by Edith Widder
As I logged off comms with Charlie, a large jellyfish pulsed into my lights. Arched white tentacles studded the rim of a glasslike central disk. As it swam out of the light and into the enveloping darkness, I attempted to give chase, pressing forward on the foot switches that activated the thrusters. My pursuit was abruptly terminated, though, as the umbilical brought me up short, like a dog on a leash. I hadn’t adjusted my ballast.
I cracked open a valve, letting compressed air into the ballast tank. At the same time, I called up to the surface to let Charlie know what I was doing and he cautioned me to keep an eye on the umbilical. During training, he had made it abundantly clear that not being aware of where your tether was at all times was a major no-no. He had reinforced this admonition with a graphic description of the umbilical looping under the suit’s arm and ripping it off, with, of course, lethal consequences for the diver inside. I was pretty sure the story was apocryphal, but that didn’t lessen its impact. At the first sign of slack in the cable, I closed the valve and returned my attention to the view outside.
The jellyfish was long gone. In its place were large, elegant shrimp that looked like they were on skis and gliding through a perpetual snowstorm. The shrimp were a type of deep-sea prawn called a sergestid (Sergestes similis), and their “skis” were their impressively long antennae that stuck out and down in front and then bent back sharply, trailing to either side of their beating pleopods. The snow was marine snow, the white flocculent material, made up of decomposing plankton and fecal pellets (a.k.a. poop), that rains down from surface waters. The procession of shrimp was mesmerizing—completely unlike the damaged specimens I had seen in trawl nets. Each had a body that looked like it was fashioned out of cut glass—crystal clear except for the section just behind the head, which was cherry red. I longed to keep watching, but there wasn’t time—Charlie wanted to finish up and go to dinner. If I was going to see the thing that had brought me here in the first place, I needed to act quickly.
It takes more than twenty minutes for the human eye to dark adapt, so I turned out the lights, prepared to wait a while before being able to see much, but no delay was necessary. Instantly I was engulfed in what looked like a field of stars. Everywhere I looked, there were glowing motes. The density was like what you might see in a desert sky on a moonless night, but these stars weren’t static; they were swirling all around me like a three-dimensional version of Van Gogh’s Starry Night. My breath caught in my throat.
It was difficult to focus on any one star long enough to fully see it, but after a while I realized that these weren’t just discrete points of light. Much of it looked like internally illuminated organic protoplasm that, upon closer inspection, consisted of strings of two to four tiny glowing orbs held together in a chain by a gossamer sheath. The phrase “mermaids’ tears” came to mind. Their light was not a steady glow or an abrupt flash, but rather a slow, deliberate illumination, like turning up lights with a dimmer switch, only these lights didn’t switch on synchronously, but in a propagated sequence. I tried to time them to see how long they stayed on, but they kept disappearing from my field of view before the light faded out.
It was hard to be sure, but it seemed as though the tears were being stimulated by the movement of the Wasp. The motion of the ship bobbing in the waves at the surface was transmitted down the tether to the suit, causing it to undulate up and down, creating an enveloping sparkly halo. It was my impression that the sheer stresses caused by that motion were activating the tears. Occasionally, one of the strings of tears would make direct contact with the observation dome and the orbs would brighten as the gossamer sheath holding them together stretched, and stretched some more, and the lights smeared and disappeared. Mixed in with all these mermaids’ tears were other, less abundant flashes and glows, small puffs of what looked like luminous blue galactic clouds, and distant orbs that would glow brightly for three seconds and then wink out. I was awestruck and baffled.
Producing light takes energy—a lot of energy. Energy is the currency of life, and it is never spent frivolously. How was this seemingly profligate expenditure possible? Why was it here? And, given that this was obviously the most important thing happening in the ocean, why weren’t more people studying it? In those days, it was possible to pick up a textbook on marine biology and find no mention of bioluminescence. And when it was written about, it was deemed of little import—akin to fireflies on land, bit players in the grand theatrical production of life. It seemed obvious to me that in this theater, the light emitters weren’t bit players; they were the stars, both literally and figuratively.
When Charlie called to say it was time to wrap it up, I couldn’t believe it. It felt like no time had passed, and I didn’t want to leave. I turned on the lights and looked out into the void, anxious to know who was making all that light and why. There were a few sergestid shrimp and one small jellyfish, but no mermaids. Nothing I saw with the lights on could account for all those tears. There’s nothing there! I thought. Well, that was certainly a problem, and might explain why more people weren’t studying bioluminescence.
As Charlie began hauling up the suit, I turned the lights off again and watched a dazzling meteor shower of luminescence whipping by as my mind raced. What part did light really play in this world? How was it similar to light’s role in the terrestrial realm, and how was it different? What kinds of experiments could I devise to answer such questions?
Intermixed with these scientific questions was the beginning of a realization: If you gain access to a portion of the planet as remote and inaccessible as this and it proves filled with glittering treasure, what choice do you have but to come back—again and again and again?
Skip Notes
*1 Much like modern politics.
*2 Although an argument can be made that a cod end and a codpiece share a similar shape and therefore a possible common word origin, I can find no connection. Cod end refers to the end of the net where the codfish accumulated, while codpiece derives from the Middle English cod, meaning “scrotum,” and refers to a male fashion accessory that has largely fallen out of favor except with certain comic-book superheroes.
*3 Most shipboard research facilities have a wet lab and a dry lab. The wet lab is kind of like a mudroom in a house, where all the messy, wet stuff is dealt with, and the dry lab is where the more sensitive electronic gear is set up.
*4 Gnathophausia ingens.
*5 And hopefully more slimming.
*6 This describes the most extreme example of sexual dimorphism in anglerfish, found in the northern giant seadevil, one of the largest species, with the female achieving gargantuan proportions of up to 3.9 feet long and further adding to her allure by being covered in warts.
*7 Good advice for anyone when it comes to mating.
*8 I’m aware that Madison has sunshine. It just didn’t appear at any time while I was there.
*9 Despite the claims of English scientist J. B. S. Haldane that a perforated eardrum allowed him to “blow tobacco smoke out of the ear in question, which is a social accomplishment,” it is a condition that most scuba divers prefer to avoid. All manner of contortionist maneuvers may be employed, such as the Toynbee maneuver, the Edmonds technique, the Lowry technique, and the Frenzel maneuver, which all have the goal of forcing open your Eustachian tubes to allow higher-pressure air to pass through benignly.
Chapter 5
STRANGE ILLUMINATION
Drifting on a glassy tropical sea at high noon, if you stare over the side of a ship you may observe sunbeams disappearing into the depths. The dancing rays, made visible in the clear water by plankton and particles that scatter light back to your eyes, appear to converge and lead downward into a dark tunnel. The effect is hypnotic, bringing to mind Alice peering through the looking glass. She wanted to see behind the fireplace to know if a fire burned there, as it did on her side
of the mirror. The more she looked, the more she wondered what it would be like to live in that other world. In Lewis Carroll’s telling, she gets to find out when the glass melts away, “like a bright silvery mist,” and she passes through to the other side. The world she finds is exceedingly strange, inhabited by peculiar beings living bizarre lives.
The mirror surface of the ocean divides our planet’s living space into two realms: the air-filled one where we reside and the water-filled one that is home to most of the life on Earth. Passing through that air-water interface unveils an equally fantastical world, populated with wondrous life-forms uniquely adapted to live there.
Attempts to understand life and life processes begin with what the Nobel Prize–winning animal behaviorist Niko Tinbergen described as “watching and wondering.” That fundamental practice is something we all engage in to varying degrees, but Tinbergen formalized it in the study of animal behavior, becoming one of the founding fathers of what came to be called ethology. Key to his approach was observing animals in their natural environment. He deemed this essential because what animals look like and how they behave is all about adaptations to the places they live.
The immense oceanic realm that covers most of our planet and extends from sunlit surface waters to the seafloor is the least understood environment on Earth. Opportunities for watching and wondering here are rare, yet even a single visit is enough to reveal its most outstanding feature: There’s nothing to hide behind!
On land, prey avoid predators by concealing themselves behind trees or bushes or burying themselves in hidey-holes. For midwater prey that inhabit the open water between the surface waters and the seafloor, there are no such options. How do prey evade detection when the only thing between the hunter and the hunted is crystal-clear water?
Imagine you are a fish hanging around in the open ocean at six hundred feet deep. In very clear ocean water, this is roughly where the light intensity drops below 1 percent of surface light levels. Below this depth, there is insufficient light for photosynthesis, but there’s still plenty of light by which to see. If a hungry shark swam into view right now, you’d be an easy target. Your best option to avoid being spotted is to swim straight down toward darkness. The trouble with this plan is that you’d be leaving your food source behind. Photosynthesis is the basis of the oceanic buffet, and even if you’re not a vegetarian, the critters you eat, be they crustaceans or smaller fish, probably are, which means the best place to find them is around the plants they eat, which is to say the phytoplankton, growing at the surface.
The solution is to use darkness as cover, traveling back up to the surface to eat after the sun has set. Every day, massive numbers of animals in the ocean employ this strategy, vertically migrating toward the surface at sunset and then migrating back down before sunrise. On a ship’s sonar, these layers of migrating animals are so dense they look like the seafloor ascending and then descending beneath your ship.
There is probably no starker testament to how profoundly animal forms and functions are shaped by their visual environment than life in the midwater. Understanding animal adaptation in this realm requires visualizing what scientists call the light field. This is defined as all the light streaming in every direction through every point in space, be it in air or water. Defining this is so important to understanding how light behaves underwater that there is a whole scientific discipline dedicated to it, called ocean optics.*1
William Beebe described the odd appearance of the underwater light field as “the strange illumination.” Those words initially appear in this dramatic description of his first deep dive with Otis Barton in the bathysphere, when they reached the heretofore unimaginable depth of seven hundred feet:
Ever since the beginnings of human history, when first the Phoenicians dared to sail the open sea, thousands upon thousands of human beings had reached the depth at which we were now suspended, and had passed on to lower levels. But all of these were dead, drowned victims of war, tempest, or other Acts of God. We were the first living men to look out at the strange illumination: And it was stranger than any imagination could have conceived. It was of an indefinable translucent blue quite unlike anything I have ever seen in the upper world, and it excited our optic nerves in a most confusing manner. We kept thinking and calling it brilliant, and again and again I picked up a book to read the type, only to find that I could not tell the difference between a blank page and a colored plate.
His descriptions of that “strange illumination” were heard again and again in the reports he sent up the telephone line. So often was it mentioned that he later confessed that “the repetition of our insistence upon the brilliance, which yet was not brilliance, was almost absurd.”
What Beebe was experiencing and attempting to describe was how profoundly sunlight is altered when it passes from air into water. Most apparent is the degree to which different colors are absorbed, causing a radical shift from a multicolored, sun-drenched palette above water, filled with warm yellows, oranges, and reds, to a submarine world bathed in a cool vibrant turquoise. But there is also the impact of scattering, which gives the expression “bathed in light” special meaning. Were it not for the effects of scattering, the only visible light would come from directly overhead, and peering into the distance horizontally would reveal only blackness. Instead, the water itself seems to be emitting light, a direct consequence of scattering.
With the combination of absorption and scattering, daytime illumination in the midwater assumes a predictable form, best conjured by imagining yourself in the center of a giant blue beach ball that’s illuminated by the surrounding water. Straight overhead, the light is brightest. Straight down, it is two hundred times less bright, and between these two extremes there is a light-to-dark gradient. What is remarkable about your view is how symmetrical it is. Pirouette in any direction and it appears exactly the same. Incredibly, this symmetry does not change, no matter where the sun sits in the sky. Be it at the peak of its arc, at midday, or traversing the horizon at dawn or dusk, the only apparent change below two hundred feet deep is in intensity. This is because the shortest path for light through water results in the least attenuation, so the brightest sector in your beach ball is constantly straight up—the point of minimum distance between you and the surface.
Now imagine yourself again as a fish swimming toward darkness in order to hide. How deep do you have to go? Light decreases tenfold for every 250 feet you descend, which sounds like you shouldn’t have to swim very far, but eyes—especially eyes adapted to seeing in the deep ocean—are incredibly sensitive, and they can enable the detection of sunlight at depths in excess of three thousand feet, well below what human eyes would deem the edge of darkness. That’s a whole lot of swimming to do on a daily basis! For a hatchetfish, it would require swimming seventy-two thousand body lengths each day, the equivalent of eighty-two miles for Olympian Mark Spitz, who even at his peak generally maxed out at twelve miles per day. Clearly, anything that could reduce the length of that swim would represent a tremendous survival advantage. If only there were some way to camouflage yourself so you could blend into the background, then you could stay closer to the surface and not have to make such an exhausting trek between your dinner at the surface and your daytime safe haven in the dark depths.
How can you possibly hide when, from whatever direction you are viewed, you are going to appear as a silhouette against a stark, illuminated background? This is why the hatchetfish has silver sides. Its scales mirror light from the featureless symmetrical orb that surrounds it, reflecting light that is a close match to the light behind it. This even works for predators that are slightly above or below, because the mirrors are roughly vertical even on curved portions of the fish. But what about when seen from directly above? Looking down on the hatchetfish, you will see that its back is darkly pigmented, the better to blend into the darkness beneath it. This kind of camouflage, where an animal is darker on its top side
than its underside, is common. Known as countershading, it renders animals such as sharks less conspicuous when seen from above or the side, making it easier for them to approach prey undetected. The word countershading is about countering the natural effects of shading. A light shining from above creates a bright back and a deeply shadowed belly. Leonardo da Vinci said, “Shadow is the means by which bodies display their form.” Countershading is the means by which bodies hide their form.
But a white belly cannot hide the most conspicuous shadow of all: the silhouette seen from below. To reduce the size of its shadow, many a fish has a thin shape. That body form is not purely a matter of hydrodynamics, as evidenced by the fact that the fastest swimmers in the ocean, such as sailfish, marlin, bluefin tuna, and blue sharks, are round, not slender. However, for a fish to really disappear, it must somehow replace the light that its body absorbs, which is where bioluminescence comes in. This type of bioluminescence camouflage is called counterillumination, and, given how many animals use it, it must be an enormously effective cloaking device.
The thing I find most astounding about counterillumination is how many different ways animals have evolved to achieve the same thing. To create perfect camouflage, the light that these animals emit must exactly match the light field above them. A predator swimming beneath a counterilluminator doesn’t see its prey because the fish replaces the sunlight absorbed by its body with a perfect bioluminescent imitation of the missing light. This means that if a cloud passes over the sun and dims the downwelling light, an animal must somehow dim its bioluminescence to match.