by Edith Widder
We did replace Justin and Erika’s clever contraption with a more streamlined system for holding the e-jelly and the bait, but as homage to their ingenuity, we continued to refer to it by the acronym that Sönke had suggested: CLAM, for “cannibalized ladder alignment mechanism.”
The e-jelly was designed to imitate a number of different displays. One of these used a single blue LED to produce a rapid repetitive flash. On the 2007 expedition, there were several instances where this triggered an elaborate response that appeared as a sequence of dabs of liquid light that were released in speedy succession by something—I’m guessing a shrimp—swimming fast in a spiral. The effect was like adding an extra corkscrew flourish to the emission with an emphasis that had a Take that! feel to it. The e-jelly would fire off a series of short flashes and something—often several somethings—would respond. It was a conversation with light. I had no idea what I was saying, but I think it was something sexy, because the response bore a striking resemblance to the sexual displays of sea fireflies.
In some cases, the strings of light that sea fireflies (bioluminescent ostracods) produce rise up off the bottom. In other cases, they may be horizontal or diagonal, like drifting strands of glowing pearls. The spacing, intensity, placement, and size of the pearls represent a complex code that permits females to identify potential mates of their own species. It is thought that all this marvelous complexity is an evolutionary response to crowding, because it enables several different species to coexist without confounding the mating game.
Obviously, using bioluminescence to attract mates has a potential downside, since the light might easily also attract predators. Therefore, the evolution of displays that involve the release of long-lasting clouds of light, where the emitters can remain physically separate from their light emissions, is enormously advantageous. With sea fireflies, the females of a particular species have co-evolved to intersect the trajectory of the male’s swimming pattern, which she calculates based on the bioluminescent landing lights he so obligingly generates.
You don’t need a submersible to experience this particular light show. In fact, you don’t even need scuba. Because it occurs in water shallow enough to observe while snorkeling, you just need to hop in the ocean shortly after sunset and wait for the performance to begin. This used to be a common occurrence around the Florida Keys, where, sadly, it has largely disappeared as a result of pollution. Nevertheless, there are still plenty of places throughout the Caribbean where it happens year-round, so you should definitely take the opportunity to see it if at all possible. The experience of being surrounded by hundreds of these displays is like being immersed in a symphony of light. It’s definitely worthy of your bucket list. Revel in it while you still can.
Skip Notes
*1 Wavelengths of visible light are measured in nanometers (nm). One nanometer is one-billionth of a meter. Visible light ranges from 400 nm (the blue end of the spectrum) to 700 nm (the red end).
*2 Or, for those who are more mathematically inclined, if you plot them on a logarithmic scale.
*3 Like a low-pass sound filter that passes sound frequencies lower than a selected cutoff while blocking higher frequencies, this filter passed low-frequency red light and blocked higher frequencies, including blue, green, yellow, and orange light.
*4 Although there are multiple brine pools in the Gulf of Mexico, this one is the most famous and most studied and is therefore capitalized.
*5 The ones in the Gulf of Mexico have smaller-diameter tubes and smaller flowers than the ones discovered in the Pacific, but they can form into very large bundles called bushes.
*6 At the end of one expedition, Sönke and Erika stole the blades out of Justin’s electric razor right before he left the ship. Trying to shave on the long flight to Australia, he was puzzled when it wasn’t working. A couple of days later, he received a ransom note with the blades displayed against that day’s newspaper and photoshopped wearing sunglasses and smoking cigarettes.
*7 Subby—slang for sub-crew member.
*8 Obvious proof that too much salt in your diet is not healthy.
*9 Little-known fact: Muscle Beach, in Venice, California, was first known as Mussel Beach, back when it was famous for bivalves instead of biceps.
*10 The cause of this flashback phenomenon is unknown. More about that later.
*11 Two red lasers mounted in parallel a known distance apart—for example, 10 centimeters—so when you see two red dots on the shark’s flank you can use that known distance to calculate the full length of the shark.
*12 Possibly because on a rolling ship a portion of your brain is perpetually diverted to calculating which way is up?
*13 Recently, it was tentatively given the name Promachoteuthidae based on three captured juveniles with similar characteristics.
*14 Just the opening bars. Not the full hour of Also Sprach Zarathustra. It was majestic, not glacial.
*15 Picture a pill bug the size of a toaster oven.
*16 Not developing a new form of proctology, as alleged by Justin, Sönke, and Erika.
PART III
HERE BE DRAGONS
Nothing in life is to be feared, it is only to be understood.
Now is the time to understand more, so that we may fear less.
—Marie Curie
Chapter 12
THE EDGE OF THE MAP
We are all explorers. Each one of us is born into this world as a stranger in a strange land. Our explorations shape our understanding of the world around us. As a baby, when you crawled away from the safety of your mother’s arms to see what was beyond the next corner, you were satisfying a natural instinct to find something new. Exploring the edge of the map, knowing that at any moment you might unveil one of nature’s hidden secrets, is such a deeply visceral thrill that it feels primal.
The happiest people I know (a category in which I include myself) are those who have managed to hang on to a childish sense of wonder at discovering new things. But hanging on is not always easy. Too often, the world is served up as a collection of facts to be learned, rather than grand mysteries to be solved. The ocean holds and hides so much dizzying complexity and wondrous weirdness that there is no end of puzzles to entice explorers. Some of the best of these involve how light—both sunlight and living light—has shaped life in the ocean.
Early investigators examining net-collected specimens at the beginning of the last century were so utterly baffled by what they found there that they declared, “Nothing seems more hopeless in biological oceanography than the attempt to explain the connection between the development of the eyes and the intensity of light at different depths in the ocean.”*1 Learning about the nature of the underwater light field went a long way toward illuminating the reasons behind the awesome strangeness of deep-sea eyes.
If you dive in a submersible to the very edge of darkness, you can see for yourself the profound changes in the visual vista that account for these eccentricities. This is where the scene transitions from an increasingly dim overhead light field to one that is defined by sparks of bioluminescence seen against pitch blackness. Many animals inhabiting this zone hunt by spotting the small silhouettes of prey swimming above them. Others detect flashes of bioluminescence directly below or in front. Some do both.
Take, for example, the deep-sea brownsnout spookfish (Dolichopteryx longipes). Its unfortunate common name notwithstanding,*2 this is an amazing fish, with a big head graced with four protruding eyes.*3 Two large eyes point up and have prominent lenses that collect dim downwelling light, while two small eyes point down and employ mirrors to reflect and focus the light from bioluminescent point sources. The cockeyed squid (Histioteuthis heteropsis) meets the same challenge using only two eyes; one, the left, looks up and is large and bulging, while the other,
the right, looks down and is small and sunken. A third solution is manifested in the Pacific barreleye fish (Macropinna microstoma). It can actually rotate its telescopic eyes inside its head! Although this exceptionally odd-looking fish has a black body, its head is transparent, creating a protective dome over the eyes, like the canopy on a fighter jet. It’s no wonder early investigators were baffled by such outlandish adaptations.
There was also bewilderment at the relative eye size of animals inhabiting different depths. With eye size, there are two key factors in play. Sensitivity is the first: Bigger eyes collect more light. Cost is the second: Larger eyes are energetically more costly to build and maintain.
Life demands energy, and energy in the deep ocean is generally in short supply. Most of the fuel for life comes from the sun. This is true despite the fact that sunlight bright enough to drive photosynthesis is found only in surface waters no deeper than about 650 feet. With the exception of the chemical energy found at hydrothermal vents and cold seeps, which provide a tiny fraction of the total available resources, the most common food source, even in the sunless depths, is derived from photosynthesis.
Plant life in the form of phytoplankton grows in surface waters, then either sinks into the depths as it dies or is carried there by consumers like jellies, crustaceans, squid, and fish that vertically migrate into deep waters and die or defecate, thereby releasing valuable foodstuffs. For deep-sea dwellers, it’s all manna from heaven. But it is not an infinite bounty, and as the shower of food is gobbled up on its way to the bottom, the downpour turns into a trickle. It makes perfect sense, then, that the numbers and sizes of animals diminish the deeper we go.
What didn’t make sense to early investigators, though, was this: As animal size decreases with depth, relative eye size increases, right up to the edge of darkness. Then the trend gets flipped upside down, because below that transition zone, eye size generally shrinks with increasing depth. Why?
Well, as I learned the hard way postsurgery, the real key to eyesight is not detecting light; it’s being able to distinguish the brightness differences between an object and its surroundings—in other words, contrast. For animals that need to differentiate a small dark silhouette against a dim background light, the best way to enhance the contrast of that image is to gather more light from the background. This is better accomplished with a larger eye. Therefore, at deeper depths within the twilight zone, where the background light becomes dimmer, the eyes of upward-looking visual predators are generally larger. On the other hand, a bright object such as bioluminescence, seen against a black background, is essentially infinite contrast, which can be efficiently detected without resorting to the expense of a large eye.
Leonardo da Vinci said, “The noblest pleasure is the joy of understanding.” To have a confusing set of facts and observations morph into a simple and elegant explanation produces a wonderful “aha” feeling. Understanding the nature of the light field and the challenges it presents for survival makes many things that once seemed inexplicable obvious.
For example, there was that silvery fish I saw hanging vertically in the water during my first dive in Wasp. It was a cutlass fish, so named because of its bladelike appearance and elongated body—as much as five feet long—which tapers to a point at the tail and is so shiny it looks like polished silver. I’ve seen many such fish during submersible dives and have often heard them described (by non–fish biologists) as “pogo fish,” because they hover just above the seafloor and every time their tails make contact with the bottom they react by lunging upward. The effect can be bizarre. When there’s a group of them together, it looks like a thicket of double-edged, hiltless swords hanging vertically in the water, doing an unsynchronized dance in which one after another darts straight up and then gently drifts down. It’s wholly nonsensical, until you grasp the role that light plays in their lives.
Cutlass fish are voracious carnivores with fanglike teeth and large eyes that they use to hunt in the twilight zone, searching for silhouettes overhead. Assuming a vertical orientation facilitates their hunting strategy, permitting them to look straight up and at the same time present the smallest silhouette possible to any predators below them. They have a preferred light zone in which they hunt, so when we show up in submersibles or ROVs with our big, bright floodlights, they adjust their depth downward to get away from the light, until they hit bottom and can’t anymore, causing them to lunge upward. Sometimes they swim away, but more often they hang around, stuck in a behavioral loop, performing their crazy pogo dance.*4
How many other bizarre activities and adaptations might be explained if we had a better understanding of the light field in the ocean—not just the solar light field, but the living light field? For example, there’s the conundrum of eye size all the way down on the deep-sea floor. This is another transition zone where the general trend of eyes getting smaller as you go deeper below the edge of darkness undergoes a changeover. Many bottom dwellers have eyes that are large relative to their body size, and for those living at depths where sunlight doesn’t penetrate, the logical explanation is that those eyes must be adapted for seeing the only light available: bioluminescence. The trouble with this notion is that, unlike in the midwater, where most of the animals are bioluminescent, the number of light emitters found on the ocean bottom is relatively low.
* * *
—
To investigate this puzzle, in 2009, Tammy Frank, Sönke Johnsen, and I put together a NOAA-funded expedition to explore bioluminescence on the deep-sea floor. We wondered if it was possible that the assumed dearth of bioluminescence could simply be the result of nobody having seriously looked for it. To this day, most sampling of bottom dwellers depends on brutal forms of deep-sea trawling that leave the critters thoroughly thrashed. Perhaps they were luminescent but we didn’t know it, simply because their light-producing chemicals were being exhausted during capture. Even samples collected by submersible or ROV might not survive getting to the surface because they are generally transported in such a way that they are not sufficiently protected from the dramatic change in temperature and so they are cooked before being examined for light production.
To test this notion, we proposed using the Johnson-Sea-Link (1) to carefully collect bottom dwellers by placing them in a thermally insulated box, called a BioBox, to keep them cold for the trip to the surface, and (2) to also literally poke around on the bottom with the sub’s manipulator, while the lights were off, to see if we could stimulate anything to light up. The question was, where should we go hunting?
Three-quarters of the seafloor appears featureless and depauperate, while the other quarter more than makes up for it by including some of the most otherworldly communities on the planet. When I encountered one of these bewildering locales for the first time, it left me dumbstruck.
It was back in 1985, when I was diving Deep Rover in the Monterey Canyon. I was running with lights off for what was supposed to be a midwater bioluminescence transect. Ordinarily, readings from the sub’s sonar would allow me to stay a safe distance above the seafloor. Unfortunately, my sonar was on the fritz, so I was depending on the surface ship to tell me how close I was, and they got it wrong. To be fair, they got it mostly right. I was tooling along for several minutes, thoroughly absorbed in the bioluminescent displays flashing all over my transect screen, when I plowed smack into the bottom. Apparently, during the second half of the transect, the seafloor started angling up, which nobody noticed.
Colliding with solid things in the dark while a couple thousand feet underwater is highly discomforting. The surge of adrenaline flooding my body as I flipped on the lights made me question the reality of what I was seeing.
I had blundered into an undersea garden straight out of the imagination of Dr. Seuss. Surrounded by enormous fans of rosy bubblegum coral,*5 I was sitting right next to a giant yellow sponge shaped like a lacy, upside-down witch’s hat. I could make some sense of the coral an
d sponge, but covering the seafloor between them was a field of outsized mushrooms that looked like they belonged in a Pixar movie or a magical kingdom inhabited by unicorns. Some were white and some were pink, and each was adorned with long, feathery plumes poking out all over their mushroom caps. Sitting underneath one of these mushrooms, an orange frog looked up at me with such fixed intensity, I half expected him to scold me for disturbing his slumber.
I later learned that the mushrooms were a kind of soft coral*6 I’d never seen before—a relative of sea pens—and the “frog” was a large-eyed rockfish that was resting on its pectoral fins on the bottom, with its tail tucked behind it in such a way that it looked far more like frog than fish. Compared with the animals I was used to encountering in the midwater, these creatures were incongruously gigantic. Tellingly, they were all—except for the fish—detritivores.
