Evolution throws countless designs at the proverbial wall and steps back to see what sticks. All that evolution really needs is a big, variable population to experiment on and a lot of different conditions.
The ocean’s size and diversity make it the ideal laboratory for evolution, yielding incredible adaptations to even the most specialized organs. The eye is a perfect example: already one of the most complex structures in the sea, it has long since been modified to grant major advantages under countless conditions.
The Loosejaw’s Night Vision Goggles
In the abyssal ocean where no sunlight reaches, darkness does not rule unchallenged. Indeed, once your eyes acclimate to the permanent twilight, blue-green flashes come fast and furious.
Using biological light projectors called photophores, fish, squid, and smaller creatures hunt prey, summon mates, and cast defensive illusions. Life in the cold deep is hard and violent: without plant life offering a free lunch, animals must generally kill to eat.
Photophores are their primary tools for both offense and defense. And the preferred color—the color of most bioluminenscence—is blue-green; physics dictate it travels furthest in water. Because the vast majority of light in the deep sea is this aquamarine bioluminescence, most deep-sea organisms can only perceive that part of the spectrum.
But the “loosejaw” group of deep-sea dragonfish sidesteps these rules. In marked contrast to most abyssal predators, large and powerful photophores just beneath their eyes beam red light through the water.
Powered by a unique fluorescent protein in some species and a simple gel-like filter over the photophore in others, this hue is nearly invisible to most prey—but not the loosejaw. Small changes in the gene responsible for the eye pigment “opsin” stretch the molecule’s structure, allowing it to react with longer and redder light waves. As a result, the eye of the loosejaw can see its red search light, but others in the deep remain blind to their danger.
In a blue-green world, loosejaws are the only predators seeing red. They’ve evolved the undersea equivalent of night vision goggles, seeing without being seen, prowling the abyss with impunity. Most deep-sea creatures can only flicker their lights, limited to quick bursts lest they be discovered and devoured. Loosejaws can afford to keep their stealthy lamps on full-time, giving themselves yet another advantage.
Just a few small mutations allow them to outmaneuver everything else in their environment. Closing in on their prey, headlights blaring yet utterly hidden, they are the withered rulers of their realms.
Marlin’s Eye Heaters
If the small, feeble loosejaw has a physical opposite, it would be the billfish. Arguably the ocean’s greatest natural athletes, they feature streamlined muscular bodies tapering to long menacing beaks.
All the world’s billfish—a family counting sailfish, marlins, and swordfish in their number—are large, fast predators who stalk continental shelves for smaller fish. Streaking at up to 80 miles (130 kilometers) per hour during jumps, they hunt their overwhelmed prey at up to 30 miles (48 kilometers) per hour. The combination of fins and muscle—geometry overlaid on physics—gifts them unparalleled efficiency even in heavy, dragging water.
But swimming quickly isn’t the same as eating quickly. Hunting at sailfish speed is akin to driving on a busy street while leaning out of your car door to snatch a coffee mug off the asphalt. Roaring through the open water, these fish plow through clouds of small prey with quick twisting movements and calculated swipes from sail and bill. Chewing is a waste of time; swordfish suck down their dazed prey with gulps quick as fingersnaps.
Billfish are cold-blooded animals hunting in cold, highly productive ocean waters where prey fish roil in glittering silver clouds. Exertion warms their powerful swimming muscles, but this does nothing to heat their brains and eyes: organs absolutely crucial for predation’s elaborate game of anticipation and reflex.
In response, these cold-water predators have evolved specialized tissues that function as heating units. Tuna fish use similar tissue as whole-body heaters, visible as dark brown flesh on either side of the spinal cord. But other parts of their bodies, like the eye-socket muscles and the reflex nerves, stay cold and function slowly. In billfish, where eye and nerve reflexes are critical, the heaters are also found where they’re most needed: right next to the eyes and braincase.
Maintained at warm temperatures, often more than 7° F (4° C) warmer than the surrounding water, the eyes in particular can operate at race-car speed and precision—the kind of suspended “bullet time” typically available only to Hollywood action stars. The retina of a swordfish—when kept warm—can process information fast enough to detect the quick flash of prey fish during a high-speed pass. But if you cool down the retina enough to match the temperature of the ocean, its scanning speed drops to the point where a fast flash of prey is invisible.
Warm retinas also see better at low light levels; eye heaters give swordfish sharper vision up to 1,000 feet (300 meters) deep, where they often forage.
Rift Shrimp’s Infrared Optic Patches
Some eyes have become so intensely specialized that they’re no longer identifiable as such. Rimicaris exoculata (literally, “rift-shrimp without eyes”) is a cocktail-shrimp-sized crustacean found exclusively near the superheated smoky pilings of deep-sea hydrothermal vents. A rift shrimp spends her whole adult life at the edge of death, dancing across black smoker chimneys. Strong chitin toe-tips tear away deposits from the vent’s walls, exposing the sulfide-processing bacteria she slurps down en masse. The shrimp relies on the “black smoker” for sustenance, but she has no proper eyes with which to see it.
But she does have two symmetrical swatches adorning her back, carrying heavy concentrations of a protein called rhodopsin: the same light-capturing pigment found in vertebrate eyes. The shrimp patches have thin corneal membranes and sensitive retinas, with optic nerves buried in the carapace beneath and hooked to the back of her brain.
This specialized optic patch is built in a unique way for a unique purpose. The shrimp sees not with sunlight or bioluminescence, but with the smoldering glow of red-hot water.
When heated sufficiently, almost any substance radiates light in the low-frequency, infrared spectrum. The exact frequency of light depends on the temperature—more heat, shorter wavelength, higher frequency. Our sun emits yellow light because of its enormous heat—11,000° F. The heating elements of a Red Giant star, or a toaster, are cooler, and emit more reddish light. The water of the deep sea vents, at 650° F, emits a light at the very lowest end of red—just bright enough for the rift shrimp’s eye patches to absorb.
Rimicaris has evolved the unique ability to perceive weak infrared radiation, emitted by any object that grows sufficiently warm. The broad rhodopsin patches are thought to increase the animal’s ability to perceive dim light sources, providing just enough data for the shrimp to interpret her surroundings. It’s likely not a coherent image; more a sense of proximity to heat.
Our heroine is never more than inches from a 640° F plume, and her food source sits on the fissure’s very lip. A single step too far will cook her instantly in her shell like a deep-sea Hors d’oeuvre, but security leads to starvation as bacteria live only where boiling sulfides rush to feed them. Though completely blind to light we’d call visible, her strange eyespots are perfectly built to perceive her only meaningful threat. Perceiving the vent’s horizon by its own ember glow, she keeps herself safe while mandibles skitter away at brittle sediment.
For Your Eyes Only
Between their razor-sharp vision and lightning-quick reflexes, sailfish use their eyes to rule the world of small schooling fish. Loosejaws, seen through the same lens of outrageous fiction, become canny, grizzled CIA assassins, or players in a darkened laser tag arena, beaming out red spot lights that only they can see. Rimicaris shrimp are intrepid coal miners, risking death for every meal far from the sun’s sight.
The eyes of all these creatures help us understand their part in the ocean, and how they are tuned to thrive abundantly in the far corners of the sea.
Stephen R. Palumbi is Professor of Biology and Director of the Hopkins Marine Station at Stanford University. His film projects include the BBC series The Future Is Wild, the History channel’s Life after People, and the Short Attention Span Science Theater. His books include The Death and Life of Monterey Bay and The Evolution Explosion. Anthony R. Palumbi, Stephen’s son, is a science writer and novelist whose work has appeared in the Atlantic and other publications.