While not all sharks are blue, several recognizable species exhibit profound blue coloration. This hue is not random but a highly refined form of natural camouflage. This evolutionary adaptation allows these oceanic predators to hunt and survive in the vast, exposed environment of the open sea. The coloration involves unique cellular structures and the physics of light rather than simple pigment alone.
Which Sharks Exhibit Blue Coloration?
The most prominent example is the aptly named Blue Shark, Prionace glauca, which possesses a striking indigo blue on its back that transitions to a vibrant sapphire along its flanks. This species is one of the most widespread and easily recognizable sharks, inhabiting temperate and tropical oceans across the globe. Their distinctive color is so pronounced that it gives the species its common and scientific names.
Other large, open-ocean species, known as pelagic sharks, also feature blue or deeply-colored dorsal surfaces. The Great White Shark (Carcharodon carcharias) often presents a dark slate-gray or gray-blue color on its back. Similarly, the Shortfin Mako shark (Isurus oxyrinchus), known for its speed, has a metallic blue-gray back that helps it disappear in the water column. This coloration contrasts sharply with the brown or mottled gray of sharks that live closer to the coastal seafloor.
The Biological Mechanism Behind Shark Color
The vivid blue of these sharks is not created by blue pigment but through structural color, which relies on how light interacts with microscopic structures in the skin. The color is generated by specialized cells called iridophores, found just beneath the outer layer of the skin. These cells do not produce blue pigment, unlike the melanin that gives humans brown or black color.
These iridophores contain tiny, reflective crystal structures, primarily composed of guanine, stacked in precise layers. When sunlight hits these nanostructures, the crystals scatter only the blue wavelengths of light, reflecting them back to the observer. The surrounding dark pigment, melanin, absorbs the longer wavelengths, such as red and yellow, enhancing the vividness of the scattered blue light.
This combination of reflective crystals and absorbing pigments creates a highly saturated blue appearance, similar to the optical effect known as the Tyndall effect or Rayleigh scattering seen in the sky. Researchers found that in the Blue Shark, these nanostructures are housed within the pulp cavities of their tooth-like skin scales, called dermal denticles. Tiny changes in the spacing of these guanine crystals, possibly triggered by environmental factors, can subtly shift the shark’s color. This shift, from blue toward green or gold, offers a dynamic form of camouflage.
Countershading: The Purpose of Blue Hues
The blue coloration serves a functional purpose in the marine environment, forming one half of an ecological camouflage strategy called countershading. This adaptation features an animal’s dorsal (top) surface being dark and its ventral (bottom) surface being light, typically white. This dual-tone appearance is designed to counteract the effects of light penetration in the ocean.
When a predator or prey looks down from above, the shark’s deep blue or dark gray back blends seamlessly with the dark, sunless waters of the deep ocean beneath it. Conversely, when viewed from below, the shark’s light-colored belly disappears against the bright, sunlit surface of the water above. The light underside effectively matches the downwelling light filtering through the water column.
This specialized form of camouflage makes the shark visually blur into its background, whether viewed from above or below, significantly reducing its silhouette. For a pelagic hunter like the Blue Shark, which operates in the open water without cover, this optical invisibility is a significant evolutionary advantage. This adaptation is essential for stealthy ambush hunting and for avoiding detection by larger predators.