by Edith Widder
I got to see some of this fantasticness firsthand during my dives with the light meter. The first of these took place at high noon so that I could make careful notes on where different animals were found relative to different light levels. There were clear delineations. A smaller species of krill (Euphausia pacifica) formed a layer between about 550 and 650 feet. At about 750 feet, another layer of krill, a larger species (Nematoscelis difficilis), occupied a light level twenty-five times dimmer. And below 1,000 feet, the sergestid shrimp resided in a murkiness that was two hundred times dimmer still, right at the limit of my eyes’ and the light meter’s ability to detect any light coming from the surface.*5 To see it, I had to look straight up, and all I could detect was a hint of gray surrounded by the blackest of black. That this is enough light to reveal a silhouette and thus warrant expending energy on counterillumination is a testament to the sensitivity of the eyes of deep-sea predators. To achieve such sensitivity, they often sacrifice acuity, with adaptations like neurally linking neighboring photoreceptors, which helps explain why the belly lights found on so many fish, squid, and shrimp, which appear as distinct points of light to our eyes, might actually blur together, forming a diffuse glow that blends perfectly with the background as seen by the eyes of deep-sea predators.
I also made dives at dawn and dusk, hanging at a depth of 500 feet while watching the commuters swim by. I thought I would see a well-ordered migration with a staggered order of appearance as animals adhered to their preferred light levels, but it was more chaotic than that. Some stuck with their light zones and some didn’t. At sunset, the small and large species of krill were mixed together, with the large krill taking the lead, hightailing it for the surface and outswimming their smaller brethren. The krill reached the depth at which I was hanging before sunset and continued to pass by for more than an hour afterwards as the light level dropped more than three-hundred-fold. Then the first sergestid shrimp came into view, and they reached their peak concentration an hour and a half after sunset, apparently following the same very dim light levels they occupied during the day. Gooseberry comb jellies and corncob siphonophore chains (Nanomia bijuga) were also racing for the surface, with the comb jellies in the lead. And mixed in with the sergestid shrimp were fish and squid. Like rush hour in most of the world’s big cities, it lasted longer than an hour, but it felt much better managed.
To be a firsthand observer of this massive daily trek was an extraordinary privilege. On my first sunset dive, as I held stationary at 500 feet, I got to watch the edge of darkness well up from the depths and pass me by on its inexorable ascent. What peculiar and exhausting lives these creatures lead, spending their existence as commuters, racing toward the surface at sunset, seven days a week, and descending back into the depths at dawn. As a result, they live forever in darkness, which helps explain why so many of them are bioluminescent. The best way to cope with perpetual night is to make your own light. That might sound like something you read in a fortune cookie, but it’s an evolutionary concept that considerably predates cookies of any kind.
As I dangled there, I kept my lights off, turning them on only briefly to survey the scene and count the commuters visible within my field of view. When the lights were off, I was watching the bioluminescence. Up until sunset, there was none, either because the downwelling sunlight made it impossible to see or because it wasn’t there. But as the sun set and the light faded, I began to notice a very few brief flashes appearing, initially at fewer than three per minute and then gradually increasing to a crescendo of both short and long flashes and a combination of point sources and short, fragile chains of mermaid’s tears. The light show reached its peak an hour after sunset, when the flashes became too frequent to count; I had to estimate their density by the spaces between them, which I guessed as two inches between the point sources and anywhere from two to six inches between the mermaid’s tears.
The more I studied the bioluminescence, the more I began to be able to guess who was who. I suspected that some of the brighter point sources were krill that were stimulated to flash by the undulations of the Wasp. Some of the point sources looked like a background of sparkling pixie dust, and I guessed these might be dinoflagellates. But there was so much I was seeing about which I had no clue. One type of flash in particular intrigued me. It was a bright, slow-on, slow-off flash that lasted about five seconds and was so far in the distance I was sure it wasn’t being mechanically stimulated by the suit. I routinely saw it associated with the sergestid shrimp layer, both deep during the day and migrating with it at twilight.
When I was first reviewing my dive transcripts and saw the entry “bright flash in the distance,” I had to sit and think for a while: How did I know it was “in the distance”? I tried to re-visualize it, and when I did, I remembered the halo that surrounded it. It was the scattering of the light that gave its remoteness away; the bigger the halo, the more scattering, the greater the gap. This must be an important clue for animals having to judge distance in the dark. If you see a flash and swim toward it, you need to have some notion of how far to swim.
If we want to understand life on our planet, we need more time for watching and wondering within its blue heart. Light evidently plays a critical role here, but often in ways we do not yet fully comprehend. As the Earth spins on its axis and the intensity of sunlight impinging on the surface of the ocean waxes and wanes, the push and pull of twilight’s threshold on myriad creatures in the depths is relentless. An overcast sky can cause multitudes to relocate to shallower depths or dim their bioluminescent belly lights. If sunlight exerts such sway, then what of bioluminescence itself? Living light dominates the submarine light field at depths below the penetration of sunlight, and at night in surface waters, but it is poorly characterized compared with the solar light field. I desperately wanted to know what the true nature of that biological light field was when there was no big mechanical suit down there stirring things up. These were clearly solvable problems, I realized; we just needed to find new ways of observing.
Skip Notes
*1 Warning: The field of ocean optics employs mathematical equations that may be offensive to math phobics.
*2 You might be a Trekkie if you know that this was revealed as a weakness of the Romulan Bird of Prey introduced in the first-season episode “Balance of Terror.”
*3 Since no such underwater light meter existed, we had to create one. This was a collaboration that involved engineer Mark Lowenstine and fellow Case Lab graduate student Mike Latz. The light meter was based on a photomultiplier tube built into an underwater housing designed with a gimballed mount that assured that it always pointed straight up.
*4 I held the depth record in Wasp for a grand total of two days, until Robi found a deeper place to dive and usurped the title. He was welcome to it.
*5 In coastal waters, particles and dissolved substances in the water increase absorption and scattering, so light doesn’t penetrate as far as it does in clear ocean waters.
Chapter 6
A BIOLUMINESCENT MINEFIELD
The day of the flood dawned calm and cloudless, with no hint of impending doom in the air—if you disregarded my journal entry scribbled the night before: “Lousy sleep. Keep having dreams of entrapment and drowning.”
A year had passed since the Wasp expedition. Although the original plan was for another mission using Wasp, the suit had proven less than ideal for our purposes. The biggest hindrance was the tether. That connection to the ship made for a wild ride when the sea was rough, transferring all the wave action at the surface down the cable to the suit’s occupant. During the course of a dive, we could reduce this to some degree by adjusting the ballast. Putting slack in the tether provided a certain amount of cushion, but I never felt fully decoupled from the tea-bag motion, so I couldn’t be sure how much of the bioluminescence I was seeing was stimulated by the suit. Also, during descents and ascents, when there
was full tension on the cable, the experience felt akin to being inside a martini shaker.
All the headaches caused by the tether, both literal and figurative, led Robi to look for an alternative. He settled on the untethered single-person submersible Deep Rover, which was the most recent brainchild of the same inventor who’d developed the Wasp, Graham Hawkes. Deep Rover appeared to have all the advantages of Wasp for providing direct access to the midwater, and none of the drawbacks.
In Wasp you are always standing and, because the metal body sucks the heat out of you, frequently cold. Deep Rover, meanwhile, is more like an underwater helicopter, where you sit in a comfortable pilot’s seat in the center of a nearly invisible five-foot-diameter acrylic sphere with five-inch-thick walls that insulate you from the cold. Even better for our purposes, it did not rely on a tether, which meant that there was no martini-shaker effect. This made Deep Rover potentially the perfect platform for answering one of the biggest questions related to bioluminescence in the ocean: How much occurs when we’re not down there stirring things up?
Virtually all the natural light that illuminates life on Earth originates from two sources: the sun and bioluminescence. Scientists know a lot about how profoundly sunlight impacts animal adaptations and behaviors in the ocean above the edge of darkness. Below the edge of darkness, the presence of so much bioluminescence and so many animals with eyes suggests that living light is equally impactful. But in what ways? Although the field of ocean optics had gone a long way to describing the solar light field, there was very little understanding of the bioluminescence light field and how it affects animal behaviors.
The first time sensitive light detectors were lowered into the ocean, in the 1950s, scientists were amazed by the amount of light they recorded. The light meters were designed to measure the penetration of sunlight underwater, but once they dropped below a thousand feet, they started recording flashes. At first, the investigators wondered if there was something wrong with their detectors, but then they realized it must be bioluminescence. These were bright flashes, and there were lots of them. At two thousand feet, flash intensities were a thousand times greater than the intensity of sunlight at that depth, and flash frequencies exceeded one hundred per minute. It called to mind a Disney light parade with an extravaganza of illuminated floats and fireworks spectacles. They wondered, What the heck is going on down there?
Since the greatest amount of flashing was observed in the vertically migrating layers seen on sonar, one suggestion was that all this light output might be connected to an increase in the animals’ metabolism needed to make such long migrations. Another idea was that maybe the flashing served to help coordinate rush hour traffic by keeping the commuters in sight of one another. Although these notions sound far-fetched now, they reflect how little was understood at that time about the functions of bioluminescence.
A number of papers were published detailing flash frequencies at different depths and different times of day, until eventually it was noticed that the frequency of flashing correlated with the sea state: Rough seas generated more flashes than calm seas. The researchers deduced that their instruments must be bumping into light emitters, causing them to flash. So the question became: What is the true background level of bioluminescence? It was surprisingly tough to answer. Decoupling any detector on a cable from the motion of the ship on the surface is a major challenge. And it doesn’t help to anchor the detector to the bottom, because currents flowing around it also mechanically stimulate bioluminescence.
Being able to determine the levels of spontaneous bioluminescence was a big deal for two reasons. The first—the one I cared about—was that it spoke directly to understanding the nature of the visual environment in the biggest living space on the planet. If I was ever going to be able to comprehend what life is like for the animals inhabiting this space, I really needed to know what the visual scape looked like in its undisturbed state.
The other reason it was important had to do with military concerns. The U.S. Navy was looking into using lasers as an acoustically quiet means of underwater communication for submarines. They wanted to know what kind of signal-to-noise ratios they might expect. If there was a lot of spontaneous bioluminescence, that equated to a high level of optical noise, and potentially muddled communications.
It seemed like Deep Rover might finally provide the means to answer this question. According to Graham Hawkes, his little sub had such good ballast control that it could essentially become one with the surrounding water.
In order for a craft to be neutrally buoyant, the force of gravity pulling down must be exactly offset by an equal but opposite force of buoyancy pulling up. The result is that you neither sink nor float. Some fish control their buoyancy by means of a swim bladder—a gas-filled sack that they can inflate when they want to ascend, deflate to sink, or adjust to perfectly offset the downward pull of gravity when they want to hang out at a particular depth. Deep Rover has something similar in the form of a soft ballast system that feeds compressed air or water into a tank to make the sub either lighter or heavier. I hoped to be able to trim the sub so perfectly that I could eliminate any mechanical stimulation of bioluminescence and then sit quietly, watching a world that didn’t know I was there and counting spontaneous bioluminescent events.
We had the same team of scientists as the year before—Bruce Robison, José Torres, Larry Madin, Richard Harbison, and me—but because Larry had to leave early and Rich had to arrive late, Robi, José, and I were going to get the lion’s share of the diving. Deep Rover was a lot bigger than Wasp, which meant we couldn’t train in a tank the way we had the previous year. Instead we were handed a manual to memorize and given some classroom training in emergency procedures, and then it was time to dive.
For this expedition, we had moved north from Santa Barbara to Monterey Bay, home to one of the world’s most spectacular submarine canyons—comparable to the Grand Canyon in depth, but with steep escarpments and multitier plateaus studded with all manner of marine life. The canyon also serves to funnel deep-sea animals from offshore up the chasm, potentially providing a greater concentration of the midwater animals that we wanted to observe.
My first dive was a training dive near the head of the canyon in water that was only sixty feet deep. They dropped me in with the ship’s crane and then held me on the hook while we did pre-dive checks of thrusters, electronics, and comms. By far the biggest drawback of not having the direct electrical link that the tether made possible was a significant degradation of communications. At the surface, we could communicate with VHF walkie-talkies, but as soon as we submerged, we had to switch to through-water acoustic communications, which use the water to carry the signal. Some of the time, this was almost as good as talking on a cellphone, except you had to push to talk, which meant that only one person could speak at a time and you had to remember to say “over” when you were done. But a lot of the time it was a noisy link, subject to dropouts and ocean noise, including the chatter of dolphins. It worked best when restricted to succinct reports of depth, cabin pressure, and oxygen level. No trading bad jokes, as was our wont with Wasp.
Once all was deemed A-OK, they set me free from the crane, with instructions to motor along the surface toward one of two buoys that were part of our training course. At the first buoy, I was told to submerge and then resurface, at which point a scuba diver removed a small fifteen-pound lead weight from the sub. When I tried to submerge again, I couldn’t. That meant they had done their buoyancy calculations correctly. The diver replaced the weight and I was instructed to submerge and proceed on a compass heading to buoy number two. I came up very close to it, and then after submerging again I got permission to “go play.”
Flying Deep Rover was like the best video game ever. It was incredibly easy and intuitive. All the controls were in the seat base or the armrests, so my view was unobstructed. Handgrips at the end of each armrest controlled two multifunction manipul
ators that responded to the lightest touch. To activate the thrusters, I only needed to slide the armrests forward or backward. I found that if I slid one forward and one back, I could make the sub spin like a top, and the manipulators were so dexterous I could pick anything off the bottom with great delicacy.
The view was panoramic and offered up plenty of eye candy on which to feast: fluffy white-plumed anemones, pink and orange sea stars, bottom-dwelling flatfish, and much more. In just this one short dive, I saw five small octopuses, a diving bird (a grebe) that I observed with amazement at a depth of forty feet, and a sea lion that swam by with such speed that it put the sub’s zippy three knots to shame.
My second dive, to 120 feet, was far less thrilling, because the visibility was almost zero and I had to head up early because of bad weather. On my third dive, though, things got really interesting. The first two had been considered training dives, and this was my first real science dive, to a depth of a thousand feet for a total of four hours. I went down at three in the morning. Inside the sphere I had a supersensitive video camera*1 that I hoped would be capable of recording the bioluminescence outside the sub. Up until this point, the only people who had ever seen bioluminescence in the deep ocean had been those lucky few who had been able to dive in a submersible and had bothered to turn out the lights. I badly wanted some way of recording it so I wouldn’t have to depend on my visual memory of fleeting flashes, and also so I could share something I considered one of the most beautiful natural phenomena on the planet with people completely unaware of its existence.
As soon as they let me off the crane hook, I cruised away from the ship and then flooded my ballast tanks and began to sink through the inky black waters. I kept my floodlights on as I descended so I could observe the animal life. Almost immediately, I entered a layer of squid and what appeared to be red octopuses. The octopuses reminded me of the red crabs I had seen from Wasp—another creature that one usually associates with the bottom. These were the ruby octopus (Octopus rubescens), which spends a longer portion of its young life drifting about in the water column and growing to a larger size than most other octopus species, before settling down to a more sedentary existence on the bottom.
