Looking at Life Through the
Two Dozen Eyes of a Jellyfish
"They Sense Their World With Not Just One But With Many, Many Faces"
It was feeding time for our new pet jellyfish. For once, the kids were interested in my world of jellies. Following the directions, we broke off a tiny bit of frozen jellyfish food and placed it in a dish. Drizzling a bit of seawater on the chunk melted it into a slurry of maroon baby brine shrimp. I thought about what a curious twist of evolution it is that jellyfish are more ancient than their food.
With a turkey baster, I showed 10-year-old Ben and Isy, who was two years younger, how to gently squeeze the food toward the underside of the jellyfish bells. I warned them to be careful not to get bubbles underneath, because they could rip the animals’ delicate tissue.
“Look, you can tell they’re eating,” I said, pointing out the brick-colored bits that were accumulating like delicate piping along the oral arms.
Isy considered our three new pets. “How are we going to tell them apart?”
“The big one has one crooked oral arm,” Ben said. “The middle-sized one is flat. The smallest is kind of round. And it’s sort of upside down.” I found that last observation worrying.
“We need to name them,” Isy said. “We need a name for three things.”
“Like the Three Stooges: Moe, Larry, and Shemp or Curly,” I offered.
“Who?” Ben asked.
Eventually we decided to go with one of Isy’s suggestions. The small one was Peanut; the fat one, Butter; and the one in the middle, Jelly. I’m sure we aren’t the only jellyfish owners who have named their pets for a sandwich.
As I walked by the tank on my way from one side of the house to the other, I found myself stopping in my tracks, watching Peanut, Butter, and Jelly swim. It was relaxing and soothing, like having a miniature spa in my dining room. Soon I moved a lone chair so that it directly faced the tank. Sometimes during the day, while the kids were at school and I was supposed to be working, I sat watching the pulsing, the resting, the floating. What was life like for a jellyfish? What did they feel? Could they hear the sounds in the house? Did they taste the water? Did the jellyfish know I was watching them?
The answers existed along the edges of moon jellies’ bells, which were scalloped like fancy lace parasols. If I looked very closely at the notches between the scallops, I could see a tiny intensification, a place where the bell looked a little less transparent. Under a magnifying glass, these spots are club-shaped structures called rhopalia (rhopalium in the singular), from the word for “bludgeon” in Greek. My moon jellies had eight, and while they tend to come in multiples of four, the number of rhopalia varies in different species.
Over the top of the moon jelly’s rhopalia, there’s a little flap, like a hood or an eyelid—rhopalia hold the gift of sight. We have just one type of eye, but ten different types of visual sensors exist in the animal kingdom. Jellyfish use at least three and probably four of them. My pet moon jellies had two types of eyes on each of their eight rhopalia—one shaped like a cup that looked up and a flat one that looked down—a total of 16 visual sensors. However, for Peanut, Butter, and Jelly, vision was not high definition; it was a hazy thing. They probably couldn’t see me as much more than a passing shadow.
Some jellies can see light without eyes. How they do it is mysterious. Scientists call it extra-ocular sensitivity, and I think of it as similar to the way we sometimes know that someone is watching us even when our eyes are closed. A jellyfish larva, a planula, doesn’t have eyes or eyespots or any obvious visual receptor cells, but if you shine a light, it will swim toward or away from it, depending on the species. Even though a jellyfish polyp, the stationary stage of a jellyfish’s life that looks like a sea anemone, doesn’t have any apparent way to see light, it contracts and waves its tentacles when light changes.
A lot of jellies make a daily vertical migration in the ocean in response to light. At night, they rise up to the surface to feed under the cover of darkness along with trillions of other planktonic organisms. At daybreak, they sink back down into murky depths. Forget about the wildebeests or even the monarch butterflies. This vertical migration of the plankton is the world’s largest migration. It occurs in all oceans twice a day. It’s thought that for moon jellyfish—and probably other species as well—the cue to migrate is visual, because on cloudy days they remain near the surface, unaware that day has broken and that they should sink for the cover of darkness in the ocean’s depths.
Changing light also makes jellies’ eggs mature and their gonads swell. Some jellies spawn after a full moon. Many jellies, including moon jellies, respond to a sudden decrease in light by pulsing faster. Scientists call this the shadow response and think that it’s a get-away-quick behavior to avoid becoming the lunch of a large light-blocking fish or turtle.
Although it’s a murky world for most jellies, it’s not quite as dim for box jellies. These hawks of the jellyfish have surprisingly complex visual systems, and they use them to hunt actively, much like fish. Box jellies are a particular class of jellyfish, about 50 species strong, that includes some of the most toxic creatures in the sea. Scientist Anders Garm from the University of Copenhagen has been studying the eyes of a Puerto Rican box jelly called Tripedalia for over a decade. Unlike its fearsome cousins, this Caribbean native’s poison is fairly mild, which makes it a reasonable creature to study in the lab. “You have to kiss them practically to feel the sting. Then your lips go numb. Then you are fine,” Anders told me.
In the journal Nature, Anders and his colleagues called the visual sensors of box jellies “a bizarre cluster of different eyes.” It’s a fair description. Most box jellies have six eyes on each of their four rhopalia, making a total of two dozen eyes. The six eyes are arranged in two rows of three, like the dots on a six domino. The four outlying eyes are fairly similar to the eyes of the moon jelly. They are cup-shaped, with pigments in the bottom of the cup that can absorb light. But the two eyes in the center column are surprisingly sophisticated, with roughly the same parts that make up the eyes you are using to read these words. These central eyes have a cornea, a lens, a retina, and an iris. What’s more, if you shine a bright light on a box jellyfish, the iris closes down, protecting the eye from the excess light. The constriction is a little slower but not unlike what happens to our eyes when we walk into the sunlight. The top three eyes peer upward and the bottom three are angled downward, even if the jellyfish gets flipped completely upside down.
Perhaps the greatest technical achievement of the box jellyfish eye is its lens. If the clear proteins that make up the lens were packed in the simplest way—uniformly like balls in a gumball machine—then the lens would form a blurry image. No big deal; jellyfish nervous systems probably can’t interpret a sharp image anyway. But the jellyfish lens doesn’t produce a blurry image. Its proteins are packed more compactly in the center of the lens and more loosely around the outside in just the right manner so that all of the light rays that pass through the lens focus on a single spot. The box jellyfish lens creates an incredibly sharp, clear image. Anders and his coauthors wrote in Nature, “For such a minute eye, it is surprising to find well-corrected, aberration-free imaging, otherwise known only from the much larger eyes of vertebrates and cephalopods [octopus and squid]. The gradient in the upper eye lenses comes very close to the ideal [optical] solution.”
What’s even harder to understand than a jellyfish owning a perfect lens is this: If you map out where the rays of light go as they pass through the lens and deeper into the eye, the retina, which detects the image, is not positioned where it should be to make use of the perfectly focused image. It’s too close to the lens. The optics are the same as trying to take a picture of your friend standing in front of a tree when the camera is focused on the tree. What you are trying to see is blurry. “The sharp focus of the lens is wasted by the inappropriate eye geometry,” Anders wrote. Evolutionarily, perfect things don’t usually happen by chance, but whatever it is that a perfect lens does for a box jellyfish is still a delightful and unsolved mystery.
Another mystery of the box jelly’s complex eyes is how its simple nervous system processes the complex information it receives. Like a psychologist studying mice, Anders used mazes to try to find out. He set up a jellyfish obstacle course, poles of different colors and thicknesses at one end of a tank, and put the jellies at the other end. The flow of the tank pushed the jellies gently toward the obstacles. When the colored poles were in the tank, no matter the color or thickness, the jellies avoided them, never once bumping into them. When transparent poles were in place, the jellies bumped into them. He concluded that the jellies could see the colored poles and that their brains had enough computing power to steer around them. Using other obstacle courses, he discovered that the jellies avoided vertical and diagonal stripes but crashed into horizontal ones.
Anders also figured out that each type of jellyfish eye has its own specific purpose. The jellyfish actually use their top lensed eye to find landmarks for navigation. Those eyes stare out of the water at the trees on the shore to make sure the animal remains in the roots of the mangroves where their food lives. The lower lensed eye did all the avoidance work, keeping the jelly from crashing into roots. Two of the other four eyes are used as depth gauges, and the other two control muscles that make sure the jellies’ eyes are always oriented correctly relative to gravity. A visual system built on “special-purpose eyes” was the way to square the complexity of the many box jellyfish eyes with their weak neurocircuitry.
“Humans have one set of eyes picking up enormous amounts of information, but then you need a complex nervous system to sort out the information afterward,” Anders said. “The alternative is that you have eyes designed to pick up a certain type of information. You can filter the information very strongly in the periphery. So the information uptake is much less and much more specific. That’s why you call it a special-purpose eye.” Each special-purpose eye supports only “one, maybe two, behaviors. So if you need more behaviors, as most animals do, then you have to add more eyes.” It’s an entirely different mental architecture from the system we humans use, which requires a lot of brainpower to process information after it arrives. Instead, in jellyfish each eye is responsible for passing on only a certain type of message, allowing the lean neural circuitry to process complex messages.
A couple of years after we were married but before we had kids, my husband Keith was out with a friend for dinner. I was at home, reading or watching TV. Suddenly I felt woozy, as though I’d had too much wine. The feeling came fast. By the time I called Keith and asked him to come home, the world was spinning madly. The only way to stop the feeling was to keep my eyes sealed shut and my head perfectly immobile so that the only inputs my brain received about my location were from the skin that rested on my bed. If I peeked upward or shifted my head a fraction of an inch, I would feel as if I were reeling and I’d puke. With my stomach empty and my eyes sealed shut, Keith managed to get me in the backseat of the car, where I could lie flat, and took off for the hospital. When he pulled up at the emergency room, I tried to look functional. I opened the door, hoisted myself out, and took one step. Whomp! I hit the curb like a domino falling over.
In school, I was taught about the “five senses”—sight, sound, taste, smell, and touch. But I don’t remember learning about what might be the most important sense: proprioception, the ability to orient yourself in the world. People can live without sight or sound or taste or smell, but I don’t believe it would be possible to live without the ability to know your orientation: what’s up and what’s down. One reason for a vertigo attack like the one I had is a malfunction in the inner ears. Behind our eardrum is a space where tiny hairs protrude inward from cells attached to nerves. The hairs are embedded in a layer of gel. Above the gel is a layer of crystals. When you move your head, the crystals shift, pulling on the gel, swaying the hairs, and sending a signal through a nerve. When the hairs bend left or right, you can tell, and you straighten up if you need to. When the hairs shift quickly, you know you’re moving fast; when they’re not moving, you know you aren’t moving, either. Sometimes, the crystals detach from the gel layer, and collect in a nearby part of the inner ear that’s also used in balance, the semicircular canals. When this happens, the crystals bang around in the canals and trigger signals that don’t match the ones from your skin, eyes, and the remaining crystals that are still attached. Your brain reads “Does not compute” and sends you into a tailspin. A short-term version of this can happen when you spin too fast on an amusement-park ride. The crystals keep moving after the ride ends, making the world continue to circle even when you’ve stopped.
We share our sense of proprioception with an ancient organ found alongside the eyes on each of a jellyfish’s rhopalia, a clump of cells known as a statocyst, which tells the animal its position in its three-dimensional world. Statocysts aren’t unique to jellies; they are found throughout the animal kingdom, from worms and sea stars to clams and crabs, evidence that your orientation in the world is one of the most important things you can know. A statocyst is a hollow ball of cells, each with many little hairlike cilia pointed inward, toward the center of the ball. Around the outside of the ball, a nerve fiber attaches to each of the cells. Trapped inside the statocyst are a few grains of a mineral—in a jellyfish, it’s gypsum—that are free to move. When a jelly is upright, the grains rest on the cells at the bottom, pressing down on the cilia there. These cilia send a signal to the nerve cells they are attached to, informing a ring of nerves around the jelly’s bell that it is upright. If the jelly gets swept into a current and tipped sideways, the grains roll inside the statocyst, pressing on cilia on the side of the ball. Signals from these cells on the side instruct the muscles near the signal to paddle slower and muscles on the opposite side to paddle a little harder. The jelly rights itself. Its pulses return to normal when the nerve signal again comes from cilia at the bottom of the statocyst. The system is more complicated, of course, because jellies don’t get information from just one statocyst on one rhopalium, but from statocysts on all the rhopalia around the edges of the bell. The jellyfish has to integrate the input from all those different signals in order to come up with a response.
Alongside the statocysts on the rhopalium, moon jellies also have a sensory spot called a touch plate, which is a field of cilia that reach out into the seawater. The cilia bend and move with the current that flows over the rhopalia, perhaps providing more information about position but also input about speed and turbulence. Perhaps they do even more. Sound is a movement of water, a wave that oscillates water molecules. Sound waves wiggle these cilia on the touch plate, just as they wiggle the hairs in our ears that help us perceive sound. Together, the touch plate and the statocyst form what might be considered a jellyfish’s very primordial ear.
In late May 1991, meteorologists at Cape Canaveral, Florida, were monitoring three weather systems, any one of which could delay the launch of the space shuttle. At the same time, engineers were scrambling to fix a leak of liquid nitrogen from the propulsion system, which would also mean delays. Such holdups would mean trouble for Dorothy Spangenberg, a jellyfish scientist from Eastern Virginia Medical School. Dorothy and her team had prepared nearly 2,500 infant jellyfish and polyps for the mission. They were to be the first—cue the reverb—jellies in space.
While it might seem odd to send jellies into space, NASA had good reasons. Although no one in the West heard about it for years, the second man in space, Russian cosmonaut Gherman Titov, reported feeling a nausea similar to motion sickness. Later, U.S. astronauts reported similar symptoms: malaise, headaches, and vomiting during the first two or three days of their flights. This affliction became known as space sickness, and even today no one really understands what causes it, though microgravity is thought to be a culprit. The parts of the ear that provide our proprioception don’t give the same signals when gravity isn’t pulling with the same strength. Because it’s not easy to get inside human ears, NASA tapped jellyfish as stunt doubles.
For Dorothy Spangenberg’s microgravity experiment on jellyfish, timing was everything. Moon jellies develop their statocysts and touch plates about seventy-two hours after they are born. She planned to induce one group of polyps to form baby jellies three days before liftoff, one group just twenty-four hours before liftoff, and one group in the microgravity of space. The experiment needed to be timed to perfectly capture the different stages of development.
On the morning of June 5, launch day, the storms swerved. The skies were clear over Florida. The problem with the leak was solved. At 9:24, the spaceship Columbia roared into the sky. Astronaut Tammy Jernigan was tasked with rearing the baby jellies onboard. They were grown in two containers holding less than three quarts of water on the Columbia’s mid-deck. The containers sat inside an incubator about the size of a microwave oven, which held the temperature at a constant, comfortable 82 degrees Fahrenheit. Eight hours after liftoff, Jernigan topped off the flasks with iodine to induce the last group of polyps to strobilate into baby jellies. She recorded video of the infants as they pulsed about in their orbiting aquaria and then preserved some of them for further study back on Earth. Jernigan told me that in space the ephyrae moved differently from their cousins swimming in identical control studies back on Earth in Dorothy Spangenberg’s Virginia lab.
On Earth, the ephyrae swam up to the tops of their test tubes, then relaxed and sank downward. In space, they swam in endless circles. The jellies didn’t resume normal behavior after reentry, either. About a fifth of the space-born jellies swam strangely after they returned to Earth. They were uncoordinated; they spasmed; their arms were out of sync. Some could swim only in circles. Jellies that were hatched on Earth but grew up in space had fewer statoliths than their Earth-bred cousins. Growing up in space gave the jellies chronic vertigo.
It makes sense that jellies born in microgravity wouldn’t develop the proper nerve and muscle connections to balance properly, but questions remained as to how and why and what we could learn about our own sense of balance from them. On a second jellies-in-space mission, astronauts observed two groups of polyps hatch into ephyrae without being induced to do so with iodine. On Earth, iodine causes the polyps to make a hormone, thyroxine. Dorothy Spangenberg speculated that thyroxine might play a role in the development of statocysts, but she never got the chance to find out whether her hypothesis was correct. Shrinking budgets scrapped future experiments.
When I sat in front of my pet jellyfish in my dining room, it was easy to become swept up in the elegance of their dance, the seemingly spontaneous motion that felt disconnected from my reality, movement driven by impulses so innate and alien as to be incomprehensible. But then I would focus on the rhopalia. It’s a myth that jellyfish are faceless creatures. In some ways, their sensory experience is broader than our own. They sense their world with not just one but with many, many faces.
From Spineless: The Science of Jellyfish and the Art of Growing a Backbone. Used with permission of Riverhead. Copyright © 2017 by Juli Berwald.