Why are so many sharks dark on top and light on the bottom?
Most animals that live up in the water have dark tops and light bottoms. This is true not just of sharks and rays but also of dolphins, killer whales, penguins, and even crocodiles. The coloration is known as “countershading.” Countershading is adaptive because it makes the animal blend into the open-water background against which it is seen by predators and prey alike. Blue Sharks and makos, which are open-ocean species, are also countershaded but tend to a dark bluish back color that is probably a match to the indigo blue background of the open seas.
A countershaded shark usually has a dark back that grades to a light belly, although in many sharks the transition from dark to light is abrupt. Regardless, the principle in operation is that the shark reflects or absorbs light that makes it blend into the background because dark colors absorb incoming light and light colors reflect it. A countershaded shark disappears because its color is opposite to the distribution of light in water, creating a target that is identical to background. The fish therefore reflects or absorbs light that is roughly equivalent to the background against which it is seen at all viewing angles, dark against dark, light against light, intermediate against intermediate.
Light coming from above is called downwelling, from the side sidewelling or spacelight, and from below upwelling. Countershading is easiest to understand when viewing a shark from above. A shark with a dark back absorbs very bright downwelling light, creating a dark target against the dark background of dim upwelling light. However, most fishes are viewed by their predators or prey from the side, and countershading works equally well then. When seen from slightly above the horizontal, the dark upper side of the shark absorbs relatively bright, mostly downwelling light, creating a dark target that is seen against the darker background of slightly upwelling light. Similarly, if viewed from slightly below the horizontal, a lighter-colored upper belly surface reflects weak upwelling light, creating a relatively bright target seen against the lighter background of slightly downwelling light.
Most sharks are countershaded: darker on top, gradually lighter on their sides, and brightest on their bellies. This color pattern makes them disappear in the water column because their colors reflect light that matches natural background light. (A) A uniformly gray shark illuminated from above, as in natural lighting, would have a relatively bright back and a relatively dark belly. (B) A countershaded shark illuminated from the side, as in flash photography, stands out against the natural light behind it. (C) In a countershaded shark under natural lighting, the gradual transition from dark back to light belly has an averaging or cancelling effect. The top of the shark is seen as dark against dark, the middle as intermediate brightness against intermediate, and the light belly as light against light. All shark color-background combinations eliminate contrast between the shark and its background. Helfman et al. (2009); used with permission of Wiley-Blackwell
Countershading makes even more sense when one realizes how conspicuous a uniformly colored shark would be. A uniformly light-colored shark viewed from above would reflect rather than absorb downwelling light and become a bright target against a dark background. Similarly, a uniformly dark shark viewed from below the horizontal would absorb weak upwelling light and appear as a dark object against a relatively bright background. Countershading is the best of all possible coloration worlds. With one minor exception. When viewed from directly below, any shark, regardless of color, is clearly visible as a dark silhouette against the background of bright, downwelling light.
Which sharks aren’t countershaded?
Sharks that spend most of their time on or very near the bottom (wobbegong carpetsharks, scyliorhinid catsharks and shysharks, swell sharks) may or may not be countershaded. Regardless, their backs, instead of being a solid dark color, are a mottled gray or brown, often arranged as lighter regions between dark saddles. In some sharks, such as triakid Leopard Sharks and scyliorhinid shysharks, the saddles and blotches wrap around almost the entire body before grading to a lighter belly. Such blotchy coloration is similar to that of benthic (bottom-living) bony fishes such as scorpion-fishes, flatfishes, and sculpins. This color pattern most likely makes the animals blend into the mottled bottom against which they are seen from above.
A Big Skate, the largest skate in North American waters. The round “bull’s eyes” on the upper surface of the pectoral fins occur on many skates, but their function remains a mystery. Photo from Wikimedia Commons, www.afsc.noaa.gov/race/media/photo_gallery/fish_files/Big_skate.htm
Skates are generally countershaded, but their white bellies are almost always pressed against the bottom, unseen. In some skates, a pair of distinctive round bull’s-eyes decorate the exposed top side; among these are the Eyespot Skate, Atlantoraja cyclophora; the Big Skate, Raja binoculata; the Starry Skate, R. stellulata; and the Longnose Skate, R. rhina. (See also the description of Epaulette Sharks in the next question.) How these “eyespots” function is a mystery. In many animals (toads, butterflyfishes, pufferfishes, Oscar cichlids, moths), eyespots on backs, fins, and wings are thought to startle or perhaps fool predators into thinking they are encountering a much larger animal. Alternatively, eyespots may invite an attack, deflecting it away from more vulnerable body areas, such as the head. Whether skate coloration serves a similar function is unknown.
Deviations from countershading in open-water sharks usually function to make a small part of the shark visible when first detected. These colored body parts probably separate one species from another, helping sharks tell each other apart, a useful feature during mating. These seemingly minor patches of color are also helpful to people who are trying to tell sharks apart. This is the case with the Porbeagle (Lamna nasus). Porbeagles differ from all other lamnids because they have a distinctive white patch in the lower rear portion of their first dorsal fin.
These variations in coloration are indicated by the common names of many carcharhinid sharks: Blacktip Reef Shark, Blacknose Shark, Silver-tip Shark (Carcharhinus albimarginatus), Whitetip Reef Shark, Whitenose Shark (Nasolamia velox). It appears that such color is meant to be obvious. The black tip on the dorsal fin of a Blacktip Reef Shark often has a white lower edge, which makes the black tip stand out even more. The White-nose Shark’s nose has a black tip with a white border behind the black, which makes it a blacknose shark, but that common name was already taken.
Tiger Sharks, with their noticeable vertical barring (a color pattern that fades with age) repeats a pattern seen in large bony fishes that often swim near the surface, such as King Mackerel, Wahoo, tuna, and marlins. The vertical barring is thought to mimic the flickering shafts of sunlight created by ripples at the water’s surface, breaking up the outline of the fish and making it harder to identify. This type of camouflage might also explain the variably shaped white spots and lines on the backs of Whale Sharks.
Even more speculative are suggestions about the function of light color on the fin tips of Oceanic Whitetips. The ends of the first dorsal, pectorals, pelvics, and both lobes of the tail are distinctly white in this oceanic species, which feeds on fast-moving offshore fishes. The idea is that an otherwise countershaded shark approaching from a distance in open water is almost invisible, except for the white tips. The tips are made even more visible by side-to-side tail movements and adjustments of the pectorals. A potential victim could be fooled into thinking the obvious white marks were something small and maybe even edible, allowing the Oceanic Whitetip to get close enough to put on a burst of speed, when it was too late for the prey to escape. If you watch videos of these sharks, or at least videos that include footage as the shark approaches from out of the gloom, this idea makes sense.
Finally, deepwater sharks—those that live well below the depth of sunlight or moonlight penetration—tend to be uniformly dark in color. The Portuguese Dogfish, the deepest living shark, is uniformly blue-black or brown-black. This lack of patterning makes sense in terms of the assumed function of countershading and its link to light distribution nearer the surface: bright downwelling, intermediate spacelight, weak upwelling. In the inky-black depths, light is equally absent in all directions. This makes countershading, or any variation in color across the body, pointless.
Once again, known exceptions generally prove the rule. Some sharks that live at intermediate depths and that move up and down day and night have dark backs while their lower surfaces are spotted with light organs called photophores. Spined Pygmy Sharks, diminutive dalatiids (28 cm, or 11 in, in length), occur around 500 m (1,640 ft) by day but move up to 200-m (660-ft) depths at night. They have numerous small photophores on their bellies right up to their snout. It is thought that these light organs put out just enough light of the correct wavelength to match weak downwelling light, thus obscuring their profile when viewed from below by their predators. This so-called “ventral bioluminescence” also characterizes many bony fishes, squids, and shrimplike animals that live at the same depths.
The Portuguese Dogfish is a deep-dwelling, 1-m-long (3-ft) somniosid shark with the uniform dark coloration typical of sharks that live in ocean depths beyond the reach of surface light. Photo courtesy of Maria Pickering
Are any sharks colorful?
Colorwise, sharks are pretty boring. Dominant colors are shades of gray and brown, some darker, some lighter. Colorful sharks that deviate from black, gray, and brown are definitely rare. Those that appear colorful— with spots, stripes, bars, or blotches—are almost entirely small bottom-living species that use their color patterns as camouflage, blending into the varied-color background on which they sit. Bold marks may break up the image of their body outline, making them unrecognizable as a shark. This kind of patterning, referred to as disruptive coloration, is common in many fishes, both predators and prey.
The squalomorphs, although taxonomically diverse, contain few sharks that could be considered colorful in any sense of the word. Some angel sharks (Squatinidae) have rings and spots on their bodies and pectoral fins. Angel sharks live on or even buried in sandy bottoms, and the spots probably serve (somehow) as camouflage.
Most galeomorphs are countershaded or unimaginatively colored, but a few benthic (bottom-dwelling) families could be considered exceptions. Heterodontiform bullhead sharks, especially the juveniles, sport bold markings. Members of the genus Heterodontus, such as the Zebra Bullhead Shark (Heterodontus zebra—not to be confused with the even more striking stegostomatid Zebra Shark, Stegostoma fasciatum), have vertical bars, wavy lines, and numerous white or black spots. The skin coloration of orectolobid wobbegongs could serve as models for Realtree camouflage patterns. Wobbegongs sit motionless on algae-covered reef surfaces, and their color and head frills easily blend into those complex backgrounds. The hemi-scylliid carpetsharks and bamboosharks are among the most colorful of all sharks, although their spots, blotches, and bars are still brown, black, or grey, though often set off with white edges. The tropical Pacific Epaulette Shark has a beige to brown back covered by brown spots and some darker bands. It gets its name from a very large black spot that is ringed by white on each “shoulder,” behind the pectoral fins. (Epaulettes are shoulder bars on a soldier’s uniform—and the red shoulder patches of Redwing Blackbirds.) The conspicuous ringed epaulette may draw a predator’s attention away from vulnerable body parts, or it might be mistaken for a large eye that belongs to a much larger organism, one that shouldn’t be messed with. We don’t really know.
Also within the Galeomorphi is the strikingly marked Zebra Shark, juveniles of which are popular aquarium animals because of their pattern of brownish-black rings on a white background. These sharks outgrow even the largest home tank, becoming leopard-spotted adults 2.5 m (8 ft) long. Scyliorhinid catsharks can be split into two major groups as far as color is concerned. Many are uniformly colored deepwater animals (living at about 1,000 m, or 3,300 feet). The shallow-water catsharks are another matter altogether, with common names that reflect their coloration (Australian Marbled Catshark, Orange Spotted Catshark, Redspotted Catshark, Tiger Catshark, Pajama Shark). Many have multiple dark circles lined in white running the length of their body. Again, these are almost entirely bottom-sitters.
Other galeomorphs with variable coloration or shading are the Harlequin Catshark (Ctenacis fehlmanni), with orange-brown blotches and spots; the aptly named Leopard Shark of California kelpbeds; and the puzzling Atlantic Weasel Shark (Paragaelus pectoralis), which has a series of striking yellow bands running from its snout to its tail. The Atlantic Weasel Shark may be the only actively swimming shark that can be considered truly colorful, and its coloration sets it apart from all the bottom dwellers that are clearly camouflaged by their color patterns. Longitudinal (lengthwise) stripes, including yellow stripes, occur in a number of actively swimming bony fishes with relatively long bodies, such as snappers, rainbow runners, goatfishes, and sweetlips, but how this color helps these fishes remains a matter of debate.
Perhaps strangest of all is the Goblin Shark (Mitsukurina owstoni), with its long, daggerlike snout, protrusible jaws, and slender, fanglike teeth. This deepwater (around 500 m) rare lamniform has a body that is soft pink to purple-gray, with blue-edged fins. Its color is due neither to pigments nor structural iridescence but to blood vessels that lie close beneath the skin, which is itself fairly transparent.
Rajid skates as a group are plainly colored, except for those with paired bull’s eyes on their pectoral fins and backs. Many other skates, including the Spotback Skate, Atlantoraja castelnaui, have small black spots. Others, such as the Undulate Ray (Raja undulata) and the Sydney Skate (Dipturus australis), sport wavy lines and white spots reminiscent of Australian Aboriginal bark paintings, which may have been inspired by these batoids or by the color patterns on a Whale Shark.
Colorful stingrays are also the exception. Interestingly, several of these have blue spots and stripes. Blue is a pigment almost entirely absent from the selachian sharks, except for some open-water species like Blue Sharks and makos, whose countershading includes deep-blue backs. The Blue-spotted Stingray (Dasyatis kuhlii) of the tropical Pacific has blue spots on a greenish background. It is also one of the few rays thought to be able to see colors (see “Can sharks see color?” in chapter 2). The Bluespotted Ribbon-tail Ray (Taeniura lymma), also a tropical western Pacific reef species, has electric blue spots on a yellow background, with two blue stripes running the length of the tail. Some of the most colorful rays are the potamotrygonid freshwater stingrays of the Amazon Basin. The backs of these animals are spotted and streaked, often in (again) bright blue. Freshwater stingrays have powerful venom in their tail barbs, tempting an interpretation that their bright, contrasting colors serve to warn potential predators of these effective defenses.
One species of electric ray, the Ornate Sleeper Ray (Electrolux addisoni) of South Africa has a dark brown back with small yellow spots and curving, concentric black stripes. When it is approached by a diver, the ray arches its back, curls its disk, and raises its tail in what might be a threat display. Bold, obvious “warning” coloration and conspicuous body postures occur in a wide variety of animals with toxic properties, from wasps to caterpillars, poison dart frogs, and skunks. Electrolux addisoni also holds the distinction of probably being the only elasmobranch (or any animal?) whose genus name honors a brand of vacuum cleaner and reflects both its electrical traits and its suction-feeding behavior.
The large, graceful Spotted Eagle Ray has distinctive white, bluish-white, greenish, pearly, or yellow spots on a black background across its back, including on its pelvic fins and the base of its tail. The spotting may continue onto the outer margins on the undersides of the wings. Individuals can apparently be told apart by their spot patterns. (Researchers also recognize manta ray individuals by the black spots on their otherwise white bellies.) Unlike most other stingrays, Spotted Eagle Rays spend little time resting on the bottom and much time “flying” around a reef.
Spotted Eagle Rays are among the few rays that occur in tropical Pacific, Atlantic, and Indian Ocean waters. Often seen in small groups in relatively shallow water, these graceful, active rays are a favorite among divers. Photo by John Norton, Wikimedia Commons, http://en.wikipedia.org/wiki/File:Spotted_Eagle_Ray_(Aetobatus_narinari)2.jpg
Several chimaeras are relatively colorful, with deep reds, purples, and blues, colors seldom found in elasmobranchs. Their names reflect these color patterns: the Giant Purple Chimaera (Chimaera lignaria), the Purple Chimaera (Hydrolagus purpurescens), and the Pointy-nosed Blue Chimaera. How this coloration might interact with habitat and color vision remains a mystery.
Color usually comes from pigments or structures that reflect light of particular wavelengths (see “What causes the different colors of sharks?” below). But at least six different evolutionary lineages of deep-sea squaloid sharks have light organs on their sides that emit a greenish-yellow light, often in patterns specific to a species. Examples include the Velvet Belly Lantern Shark (Etmopterus spinax) and the dalatiid Cookiecutter sharks, the latter discussed in chapter 2 with respect to teeth and in chapter 7 on feeding habits. Cookiecutters can be considered micropredators or megapara-sites, depending on your thinking. They live at middle ocean depths, where many squid, shrimp, and deep-sea fishes also have light organs. Tunas, dolphins, sperm whales, Megamouth Sharks, White Sharks, and other predators descend into this cloud of illuminated prey to feed. Cookiecutters are thought to hide among these other light producers, blending into the background of flickering bioluminescence, then sneaking up on the larger predators and removing plugs of flesh.
The Megamouth Shark, one of the victims of Cookiecutters, was at first considered a light emitter, producing a glow from its upper lip region that might attract plankton prey. This idea is unproven, either by observation of healthy live animals (unlikely) or by histological (microscopic) examination of mouth tissues, yet to be done. It is more likely that the distinctive white band of tissue along the upper jaw reflects dim light at midwater depths. The glow could attract prey animals, or it could be a species identification tag used by other Megamouths. In truth, we don’t know the function of the unique white oral band in this species.
Are any sharks colorless?
Albinism occurs in animals because of a malfunction in the gene responsible for producing melanin pigment. A true albino shark is almost white (in underwater photos albinos appear to glow) and has pink eyes. Such specimens are rare and most often involve embryos or newborns. Few true albinos live long because they lack the coloration necessary to protect them from a variety of large, visually hunting predators. The rarity also means that just about anytime someone finds an albino shark, or even a partial albino, the discovery gets published in a scientific journal.
Albino embryos (often removed from a dead shark’s uterus) or very young albino individuals are known in the Gray Smoothhound Shark (Mustelus californicus), the Whitespotted Bamboo Shark, the Leopard Shark, the Sandbar Shark, the Tiger Shark, the White-Spotted Marble Electric Ray (Torpedo marmorata), and the Common Torpedo (Torpedo torpedo). Albino Brownbanded Bamboo Catsharks have been born to a captive, normally colored female at the Denver Downtown Aquarium. The fish collection at Rhodes University in Grahamstown, South Africa, has a real white shark, or rather a very young albino White Shark 1.7 m (5.6 ft) long. Most bizarre is a 22-inch-long (56-cm-long) albino Dusky Shark (Carcharhinus obscurus) embryo that was removed from a pregnant female by a fisherman in the Sea of Cortez, Mexico, in 2011. What made this find truly strange was that it was a cyclops. It lacked normal eyes but had one fully developed, forward pointing eye between its mouth and snout.
Full-grown albino sharks do occur and are even sometimes photographed in the wild. Species include a Narrownose Smooth-hound (Mustelus schmitti), an adult Tawny Nurse Shark (Nebrius ferrugineus), and a Halave’s Guitarfish (Rhinobatos halavi). An albinistic adult male Sand Tiger was filmed off the New South Wales coast of Australia in 2007, and the same month, a 10-m (33-ft) female albino Whale Shark was glimpsed and even videotaped near Darwin Island, Galapagos. That same year—a seemingly big year for albinos—a 12-inch-long (30-cm) juvenile female albino Spotted Ratfish was caught not far from Seattle, Washington. An adult male albino skate (mislabeled as a stingray in the posting) was captured by a fisherman in British waters in 2011. Albino Cownose Rays (Rhinoptera bonasus) show up often enough that SeaWorld Orlando has a live one on display. A few albino Short-Tail Stingrays (Dasyatis brevicaudata), perhaps the world’s largest stingray, have been seen and photographed. Some other photos appearing on the web include an albino Zebra Shark (size not given) photographed at East Wallabi Island, Western Australia. A presumed albino juvenile hammerhead shark in the Cebu Zoo, Philippines, turned out to be a pangasiid catfish.
Yes, Virginia, there really is a white shark. This 1.7-m-long (5.6-ft) albino White Shark was caught off Boknes in Eastern Cape Province, South Africa, in 1996. It is the only known albino White Shark and in fact the only known albino member of the family Lamnidae. Normally colored White Sharks get their name from their very white bellies. Photo by Gene Helfman
What causes the different colors of sharks?
Sharks have skin coloration of two types. Most common are skin colors due to pigments in the skin cells. Melanin pigment results in grades of black; guanine results in grades of white. The color-containing cells are called chromatophores (literally, “color bodies”), defined by the pigments they contain. For example, melanin is concentrated in color cells in the skin called melanophores. Color change occurs when the shark expands or contracts its chromatophores. When melanophores contract, those regions of the shark become lighter in color. Pigments are either produced by the shark itself or somehow transferred from colorful food organisms.
Less common are structural colors that cause iridescent (shiny, glowing) color. Iridescence results from light passing through and reflecting from thin surface tissue. Iridescent colors change depending on the direction of incoming light and the angle at which a shark is viewed.
The Smalleye Hammerhead (Sphyrna tudes) of eastern South America is sometimes called the Golden Hammerhead. Juveniles sport both structural and pigment coloration that makes them gold. Orange and yellow pigments in the skin cells add brightness to the color, which is also enhanced by iridescent structural tissues. The result is that live animals have glowing, metallic golden heads, sides, and fins that lose their glow upon death. Even more interesting, the yellow and gold pigments are not produced by the animal itself but apparently come from its diet, in particular, a kind of shrimp and a marine catfish and its eggs. The Yellow Smoothhound (Mustelus higmani) feeds on the same shrimp and is also yellowish. Horn Sharks in California are another example of shark “coloration” obtained from an external source. Horn sharks feed heavily on purple sea urchins and often have purplish-pink teeth and fin spines.
After death, the drying and shriveling of the skin alters the thin surface tissues. Hence, iridescent colors quickly disappear in dead sharks. Pigment-based colors fade more slowly as the pigment granules decay with time. The fairly uniform yellowish-brown color (with some darker and lighter areas) of sharks that have been dead for a while, such as those in museum displays, doesn’t do justice to their colors in life.
One interesting twist on color change occurs in deepwater sharks like the Velvet Belly Lantern Shark. Lantern sharks are uniformly dark but have small light organs (photophores) on their bellies (see “Which sharks aren’t countershaded?” above). Lantern Sharks change the amount of light they emit in much the same way that shallow-water sharks change their color by controlling melanin distribution—except that Lantern Sharks use melanin to control light emission. On top of each photophore is a closeable diaphragm filled with melanin that works like the iris diaphragm in our eyes. The light stays on all the time, but the Lantern Shark contracts and expands the melanin in the overlying diaphragm to allow light to shine out or not.
Is there a reason for the color patterns of sharks?
Most fishes, either predator or prey, are colored in a way that makes them difficult to see underwater. “Invisibility” is a good explanation for the dominant color patterns of sharks, whether countershaded or bottom-matching. Countershaded sharks blend into the background of the water column, and mottled-back sharks and rays are camouflaged against the background of the ocean floor. In benthic sharks, skin growths and folds increase the resemblance to the bottom, as in the orectolobid wobbegongs.
If sharks are so well camouflaged by countershading, you can reasonably ask why photographs of sharks and rays often make them seem quite visible. The answer is that photos are usually taken with artificial lighting distributed in unnatural ways, at least as far as the evolution of shark coloration is concerned. Underwater photographers usually use strobe lights, which shine or flash very bright light on the side of a shark and thus illuminate it unnaturally, and conspicuously. The distribution of light on a dock or boat deck also misrepresents the real visual world of sharks. We live in a terrestrial world where light is more evenly distributed above and to the sides, and is often brighter from the side than from above. Underwater light, by comparison, is predominantly (95%) downwelling, with only about 4% of light coming from the side. Countershaded sharks are colored to match this light distribution and thus become invisible (see “Why are so many sharks dark on top and light on the bottom?” above). Add to this the fact that photographers seldom take pictures of well-camouflaged sharks, the ones that you can’t see. Or at least those photos don’t make it into books.
What color are a shark’s eyes?
Eye color in sharks and all vertebrates is primarily a function of structural colors in the iris, the area surrounding the pupil. The iris of many sharks (Tiger, White, Porbeagle, makos) is large and black, or whitish (many carcharhinids), although a few species (Milk Shark, Rhizoprionodon acutus) have a yellow iris, and others (Leopard Shark) have a lighter, gray iris around a black pupil.
The pupil itself in most sharks is black and round (sixgill sharks, Blue Shark, White Shark, hammerheads, Sand Tiger), although some sharks have a vertical, slit-like pupil (Lemon Shark, Blacktip Reef Shark, Silky Shark), and in others the slit is horizontal or nearly so (some heterodontid bullhead sharks, squatinid angel sharks, Leopard Shark, Small-Spotted Catshark). Exceptions to black pupils are found in several deep-sea sharks that have large green pupils, such as centrophorid gulper sharks, etmopterid lanternsharks, and somniosid dogfish and sleeper sharks.
The “eye color” of some batoids is confusing because the pupil may be covered by a variously pigmented curtain called the operculum pupillare (literally “pupil cover”), an eyelid-like finger of tissue that expands over the eye in bright light conditions. Examples include the Banded Guitarfish (Zapteryx exasperata), many skates, sawfishes (Pristidae), and dasyatid stingrays. The highly reflective eyes of holocephalan chimaeras generally have a dark pupil surrounded by a lighter iris.
Do a shark’s colors change as it grows?
Some sharks change color as they grow, with younger sharks often but not always more colorful than adults. Color change can be subtle and may involve loss of bars or loss of color in the tips of fins. For example, the vertical barring that gives Tiger Sharks their name is much more obvious in younger animals. The Zebra Bullhead Shark has zebra stripes at all stages, but the stripes are reddish-brown in juveniles and become black to dark brown in adults. Among the more impressive color changers are the hemi-scyliid bamboo and carpetsharks, which are banded and spotted as juveniles but more uniform brown or only slightly banded as adults. For example, juvenile Epaulette Sharks have light and dark bands over their body and fins; these bands break up and become more spotlike later. All stages have the distinctive white-ringed epaulette on the shoulders. The most dramatic color changer may be the Zebra Shark, which changes enough with age that juveniles and adults were once thought to represent different species. Juveniles have distinctive light or yellow zebra stripes or bars against a dark background, whereas adults have dark spots on a light background.
An exception to the trend of decreased coloration is the Leopard Shark of California kelp beds, which is slightly more colorful as an adult. Adults have black saddles and large black spots along the back; juveniles have fewer spots and saddles.
Nurse Sharks darken as they grow. There is some evidence that female Nurse Sharks pay attention to color differences when choosing mates, at least to the extent that they avoid lighter-colored males during mating activities. The role played by absolute male size in these observations may complicate the issue.
Juvenile Zebra Sharks have a banded color pattern; adults are covered in spots. This fish is a large juvenile that is changing from one to the other and is intermediate in color, showing both bands and spots. From Wikimedia Commons, http://en.wikipedia.org/wiki/File:Stegostoma_fasciatum_01_by_Line1.JPG
Do a shark’s colors change at different times of the day or in different seasons?
Bony fishes, especially tropical marine and freshwater species, change color on a moment’s notice, or exhibit very different colors during the day than at night, being mostly blotchy at night regardless of their daytime hues. Background matching is well known in bony fishes, a classic example being a flounder’s ability to almost mimic a checkerboard when placed on that kind of surface.
Most sharks, however, remain the same basic color throughout the day, at night, and in different seasons. A few can change color slowly, perhaps to better match their background, becoming darker or lighter or more blotchy. Obviously, this is more useful among benthic sharks than in species that swim actively in the water. Parascylliid Collared Carpetsharks (Parascyllium collare) are reported to change their skin color to better match seafloor coloration. Juvenile Nurse Sharks held in tanks lightened in skin color over a period of minutes when the tank they were kept in was covered and darkened. Little Skates held in illuminated tanks with white or black walls also changed color, becoming light brown in white tanks and dark brown in dark tanks. The color change took on the order of 9 to 12 hours, much longer than in color-changing flounders and other bony fishes. Spiny Dogfish also get lighter when kept in white-walled tanks, but the color change takes even longer, about two days. Physiological studies in skates and bottom-living sharks such as triakid smoothhounds and Small-spotted Catsharks showed that the animals sensed outside light using their eyes and the so-called third eye or pineal organ on the top of their heads. Skin color changes were due to a melanophore-stimulating hormone secreted by the pituitary gland that caused melanophores in the skin to expand or contract.
Is there much geographic variation in the color of a shark species?
Geographic variation in color occurs in sharks, although many species that were thought to be geographic variants turn out to be different species. Our increase in understanding comes from additional study, including improved genetic techniques that reveal differences that weren’t obvious from external anatomy alone. For example, the Puffadder Shyshark of South Africa was thought to come in two forms, Cape and Natal, that differed mainly in the color of their dark saddles and white spotting. A study of mitochondrial DNA genes showed that these are in fact two species, H. edwardsii and the Natal Shyshark, H. kistnasamyi.
Just the opposite applies to another scyliorhinid, the Saddled Swellshark (Cephaloscyllium variegatum) of the east coast of Australia. When the discoverers originally described this shark, they thought it was part of a three-species complex, with the species differing in their saddles and spots. But the coloration followed a latitude trend from north to south, with darker saddles in tropical individuals than in temperate animals. C. variegatum is now recognized as one species. Its species name, variegatum, refers to this within-species color variation.
Porbeagles are high-latitude lamnids that seldom enter tropical waters. Their coloration differs in northern and southern areas, evidence that little genetic exchange occurs between populations. Porbeagles in the Northern Hemisphere are mostly white under their head and abdomen, whereas the undersides of the heads of Southern Hemisphere adults may be dark, with dark blotches on an otherwise white abdomen.
