 What
is the most beautifully colored fish in the
world? This is a question that can be
passionately debated for hours. Surely the
Harlequin tuskfish, mandarin fish, coral trout
(which despite its common name is really a
member of the family of seabasses), fairy
basslet, Catalina goby, lined fairy wrasse,
flasher wrasse, Scott’s fairy wrasse, regal
angelfish, ornate ghost pipefish and stoplight
parrotfish would all have their supporters. So
would dozens of other species. Perhaps the more
relevant questions are, “Why are some fishes so
colorful while others are rather drab,” and,
“How do fishes use color?”
Clearly, coloration in fishes is something that
gets our attention. But the questions posed here
ponder the issue of how various species of fishes
can possibly utilize their coloration, especially
considering the fact that at depth our eyes perceive
the underwater world as one that is illuminated by
predominantly blue light. We all learned in our
basic scuba class that the red part of the spectrum
is quickly filtered out in extremely shallow water,
with oranges and yellows following not too far
behind. At a depth of 100 feet/30 m, most fishes
appear to be blue fish swimming in a blue world
unless we “paint in the colors” by illuminating them
with a dive light, strobe or video lights, or use a
mask with a color-correcting filter. You can’t help
but wonder why so many fishes are so colorful if
their world is so blue.
What Colors Do Fishes See?
For many years it was believed that the eyes of
most fishes were very similar to human eyes, meaning
that fishes see the same part of the spectrum we do.
However, some specialists questioned the theory
based on the fact that the riotous colors of so many
fishes would be heavily muted at comparatively
shallow depths and in low light levels. Why, they
questioned, would any fish that lived below 50
feet/15 m, or even 30 feet/9 m, display brilliant
reds, oranges and yellows? And why would fishes like
tuskfish, mandarin fish and many other species so
active during dusk and dawn be so colorful if these
colors served no purpose? Why would frogfishes, sea
horses, flounders and so many other species alter
their coloration to match that of their immediate
surroundings if other marine creatures could not see
colors?
 These
specialists simply did not accept the premise that
fishes do not use their coloration in meaningful
ways. After all, some studies had shown that in many
cases in the competition for mates, the most
brightly colored fishes were the most successful.
Color seemed to be so significant in mate selection
that the results couldn’t possibly be coincidence.
It was clear that other members of their species as
well as other animals could see the colors.
Continuing studies soon demonstrated that the
eyes of many fishes are very sensitive to a large
part of the color spectrum, including ultraviolet
light, and that ultraviolet light penetrates far
deeper into the water column than previously
thought. In fact, ultraviolet light penetrates well
below 100 feet, and because of its comparatively
short wave- length, ultraviolet light tends to
scatter less than light waves from other parts of
the spectrum. While human eyes are not sensitive to
ultraviolet light, the presence of ultraviolet light
combined with the sensitivity of some fishes to some
other parts of the spectrum opened the door to
understanding much more about how fishes use color.
 Experiments
have shown that some fishes can locate and capture
prey when ultraviolet light is the only light
present, while other studies have demonstrated that
lemon sharks and some snappers can discriminate
between widely varying colors outside the
ultraviolet range. Lemon sharks have shown a
preference for yellows, while snappers are able to
distinguish between the same species of baitfish
that have been dyed different colors. In the snapper
experiment, some baitfish were dyed red and treated
with foul-tasting chemicals before being presented
to the snappers. The dyed bait repelled the
snappers, while they feasted on untreated bait of
the same species. For a long time afterward, the
snappers continued to avoid baitfish that had been
dyed red, even though the foul-tasting chemicals
were not present.
Color-Coded
Current thinking suggests that while color does
indeed play a prominent role in the lives of many
fish species, different species are thought to use
colors in a variety of ways. Color has proven to be
an important element in species recognition and mate
selection in many fishes. Some groupers and other
fishes alter their color as a signal that they are
ready and willing to spawn. On a reef inhabited by
dozens, or perhaps hundreds, of species, it would be
a big waste of energy to court any fish that swims
past. It is vitally important for members of a
species to be able to recognize their own kind, and
to be able to distinguish adults from juveniles and
males from females, so that potential mates can be
more easily recognized.
In many species, such as Anthias, sheephead and
parrotfishes, there are clear color differences
between adults and juveniles, as well as between
males and females. However, this is not always the
case with all species. In garibaldi, blue sharks and
many other species, color alone is not enough to
distinguish males and females, though the
spectacular blue spots found on the orange body of
juvenile garibaldi do enable even casual observers
to recognize juveniles and adults. Other factors,
from size and shape to deep singing voice to
nest-building skills, can play important roles in
mate selection.
Color as Camouflage
Not all fishes use color as a means of standing
out. Many species use color in an effort to
camouflage themselves. Flatfishes such as peacock
flounders and turbot are masters of camouflage. When
resting on the sea floor, they alter the coloration
and pattern of their skin to match that of the
surrounding bottom. As is the case with sea horses,
trumpetfishes, frogfishes, clingfishes,
scorpionfishes and many other species, flatfishes
alter their color by expanding or contracting sacs
of pigments and different kinds of cells in their
skin. In controlled tests, flatfishes have
demonstrated a remarkable ability to match complex
patterns such as checkerboards and polka dots in a
wide array of colors. Of course, shape also plays an
important role in camouflage.
 Disruptive
coloration is the use of color to break up the shape
of an animal so that its form or outline is more
difficult to see. Some scientists suspect that
fishes ranging from striped emperor angelfish to
polka dot-covered whale sharks use disruptive
coloration to make it difficult for potential
predators and prey to perceive them as an individual
animal. Upon first consideration, this theory can
seem to be a bit of a stretch, but if you spend a
dive looking at many coral reef fishes as they swim
in, over, through and around a reef, it seems a lot
more plausible.
Vertical bands and horizontally oriented stripes
are the tools that are most commonly used in
disruptive coloration. Many specialists suggest that
stripes are most effective for open-ocean fishes,
while bands serve as a better disguise for resting
fishes. Some fishes change their color pattern
between stripes and bands depending upon whether
they are swimming or resting.
Some sort of facial “mask” or stripe that
de-emphasizes or breaks up the face and hides the
eyes is usually another important element in
disruptive coloration. A dark stripe through the
eyes of many fishes is thought to make it more
difficult for potential predators to distinguish
their head from their tail, and thus more difficult
to anticipate the direction of an intended escape.
Add a false eye spot near the tail, as is the case
with the four-eye butterflyfish, and you have a
design that has proven to be very successful over
time.
Still other fishes, such as a variety of nearly
translucent gobies and many larval fishes, are
believed to use the absence, or near absence, of
color to help camouflage them. These fishes are
difficult to separate from their background, because
in many instances you can see their background
through their bodies.
Many groupers, barracudas and a variety of
others are known to use color as part of their
display to advertise to potential cleaner shrimp and
fishes that they would like to be serviced. When
being cleaned, these fishes often blanch or darken
their skin to increase the contrast in color between
themselves and pesky ectoparasites that they want
the cleaners to remove. When they have had enough,
the hosts (the animal being cleaned) will sometimes
alter their color to communicate that they desire no
further cleaning at that time.
But as is so often the case, there is a fly in
Mother Nature’s soup. Mimic cleaners such as
saber-toothed blennies appear very similar to some
cleaner wrasse. After carefully picking their
moment, the mimic cleaners surprise their hosts by
biting a chunk of flesh from their skin instead of
removing an unwanted parasite.
Warning Coloration
Color is also used by some fishes as a warning.
In some instances, fishes that are not particularly
gifted swimmers, such as lionfishes, are thought to
use their bright colors as a means of communicating
to other animals that it is best to stay away from
them and their painful spines. In essence, the
theory of warning coloration states that this
defensive coloration works best when advertised. It
isn’t as beneficial to be protected by sharp or
poisonous spines if potential predators are not made
aware of it until after they attack.
Caribbean barjacks usually display a silver
coloration when they gather in schools. However,
when individual barjacks associate with feeding
stingrays, they often alter their skin color to
black and become highly protective of the moving
“territory” around the feeding ray. It is believed
that the barjacks alter their color as a form of
intraspecific communication (communicating to
members of the same species) to tell other barjacks
to stay away from “me and my ray.” Feeding stingrays
often uncover prey items that are eagerly pursued by
the barjacks, and sharing does not seem to be part
of their nature.
Startle coloration is still another effective
use of color. Fishes like scorpionfishes and sea
robins (flying gurnards) are rather drably colored.
However, when threatened they unfurl large, brightly
colored pectoral fins in an attempt to startle
potential predators long enough to allow them to
make a quick escape.
In some instances, body pigments help to protect
the fragile internal organs of larval fishes and
shallow-water fishes from damage from sunlight. Skin
that is positioned over the brain tissue of many
larval fishes is often darkly hued, a characteristic
that offers protection from sunlight.
Countershading
 While
many reef fishes are celebrated for their striking
colors, many open-ocean fishes are rather blandly
colored by comparison. However, this does not mean
that coloration is unimportant in the lives of
open-ocean fishes. In fact, strong evidence supports
quite the opposite. Open-ocean fishes such as blue
sharks, mako sharks, oceanic whitetips and many
tunas utilize a color pattern known as
countershading to help them blend in with the
surrounding water, an adaptation that helps them
avoid potential predators and capture prey. In
general terms, these sharks and other countershaded
fishes are darkly colored on the upper portion of
their bodies and lightly colored underneath. This
type of coloration is referred to as countershading,
a form of camouflage in which the color tone of an
animal’s skin closely matches the tone of its
surroundings.
Camouflage is often thought of as a defensive
mechanism for prey that is used to avoid detection,
but it is also an important weapon for many
predators. Camouflage helps predators go undetected
by their prey. Even a slight delay in detection can
make the difference between a successful attack by a
predator and wasted effort.
Countershading enables many reef sharks,
including great white sharks and tiger sharks, to
blend into an environment that is a combination of
lightly hued surface waters and the darkly shaded
bottom of the sea floor. For example, if a
loggerhead turtle, a favorite prey item for a tiger
shark, is swimming above a tiger shark, the shark
has a better chance of going undetected because the
top of its body, the portion the turtle is looking
down on, blends in with the dark water background or
dark reef below. On the other hand, if the shark is
swimming above the turtle, the shark’s light
underbelly blends in with the light-colored surface
water.
It might be difficult to believe that a
15-foot-/5-m-long, 2,500-pound/1,130-kg great white
shark needs to rely on camouflage as part of its
arsenal. However, what is even more amazing is how
effective the countershading really is. From a
relaxed vantage point in a shark cage, I have
repeatedly seen great white sharks absolutely
“disappear” from view when only 30 to 40 feet/9 to
12 m away by blending into their surroundings, even
though water visibility is in excess of 100 feet.
Some comparatively small fishes also utilize
countershading. For example, surface-dwelling
species such as needlefishes and halfbeaks use their
silvery-blue coloration to blend in with the
underside of waves.
Bioluminescence, light produced by living
organisms, takes the place of color in some
cave-dwelling species and in the dark environment of
the deep sea. Perhaps more surprising is the fact
that a variety of shallow-water fishes such as
flashlight fishes, lanternfishes, some
cardinalfishes, pineapple fishes, ponyfishes and
bullseyes possess a light-emitting organ, usually
located on their head. In some cases this organ is
associated with the fish’s digestive tract, but it
is usually a colony of luminescent bacteria that
produces the light. Bioluminescence is thought to
serve a variety of purposes, from breaking up the
silhouette of potential prey to mate recognition to
attracting potential prey.
A Curious Rainbow
As convenient as it might be to make the
definitive statement that “all fishes use coloration
in this way or that way for these specific
purposes,” it is easy to see that not all species
use colors in the same way or for the same reasons.
However, as scientists continue to pursue the
question of how various fishes use color, it becomes
more and more obvious that body color is an
important element in the underwater world.
For More Information
Watching Fishes by Roberta and James Wilson
(1992, Pisces Books) includes a chapter on color;
the text is geared to the casual fish-watcher.
A good scientific reference is The Ecology of
Fishes on Coral Reefs by William McFarland (1991,
Academic Press). Chapter 5, authored by Peter Sale,
is titled “The Visual World of Coral Reef Fishes.”
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