by Edith Widder
Some lanternfish have a photophore just above each eye that the fish can use to compare its bioluminescent output to the overhead light it is trying to match. If the photophores’ light outputs look too dark against the background, then it either brightens its bioluminescence or swims deeper until the light matches. But there are many examples of animals that don’t have light organs whose output they can see, and it’s unclear how they achieve such a perfect match. Perhaps their eyes measure light with much greater accuracy than ours do. Because our eyes can adjust to different light levels, our assessment of brightness is highly dependent on our most recent experience. Just think about how differently you perceive light when you first enter a darkened room compared with how you can see after you’ve been there for twenty minutes. A light that appears bright after you are fully adapted to the darkness might be invisible after you’ve been in direct sunlight. This is a problem if you need your eyes to perform as reliable light meters.
Color, too, must match, and many animals have evolved elaborate optical filters that serve to narrow the spectrum of their bioluminescence, producing a purer blue to look like that found in the deep ocean. The hatchetfish’s fingernail-like light organs with their magenta lunules do not produce pink bioluminescence, as I have seen suggested in some popular literature, but rather they absorb some of the shorter and longer wavelengths of the fish’s natural blue bioluminescence to create a perfect color equivalent. The magenta seen under white light is the result of a combination of red and blue light transmitted through the filters and reflected back to the viewer. That these filters also transmit red light is meaningless, because there is none to transmit down where they live. A filter can’t create color; it can only subtract it.
One further way that animals must blend with the background light, besides intensity and color matching, is directionality. There is an artist named Larry Kagan who creates 3D sculptures out of thick metal rods, sort of like rebar. Each sculpture looks like an abstract doodle in space, until you turn on the spotlight that illuminates it. Then there is a moment of epiphany as that light casts shadows on the wall behind the sculpture, revealing the image of a chair in one case, an insect in another, and Che Guevara in another. To appreciate these artworks, you need to embrace shadow as a volume in space, one that is created by a highly directional light source.
Counterilluminators use this concept and have evolved all kinds of tricks to ensure that the light they produce with their photophores has the same directionality—or, more specifically, the same angular distribution in space—as the light they are replacing. Some do it with lenses, and some do it with very clever use of concave mirrors. The hatchetfish does it with fiber optics—mirrored tubes that carry the photons from inside the fish, where they are emitted by light-producing cells called photocytes, through the magenta-colored filters, down to the fish’s underside. Where the light emerges, the photophores look like tubes cut on a steep angle that create the fingernail-like appearance of the light organs and help shape the angular distribution of the emitted light.
As any Star Trek fan can tell you, cloaking devices are a huge energy draw. This is why, for starships, the moment of cloaking or decloaking provides a window of vulnerability as energy is transferred from shields and weapons to or from the cloak.*2 The greatest window of vulnerability for counterilluminators is sunset. Since food is concentrated in surface waters, the migration becomes a race to the top. First come, first served. But in order to get to the dinner table without being seen (and devoured yourself), you need to be pumping out enough light to match the light coming from above.
Those that produce the brightest bioluminescence can take the lead in the race to the dinner buffet above while still remaining cloaked. But the more you push the upper limits of that envelope in an attempt to score more food, the more food you have to eat in order to compensate for all the energy you’ve burned. The daily energy requirement of counterilluminating at 1,300 feet deep in clear ocean water is the equivalent of adding a brisk half-hour walk to your daily routine. At 1,000 feet, it is equivalent to adding a one-hour swim. But at 650 feet, pumping out enough bioluminescence to match the sunlight found at that depth would be equivalent to running one and a half marathons every day—which may not be the best use of your resources. Clearly, animals have a range of environmental variables to which they are adapted. Pass too far outside that range and bad things happen—like using up your energy stores before you can replenish them.
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At the time we were making our Wasp dives in 1984, most of what was known about animal adaptations to the midwater light field was based on net sampling. Measuring light at a particular depth, usually by lowering a light meter on a wire from a ship and then dragging a net at that depth, provided some indication of which animals lived in which light zones, but it was a coarse view, with no insight into individual animal behaviors.
Wasp provided an opportunity for “watching and wondering” to a degree never before possible, because not only could I observe the animals in their natural habitat, but I hoped to quantify light’s influence on their behaviors using a couple of purpose-built underwater instruments. The first of these was a light meter sensitive enough to measure very dim levels.*3 The second was something I dubbed the “light wand,” which was simply a small blue light in a pressure housing that I had stuck into the end of a three-foot length of PVC pipe. I planned to wield it in front of the Wasp like a light saber, blinking the bulb on and off by using a switch inside the suit, in the hope that it would permit me to talk to the animals, Dr. Dolittle style. Since it seemed obvious that the language of life in the midwater was expressed through light, I wanted to see if I could crack the code. I had visions of either animals flashing in response or predators attacking out of the darkness, and I couldn’t wait to try it out.
Everything about my first dive in Wasp was so mind-blowingly awesome, I assumed my next would be even more so. Having passed the psych test with ease, I felt confident that I wasn’t going to have any kind of claustrophobic meltdown and so I’d be able to focus on my observations. That turned out to be a bad assumption.
Panic is not conducive to sound decision-making, which is why the admonishment “Don’t panic!” is so often offered up to those behaving like their hair is on fire. It’s great advice, but singularly unhelpful if unaccompanied by how-to instructions. The first time I experienced full-blown panic was in the hospital. Three weeks after the spinal fusion, the discovery of the massive infection in my surgical site required emergency surgery, which was carried out without benefit of general anesthesia because of concerns about retriggering the blood disorder. Before the procedure, a nurse administered a hypo with a standard anti-anxiety cocktail intended to relax me. Instead, it left me feeling like I had been robbed of the only tool I had to fight the pain and fear: my brain. Even worse than the pain was the extreme anxiety I felt every time someone from the surgical team would ask me how my breathing was doing. I was overwhelmed by this rising sense of panic that I couldn’t control because my brain was drugged. The surgery lasted an hour and a half. It felt like eons. When the ordeal was finally over, I begged the doctor to promise me that I would never have to go through anything like that again. All he would say was “We’ll see.”
Two days after the procedure, a nurse showed up in my room, hypo in hand, ready to prep me for another surgery. Her arrival ripped my fingers off the metaphorical cliff face to which I had been clinging, and I was in free fall. There had been no warning from anyone that this was coming. I had no time to prepare mentally for it, and I wigged out.
The fear overwhelmed my capacity to think rationally. I was hysterical, begging my poor mother not to let them take me. The doctor came in to try to calm me down and explained that some more dead tissue needed to be cleaned away but that it would not take nearly as long as the last time. I finally agreed to go quietly if they promised not to give me the drug co
cktail, because, as I tried to explain in the calmest voice I could muster, to deal with the panic I needed to be able to think. He was dubious. Anti-anxiety meds must have seemed like just the ticket, given my behavior, but he acquiesced, although the hypo was kept next to my gurney as an unspoken threat.
The procedure was not pleasant, and I had to fight back waves of immense fear and pain, but I got through it. In fact, I got a lot of practice getting through it, because it then needed to be repeated every other day for a month. So here are my how-to instructions for not panicking: Refocus. That’s it.
It’s actually the same advice the White Queen gave Alice to keep from being sad, only instead of calling it refocusing, she called it “considering things.”
“Can you keep from crying by considering things?” [Alice] asked.
“That’s the way it’s done,” the Queen said with great decision: “nobody can do two things at once, you know. Let’s consider your age to begin with—how old are you?”
“I’m seven and a half, exactly.”
“You needn’t say ‘exactually,’ ” the Queen remarked: “I can believe it without that. Now I’ll give you something to believe. I’m just one hundred and one, five months and a day.”
“I ca’n’t believe that!” said Alice.
“Ca’n’t you?” the Queen said in a pitying tone. “Try again: draw a long breath, and shut your eyes.”
Alice laughed. “There’s no use trying,” she said: “one ca’n’t believe impossible things.”
“I daresay you haven’t had much practice,” said the Queen. “When I was your age, I always did it for half-an-hour a day. Why, sometimes I’ve believed as many as six impossible things before breakfast.”
Rather than consider “impossible things,” my preference is for intriguing things, but the Queen was right: The key is practice. The ability to refocus your brain when it tries to go off half-cocked in some counterproductive fashion is such a valuable skill, I think it should be taught from an early age.
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My second dive in Wasp came two days after the first. It was about an hour before sunset. I was descending through the water column as dusk fell, which meant the edge of darkness was rushing up to meet me. All traces of sunlight were nearly gone at about 350 feet below the surface, where I encountered a distinct layer of krill. This was the vanguard of migrators heading for the surface. It was as if they were raising the curtain on the fireworks display, because from there on down, the luminescence was spectacular.
I had Charlie stop me at 880 feet and then again at 1,400 feet so I could study the fireworks and try out the light wand. There was so much luminescence swirling around the suit, it was difficult to tell if my little light was having any effect. It was like striking a match in the middle of a brushfire: My paltry ember was lost in the blaze. Certainly, no predators were attracted to it. There were a couple of times when I thought something flashed a response, but, given how much other flashing was going on, there was no way to be sure.
As I proceeded down through the water column, I focused on the luminescence, which remained intense until I was about 30 feet off the bottom, at which point it almost disappeared. Several times during my descent, I was distracted by a popping sound. When I described what I was hearing to Charlie over comms, he said, “It’s probably the syntactic foam on the outside of the suit.”
Syntactic foam is a common solution to the problem of providing flotation under such crushing pressures. Styrofoam floats because of air spaces inside a polystyrene matrix, but when they’re put under pressure, those air spaces collapse. Syntactic foam floats because of air spaces inside hollow glass microspheres embedded in an epoxy matrix. Glass is actually quite strong when it’s in the form of a sphere, and it stands up well under pressure. It wasn’t the glass that was cracking, but the epoxy holding it together, presumably because there were microbubbles in it that were collapsing. At least that was Charlie’s theory. He explained that the big form-fitting block of syntactic foam attached to the Wasp’s back had nothing to do with the suit’s pressure integrity, so I shouldn’t worry.
Although the increasingly frequent pops were making me edgy, I trusted Charlie’s explanation and was okay with it until I touched bottom at 1,831 feet. That was when Charlie called down to say, “Congratulations! You just broke the world depth record for the Wasp.” “What the hell do you mean?” I shot back. “I thought this thing was rated for two thousand feet!” His response, “Yeah, but nobody’s ever been,” was not reassuring. At that moment, the syntactic foam gave forth the loudest pop yet and I suddenly had a very clear image of how much water was over my head. I might have passed the claustrophobia psych test on my first dive, but today all bets were off.
A column of water one foot by one foot square and 1,831 feet high weighs more than 100,000 pounds. That translates to a staggering amount of pressure. At that depth, the tiniest leak could create a high-pressure jet that would cut through my flesh like a hot knife through butter. It had taken eighty minutes to reach this depth. Even if they pulled me up at full speed, it would take at least thirty minutes to get to the surface. I felt panic start to gain a stranglehold, and all I could think was GET ME OUT OF HERE! Just as I was about to give full-throated voice to that thought, a jellyfish caught my attention. It had an iridescent thimble-shaped bell and long, flowing tentacles, and it was swimming fast by pulsing its umbrella. Suddenly it dropped its tentacles in a swirling, tangled mass as it disappeared into the darkness.
Focusing on that jellyfish helped pull me out of my vortex of panic and back into a far more palatable reality. As I had learned to do in the hospital, I controlled the fear by refocusing, putting all my attention on the exquisite beauty encompassed in that fragile being, apparently perturbed by my presence but blissfully unaware of the weight on its back. Buddhists may claim that the best way to soothe the monkey mind is by asking yourself, What’s the worst thing that can happen? But I have found that this is not the best strategy while on the bottom of the ocean. It’s far better to simply focus on something else, and if that something is simultaneously magnificent and mysterious, so much the better.*4
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Throughout every one of the dives I made in the Wasp, there was no shortage of mysteries to draw my attention. Besides the bioluminescence, which continued to baffle and mesmerize, there was also the daily cavalcade of migrators to ponder. I had a series of dives planned using the light meter to establish the light levels where different animals were found at midday compared with those at dawn and dusk.
There were two populations of shrimp I was especially interested in observing. These were the shrimp on skis that I had seen during my first dive and the krill I had encountered on subsequent dives. Both are bioluminescent counterilluminators, but their light organs are radically different in both form and light-producing capacity. Krill generally have ten photophores—one under each eye and eight on the underside of the body—all capable of very bright light emissions. The body photophores are exquisite optical constructions almost like little eyes, except that instead of collecting light, they emit it. In place of a retina, there is a group of light-producing cells called the lantern, beneath which a lens and an iris help to collimate the light before it shines downward. Around the back of the photophore, above the lantern, a reflective layer of cells helps focus the light downward, and around that, a layer of red pigment further assures that no light escapes upward. Intriguingly, the photophores on the eyes lack the lenses found in the body organs, perhaps because they are used as flashlights. By contrast, the shrimp on skis, better known as sergestids, have far less elaborate light organs, capable of much dimmer light emissions. These light organs are actually formed from modified liver (hepatopancreas) cells and, although simpler in design than krill photophores, they, too, include a lens to assure that the direction of the emitted bioluminesc
ence matches that of downwelling sunlight.
Of course, all this careful matching to the background light is for naught if the animal tilts its body relative to the downwelling light. This is potentially a problem for vertical migrators like these shrimp, which must incline their bodies up or down to change depth; to compensate, their light organs counter-rotate in order to always maintain their vertical orientation. Krill can rotate them as much as 180 degrees, which allows them to swim nearly straight up or straight down without risking any misalignment of their bioluminescence with the background light they need to match. Sergestids, too, can rotate their light organs, in their case as much as 140 degrees. Hatchetfish, on the other hand, have an equally amazing adaptation: They can swim diagonally upward or downward without tilting their bodies. It is this capability that accounts for its ungainly shape, a necessary adaptation that allows it to present a streamlined profile for both horizontal and diagonal swimming.
It was Richard Harbison, one of my fellow Wasp pilots, who discovered the hatchetfish trick a year before our Wasp dives. While descending in a submersible, he noticed several hatchetfish swimming in a straight horizontal line across his field of view. He watched them for a little while before it dawned on him that this made no sense, because the sub was dropping at a rate of almost two feet per second. The only way the fish could appear to be swimming horizontally, he realized, was if they were actually swimming diagonally at a pretty impressive clip. The necessity of living in the ocean’s strange illumination is the mother of some mighty fantastic inventions.
