy sea dragon (Phycodurus eques), with its profusion of decorative “appendages,” looks like nothing more than a loose cluster of drifting plant bits.

Some fish aren’t above transparency as a strategy. The fry of many species are all but invisible at hatching, only developing pigmentation as they grow. Pseudepiplatys annulatus fry are tiny and transparent — nearly impossible to see but for the shine of their eyes. Indian glassfish (Parambassis ranga) and glass catfish (Kryptopterus bicirrhis) remain all but transparent even as adults; their organs and skeleton are clearly visible through their flesh.

Confusion, deception or distraction. Color and pattern can also serve to confuse, deceive or distract. Schooling and shoaling species are often visually alike. This can confuse potential predators, many of which build visual “search patterns” based on prey appearance. If all the fish in a school look and move as one, it’s not only harder for a predator to single out a specific individual, it may be harder for it to even perceive just one fish; it may actually “see” a single very large fish. Think the strategy doesn’t work? Just watch a large, moving, shifting school of like-colored fish, such as neon (Paracheirodon innesi) or cardinal tetras (P. axelrodi), or a tank full of lemon tetras (Hyphessobrycon pulchripinnis). Which individual did you say you wanted?

Some of the better-known examples of deception occur among the saltwater butterflyfishes, which often display several camouflage techniques at once: overall brilliant color, misleading patterns, and various combinations of false eye spots (ocelli) and dark stripes or patches that “mask” the eye. A good example of this is the longnose butterflyfish (Forcipiger flavissimus). Its black head marks and blue chin hide the eye; a false eyespot on both sides of the anterior anal fin and a black tail make the fish look as though it’s swimming in the opposite direction.

Disruptive coloration actually draws attention to an individual while visually breaking up its outline, thus startling or confusing predators. The bluegirdled angelfish (Pomacanthus navarchus) and others of the family Pomacanthidae are good examples of this. The much smaller (and plainer) characins of South America tend to have unremarkable silvery body coloration, but many display just a touch of an attractive bright color or pattern. The dawn tetra (Aphyocharax paraguayensis) displays a single black blotch bracketed by two small white marks at the base of its tail — colors echoed in its anal fin. The rummynose tetra (Hemigrammus bleheri) startles viewers with a blushing red face and head.

Stripes of any kind help to visually break up a fish’s outline, making its shape harder to discern, particularly in visually “busy” areas, such as dense vegetation. Think of the easy-to-see finescale tigerfish (Datnioides microlepis) or the perennial aquarium favorite, the freshwater angelfish (Pterophyllum scalare). Vertical stripes render them nearly invisible among swaying plants and dangling roots.

Horizontal stripes also can fool the eye. The penguinfishes (Thayeria boehlkei and T. obliqua) have bold stripes that begin at the snout and run through the eyes to the end of the caudal fin. The pencilfishes (Nannobrycon spp.) take this technique a step further. By maintaining an angled position in the water they actually appear to be facing down.

Mimicry. More than one species has learned to imitate (or mimic) the color and attributes of its background or of another fish. Not all fish that look like a given species actually are. The bluestriped fangblenny (Plagiotremus rhinorhynchos), which feeds on the skin, mucus and scales of other fish, has learned to use aggressive mimicry to provide it easier access to prey. The fangblenny’s normal ruddy brown base color with two distinctive blue horizontal stripes is designed for camouflage. When hunting, it will actually change colors to closely mimic those of the cleaner wrasses. In Batesian mimicry, one species actually copies the colors and patterns of another species that is distasteful or venomous.

Warning. Color can also be used as warning. Many fish having dangerous venom, venomous spines or other chemical defenses use bright colors or patterns as a warning of their toxins. The volitans lionfish (Pterois volitans) uses its bold, vertical striping and long, filamentous fins to warn of its venom-filled spines.

Although not toxic, the bright orange color of the Garibaldi damselfish (Hypsypops rubicundus) also serves as a warning signal, alerting others that it will vigorously defend its territory. Males (and some females) of many other species also use bright color and aggressive stances to defend their territory, nests and fry.

Identification and Sex
Since all fish species have their own “typical” patterns and colors, they (and we) can often use color and pattern to identify species, sexes, breeding readiness and sometimes stages of growth.

Species recognition. Color, pattern, and overall body and fin forms usually serve as clear species markers among fish. Neither fish nor aquarist is likely to confuse the reed tetra of South America (Hyphessobrycon elachys) with the Peruvian or blue loreto tetra (H. peruvianus). Although similar, they are distinctly different. Now compare H. elachys, H. luetkenii and Psellogrammus kennedyi, which display similar markings. Proper identification here is a bit more difficult.

Sexual dimorphism. In true sexual dimorphism (“di” meaning two, “morph” meaning form), the two sexes of the same species appear distinctly different. Clues may include size, body shape, coloration, the presence (or lack) of external reproductive organs or other gender-specific characteristics.

For example, there’s little question of gender when it comes to the common guppy (Poecilia reticulata). Males are always smaller and display much brighter colors and patterns. Females are larger and fairly plain, though in some fancy varieties, they may show some color in the fins. Sexing most of the other livebearing fish is also relatively easy, since the anal fin of a mature male becomes modified into a gonopodium or similar organ used to transfer sperm to the female.

Male and female golden mbunas (Melanochromis auratus) look like different species. The genders are identically marked — solid-colored bodies with two narrow horizontal stripes on each side — but females are golden yellow with dark stripes, while males are brownish-black with pale stripes.

Sexual signaling, hierarchy and spawning readiness. Some species use color to signal dominance or sexual receptivity. Three-spined sticklebacks (Gasterosteus aculeatus) are fairly unpretentious little silver fish, but males in breeding condition display a mosaic pattern, a red throat, blue iris, and high-contrast dark green and tan stripes on the flanks. Sexually receptive females develop a similar barlike pattern.

Adult male guppies (P. reticulata) display a variety of distinct color patches and patterns. These patches and patterns use colors such as orange, red, yellow, black and brown, as well as highly reflective structural colors, such as blue, green and silver iridescence.
Reflectance testing on the bright colors displayed by these species revealed strong peaks in ultraviolet, indicating enhanced visual contrasts. Under controlled laboratory conditions, females also showed distinct preferences for the more colorful males, particularly when viewed under lighting that included UV.

A number of other species studied by researchers have also been found to possess UV sensitivity, including the goldfish (Carassius auratus), the mummichog (Fundulus heteroclitus), and several of the salminid species, including brown or sea trout (Salmo trutta) and Atlantic salmon (S. salar).

In colonies of fish, often only the dominant male will show much color. This behavior has been well-documented in colony setups of the African Nothobranchius annual killifishes. Subdued male coloration appears to be a protective strategy — if a subordinate male doesn’t look like he’s competing, he may manage to avoid attack and may even be able to sneak in to breed with a female.

Age. Most fish show little or no difference in color or pattern according to age. But in the species that display sexual dimorphism, males will develop color when they reach sexual maturity. Females and juveniles usually display a neutral coloration, but the males “color up.” Even then, the males may not change colors completely until they are ready to challenge a dominant male.

But in some species color patterns change markedly from juvenile to adult. Young emperor angelfish (Pomacanthus imperator) are dark blue with a pattern of concentric white or pale blue lines focused on either side of the caudal peduncle. In adulthood, the fish become purple-brown to dark blue-green, and they develop black patches edged with blue across the forehead and cheeks. A series of yellow stripes on the flanks and a bright yellow tail complete their attire.

As aquarists, we can selectively breed our fish for characteristics that we deem of value, in the process developing many new forms and color patterns. As desirable as many of these forms may be, we need to remember that fish form and color originally developed to address very real survival needs.

D.M. (Donna) Recktenwalt has successfully bred a number of fish species. She has also authored numerous articles that have appeared in FAMA magazine, on the web and in various newsletters.

Article Categories:
Fish · Freshwater Fish