In Search of Light in the Deep Sea

The Atolla wyvelli, a Scyophozoan jellyfish, lives in the ocean’s midnight zone, between 1000 to 4000 meters below the surface. The gleaming bioluminescent flash it emits sparked the interest of thousands of scientists around the world. The jelly is hardly visible in the dark depths because when it’s not lit up, it is a red/brown color. Red light is absorbed and shattered by water molecules before it reaches ocean depths. When threatened by a predator, the jelly radiates a bright blue light to reveal the attackers to the sensitive eyes of bigger predators. Clever, because blue light, being a shorter wavelength, penetrates the deepest depths.

From the midnight zone down to the realm where no light exists, an abundance of creatures have evolved to manipulate light in fascinating ways. From jellies and anglerfish, to bacteria and even sharks, many organisms emit shimmering curtains of light and vibrant colors without burning an ounce of fossil fuel.

Countless expeditions have set out to uncover how it’s possible for animals to emit huge amounts of light where no light exists, and why, in the first place, would these animals want to stand out in a realm teeming with ferocious predators.

Shedding Light on Bioluminescence

deep sea creaturesBioluminescence occurs across the tree of life. From fireflies and glow worms, to mushroom and shrimp, we know by now why certain species on land and in shallow waters bioluminesce. But very little is known as to why living light is so pervasive in the deep, dark abyss.

In the 1870s, Scotish naturalist Charles Thompson trawled as deep as 26,000 meters in oceans around the world and took more than 4000 unknown specimens to seek answers. Unfortunatley, many of the creatures didn’t survive as their gelatinous, boneless bodies would melt when they crest the surface of the ocean. The only way to get sufficient information is to study these creatures in their natural environment.

The first deep-sea vessel created, a submersible called “Alvin” by Woods Hole Oceanographic Institution (WHOI), is capable of plunging to a maximum depth of 6400 meters and withstanding frigid temperatures and crushing pressure. Among its many other uses, the vessel helps scientists observe the pitch-black environment of the deep sea and collect live biological samples. Through the thousands of video footages, live observations, and collected specimens done aboard “Alvin,” scientists were able to rule out the main reasons deep-sea creatures light up.

Adaptation as the Name of the Game

 The ocean, which contains 99 percent of all living space on earth, hosts a kaleidoscope of life. Romantic as it sounds, it is the most competitive habitable biome on earth. And unlike the pelagic zone where creatures can squeeze into corals and plants to evade predators, deep sea creatures have nowhere to hide.

deep sea creaturesMuch like fireflies, deep sea species use light to communicate with potential mates. Some species can even make these mating signals on cue. Deep sea animals also spew  bright flashes of light, sometimes in multiple directions, to confuse attackers. This response can be triggered by a mere disturbance in the water. And predators, like the ferocious angler fish and adundant siphonophores use light to lure and then attack unsuspecting prey.

Then there are animals that simply use light to hide. This mechanism is called counter illumination. At a certain time of day, season, or geographic location, light can penetrate the ocean’s midnight zone. When animals pass through these well-lit areas, they become more visible to predators. To protect themselves, their bioluminescent structures lit up to blend in the bright blue color of the ocean when light penetrates it, much like camouflage.

While the light-making mechanism varies from species to species, the ingredients are the same. It starts with luciferin, a light emitting compound that is oxidized by the presence of luciferase (enzyme) to produce energy in the form of light. Luciferin is a photoprotein whose light-making process is activated by a charged ion.

Because this reaction is pervasive across the tree of life, many scientists have gone through lengths to extract the protein from thousands of species. From this stemmed another grounbreaking discovery, one that would benefit humans, too.

Deep-Sea Biofluorescence Lights Up Brain Science

Just recently, scientists have discovered that the deep sea is not entirely enveloped in darkness; in the eyes of other creatures, that is. Some marine organisms have the ability to absorb light, transform it, and then emit it as different colors. This trait is called biofluorescence, a phenomenon invisible to the human eye but so common in  the deep sea. It was first discovered by a team of scientists at the American Museum of Natural History (AMNH).

When they saw that some fish species had yellow intraocular filters in their eyes, they custom-built underwater cameras with yellow filters to see how these species see at great depths. This technique enabled researchers to discover over 180 species of biofluorescent fish. Using the special cameras, the scientists witnessed fantastic displays of color and light that rival the best fireworks shows on earth.

One neuro scientist teamed up with the AMNH to collect significant samples of these ligh-giving proteins to engineer genetically encoded fluorescent sensors. These sensors will be used to light up the human brain and detect a large repertoire of neutransmitters or neuromodulators at subcellular resolution.

While its in vivo application is still limited, the technique has proved to be successful in lighting up proteins, viruses, parasites, bacteria, and even the brains of flies. More deep-sea dives and cell engineering are needed to refine fluorescent protein technology.

Diet Secrets of the Deep

deep-sea bioluminescence While deep-sea bioluminescence has been systematically studied for over 100 years, very little is known about the food webs in this biome. Light, it seems, is more than a survival tool. The colors deep-sea creatures emit also say a lot about what they eat. Marine biologist Steve Haddock and his colleagues at the Monterey Bay Aquarium Research Institute embarked on an ROV (remotely operated vehicle) expedition, equipped with high-output lighting systems, broadcast quality cameras, and special filters, to find out. Through these dives, the scientists were able to produce the most comprehensive study on deep-sea food webs.

It turns out that scavengers in the ocean’s floor feed off marine snow, or a shower of organic material (mostly decaying pelagic plants and animals) falling from the water’s surface. Some flakes, also called “ooze,” include fecal matter, soot, sand, and inorganic dust.

To know other creatures’ potential sources of food, biologists collect organisms, dissect their bodies, and observe the contents of their stomachs. The problem with this method is that it’s difficult to collect large numbers of animals. Either they get destroyed when using trawl nets or their soft, gelatinous bodies rapidly decompose after being eaten. The MBARI researchers turned to a more direct approach to get more precise data. They used the ROV to observe deep sea animals as they feed off each other.

jellyfishBy painstakingly studying every minute of the 23,000 hours worth of deep sea video footage, the researchers tallied that jellies and other siphonophores are key predators in this realm. The medusa jelly, alone, eats over 22 different types of animals. Who would’ve thought that the easiest creatures to get trapped in trawl and fishing nets are just as important as the ferocious predators in the open ocean? The most unassuming of all, the Nanomia bijuga, a small, gelatinuous planktonic organism competes with blue whales for food.

Steve Haddock is confident that the more clever a species is at manipulating light, the likelier it is to survive and even be at the top of the food chain. But with nearly three quarters of deep sea creatures being bioluminescent or biofluorescent, it must still be tough to compete or stand out.

Exploration of deep-sea biomes is always a challenge for scientists, which is why much remains to be discovered. But institutions and independent researchers are setting out fpr more global oceanographic expeditions, equipped with more advanced technologies and high-resolution cameras – all in the name of science.

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