National Geographic : 2015 Mar
Luminous Life 81 It’s 7 a.m., and the ROV is about to launch. Men in hard hats scurry about, making final checks. Then an enormous metal arm lifts the ROV off the floor of the boat. Next the floor where it had been sitting folds open, revealing a square of ocean several feet below. The metal arm lowers the ROV into the water; a moment later, the vehicle disappears beneath the waves. As a place to live, the ocean has a couple of peculiarities. The first is that in most of it, there is nowhere to hide. This means invisibility is at a premium. The second odd thing is that as you descend, the sunlight disappears. First red light is absorbed. Then the yellow and green parts of the spectrum disappear, leaving just the blue. By 700 feet deep, the ocean has become a kind of perpetual twilight, and by 2,000 feet, the blue fades out too. This means that most of the ocean is pitch-dark. All day, all night. Together these factors make light uniquely useful as a weapon—or a veil. Consider the problem of invisibility. In the upper layers of the ocean—the part where light penetrates—any life-form that does not manage, somehow, to blend in with the water is in dan- ger of being spotted by a predator—especially a predator swimming beneath, looking up. To get a sense of this, imagine that you’re scuba diving in the middle of the Pacific. Above you, the place where the sea meets the sky looks silver. Below you, the water shades into a dark blue. In all other directions, it is a murky green- ish gray. The seafloor, though you can’t see it, is a vertiginous 11,000-plus feet below you. And wait—what’s that shadow down there? Is it a shark? All of a sudden you become aware of how vulnerable you are: a great dark silhouette against the silvery surface, visible to any hungry animal that might be swimming about below. Many life-forms solve this problem by not be- ing there at all. They avoid the light zone during the day, rising toward the surface only at night. Many others solve it by evolving into transpar- ent, ghosty creatures. On the dive, the first thing you’d notice is that nearly all the life-forms you meet, from jellyfish to swimming snails, are see- through. In another approach, some fish—think sardines—dissolve their silhouettes by having silvery sides. The silver functions as a mirror and allows the animal to blend in by reflecting the water around it. And some creatures—such as the shrimp Sergestes similis, certain fish, and many squid— use light. How? By illuminating their bellies so as to match the light coming down from above. This allows the animals to mask their silhouettes, donning a kind of invisibility cloak. The cloak can be turned on and off at will—and even has a dimmer switch. S. similis, for example, can alter how much light it gives off depending on the brightness of the water around it. If a cloud passes overhead, briefly blocking the light, the shrimp will dim itself accordingly. But if the aim is to remain invisible, why do so many creatures, from ctenophores to dinoflagel- lates, light up when they are touched or when the water nearby is disturbed? A couple of reasons. First, a sudden burst of light may startle a preda- tor, giving the prey a chance to escape. A deep- sea squid, for example, can give a big squirt of light before darting off into the gloom. The green bombers can throw their light grenades, and then disappear into the darkness while the predator is distracted by the light. The ctenophore can van- ish while the predator lunges at its ghost. Second, on the principle of the enemy of my enemy is my friend, giving off light may serve to summon the predator of the predator. Known as the “burglar alarm” effect, this may be especially important for tiny life-forms, such as dinoflagel- lates, that cannot swim fast: For such extremely small beings, water is too viscous to allow a quick getaway. (It would be as if you were trying to swim through molasses.) The chief defense for these creatures is not fight or flight—but light. Their flashes summon fish, which hang out in the water, waiting. And when little shrimplike critters (eaters of dinoflagellates) disturb the wa- ter, causing the dinoflagellates to light up, the Olivia Judson wrote on cassowaries in the September 2013 issue. David Liittschwager’s portraits of life- forms appear frequently in National Geographic.