Deep beneath the ocean’s surface, where sunlight can’t reach, life has found a way to thrive in complete darkness. Some creatures have adapted by becoming blind, while others have developed large eyes to spot predators above them. Despite the lack of sunlight, the ocean isn’t entirely dark. Many sea creatures have evolved to produce their own light, a phenomenon known as bioluminescence.
Bioluminescence is a common trait in the ocean, with over 75% of marine organisms capable of producing light. This includes everything from tiny bacteria to large sharks. This ability has evolved independently many times, showcasing a concept called convergent evolution, where different species develop similar traits. Given that the ocean covers 71% of the Earth’s surface, bioluminescence is one of the most widespread adaptations on our planet.
While bioluminescence is common in the ocean, it’s rare on land. Only a few fungi, insects like fireflies, and some larvae can glow. This raises the question: why is it so rare on land when the terrestrial world is also dark half the time? The answer lies in the differences between living in water and on land.
In the ocean, there’s often nowhere to hide, so many creatures use light to protect themselves. Jellyfish and other sea creatures use bioluminescence to startle predators or to camouflage themselves. Some even use light to attract prey, like the anglerfish, which uses a glowing lure to catch its meals.
On land, bioluminescence is used by some insects for communication and mating. Fireflies, for example, use light to find mates. However, the chemicals needed for bioluminescence can be toxic, which might explain why it’s less common on land. Additionally, life has existed in the ocean much longer than on land, giving marine organisms more time to evolve this trait.
Bioluminescence is a fascinating adaptation that scientists are still trying to fully understand. If we ever find life on other planets, bioluminescence might be a key trait to look for, especially in environments with water and air. As we continue to explore the mysteries of bioluminescence, we gain a deeper understanding of the incredible diversity of life on Earth.
Imagine you are a marine biologist discovering a new bioluminescent creature in the ocean. Design a model of your creature using art supplies or digital tools. Consider how it uses bioluminescence for survival. Present your creature to the class and explain its adaptations.
Conduct a simple experiment to understand how bioluminescence works. Use glow sticks to simulate the chemical reaction that produces light. Discuss how this reaction is similar to and different from the natural process in marine organisms.
Research the benefits and drawbacks of bioluminescence in marine versus terrestrial environments. Form two groups and hold a debate on why bioluminescence is more prevalent in the ocean than on land. Use evidence from your research to support your arguments.
Explore the concept of convergent evolution by identifying other examples in nature where different species have developed similar traits. Create a poster or presentation that compares these examples to bioluminescence in marine organisms.
Write a short story from the perspective of a bioluminescent sea creature. Describe a night in the ocean and how you use your light to navigate, find food, or avoid predators. Share your story with the class and discuss the role of bioluminescence in your creature’s life.
Sure! Here’s a sanitized version of the transcript:
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Far below the ocean’s surface, life has adapted to a world of darkness. Strange creatures inhabit the crushing depths, some completely blind, while others have eyes that point upwards to help them avoid predators from higher levels. Some have eyes so large that one might wonder what they are designed to see in such darkness.
No sunlight penetrates the deep waters of the ocean, but this ecosystem is not devoid of light. Instead, it shines with one of the most widespread adaptations in the ocean: bioluminescence. Many creatures have evolved the ability to glow in the dark, either by harnessing bioluminescent bacteria or initiating a chemical reaction within their own bodies. These unique adaptations have intrigued scientists for decades. More recently, researchers have realized that bioluminescence isn’t limited to the darkest parts of the ocean; in fact, the proportion of animals that glow is similar at all levels of the ocean, from the surface down to the seafloor.
Bioluminescence is less a special ability than a widespread adaptation in the sea. More than 75 percent of organisms in the ocean can produce their own light, from the smallest bacteria to large sharks. Fishes and rays have evolved this feature at least 29 different times, making it an impressive example of convergent evolution. Considering that 71 percent of the Earth’s surface is water, bioluminescence is one of the most widespread traits to have ever evolved.
This raises the question: why is bioluminescence so common in the ocean and so rare on land? The only terrestrial organisms with the ability to produce their own light are a few species of fungi, arthropods like fireflies, and some larvae like glowworms. While the ocean is certainly dark, our terrestrial world is also bathed in darkness half the time. If bioluminescence isn’t limited to the pitch-black depths of the ocean, why does it seem to be constrained on land?
When 18th and 19th-century travelers wrote about glowing water in the ocean, their accounts were often dismissed as fantasy. Jules Verne described a milky light coming from the ocean in his popular novel “20,000 Leagues Under the Sea.” It wasn’t until the 21st century that we understood that glowing phenomena can illuminate vast patches of the ocean.
Today, we know that bioluminescence has emerged independently more than 94 times within hundreds of species. Organisms that use bioluminescence have two options for producing their own light: intrinsic bioluminescence, where they mix chemicals in their own bodies to produce photons, or symbiotic bioluminescence, which involves cultivating colonies of glowing bacteria in specialized organs. The chemical process involves a molecule called luciferin, which generates light when mixed with oxygen and a luciferase enzyme.
Researchers have found that intrinsic bioluminescence evolved only eight times in fishes, while symbiotic bioluminescence has occurred at least 17 times, always with the same family of bacteria. On land, the trait seems to have taken longer to appear. A fossilized ancestor of fireflies from 100 million years ago was discovered to have a light-producing organ, suggesting that this adaptation has long existed among that group.
Another group of researchers suggested that other bioluminescent terrestrial insects evolved from a single ancestor that lived 65 million years ago, around the time dinosaurs went extinct. However, this doesn’t explain fungi, which belong to a different branch on the tree of life. Researchers analyzed the genomes of living fungi to trace the roots of glowing mushrooms back to 160 million years ago. They theorized that this ability evolved from a common ancestor that was either lost, retained, or regained across time and many different species.
It’s possible that bioluminescence makes spores more attractive to insects, which might help in spreading them. However, it might also be a byproduct of other chemical reactions, explaining why it has disappeared multiple times in the evolutionary history of fungi. The fact that bioluminescence has evolved separately on land and has been retained in several species indicates that it is a useful adaptation.
So why is it still so rare? Living in the ocean is not at all like living on land. The most obvious difference between marine and terrestrial environments is the medium: water versus air. Both present vastly different challenges and opportunities for life forms. A less obvious difference is that in most of the ocean, there’s nowhere to hide. Rodents and other small animals on land have trees, shrubs, rocks, and burrows for safety, while most aquatic life lacks such cover.
Reef systems are exceptions, occurring in only about one percent of the ocean, and only some species live on the ocean’s floor. In the open expanse of the ocean, many species have had to find other ways to keep themselves safe from predators, and light is one of the best methods. Jellyfish and siphonophores are excellent examples of creatures that use light to evade capture.
Nearly all of them have some form of bioluminescence. Comb jellies flash brightly to startle predators, while siphonophores can produce small lights to confuse them. Some jellyfish even release glowing slime that sticks to their predators, a strategy also employed by tiny crustaceans called ostracods or sea fireflies. When fish try to swallow them, they release one of the brightest lights in the ocean, momentarily blinding the fish.
By illuminating themselves or their predators, these organisms might also be calling over larger predators that could eat the smaller ones. Consider dinoflagellates, those single-celled plankton that can produce glowing waves. Many species of dinoflagellates capable of bioluminescence are less likely to be eaten, as their bright flashes act like a burglar alarm, announcing to nearby ocean dwellers that a crustacean is trying to eat them.
Another way swimmers use light for defense sounds counterintuitive: light as camouflage. The deeper you go in the ocean, the less light filters down. Between 200 meters and 1,000 meters lies the mesopelagic zone, also known as the Twilight Zone. A fish swimming above a predator would have its dark silhouette outlined against the light. To prevent this, they use counter-illumination, with spots of light on their underside breaking up their silhouette and confusing potential predators.
This strategy is so refined that creatures like the Hawaiian bobtail squid can change the amount of light they emit depending on the time of day, like their own glowing circadian rhythm. The story on land is more complicated, as lighting up in darkness is more likely to draw attention than obscure outlines. However, some organisms use this glow as a defensive strategy by advertising that they are poisonous, similar to the vibrant skin color of poison dart frogs.
This strategy is used by some worms and millipedes. But it’s not only prey using light for survival; predators have their own tricks to lure in meals. Members of the anglerfish family are well-known deep-sea dwellers that use glowing lures to attract prey. This strategy is also observed in giant squids, which have been seen flashing bright lights while hunting, either to stun prey or draw them closer.
Many bioluminescent insects on land also employ similar hunting techniques. For example, glowworms in Australia and New Zealand create starry blue constellations in their caves, but this light is part of a hunting strategy. The worms spin nets of glowing silk to trap other insects. The larvae of click beetles operate similarly, hiding in holes and emitting a soft glow to attract insects, snapping shut around their meal when they approach.
On land, organisms have turned bioluminescence to their advantage for hunting, but perhaps the most mysterious use of bioluminescence is for mating and communication. Scientists recently discovered that the Humboldt squid uses various light patterns depending on the context, which may reflect messages communicated to other squids, such as dominance displays or claiming prey.
Lantern fishes, which include over 250 species swimming in the mesopelagic zone, have lights on their stomachs for camouflage and glowing spots on their heads. Researchers hypothesize that different species have subtly different glowing signals to find mates. This strategy may sound familiar, as many have watched lightning bugs dance through the darkness on summer nights, using bioluminescence for mating purposes with different species flashing at different intervals.
However, light displays like firefly dances are the exception rather than the rule for life on land. Despite its clear utility for some creatures, researchers believe it might be harder for terrestrial life to develop bioluminescence because the chemicals involved can be toxic and harder to cleanse when not swimming through water. However, toxicity is unlikely to fully explain why bioluminescence is rare on land, as it is also nearly non-existent in freshwater environments.
The answer may be simpler: life hasn’t existed on land and in freshwater as long as it has in the ocean, and evolution is a slow process. Perhaps it’s not that land-based organisms don’t have bioluminescence; it’s that they don’t have it yet.
It’s clear that we have much work to do to understand the past and future of bioluminescence. If we ever discover life on other planets, scientists suggest we should look for bioluminescence, as where there’s water and air, there’s a good chance we’ll find glowing life.
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If you’re looking for something else to watch right now, you can check out our previous video about the fascinating biology of sloths or watch Real Engineering’s latest video about the surprising way seat belts work.
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Bioluminescence – The production and emission of light by living organisms, often seen in marine animals like jellyfish and some types of fish. – Example sentence: The deep-sea squid uses bioluminescence to attract prey and communicate with other squids.
Ocean – A vast body of salt water that covers almost three-quarters of the Earth’s surface and is home to a diverse range of organisms. – Example sentence: The Pacific Ocean is the largest ocean on Earth and supports a wide variety of marine life.
Organisms – Living beings, including animals, plants, fungi, and microorganisms, that can grow, reproduce, and respond to their environment. – Example sentence: Coral reefs are ecosystems that host thousands of different organisms, each playing a role in the marine environment.
Adaptation – A change in a species over time that helps it survive and reproduce in its environment. – Example sentence: The thick fur of polar bears is an adaptation that allows them to stay warm in the Arctic climate.
Evolution – The process by which different kinds of living organisms develop and diversify from earlier forms during the history of the Earth. – Example sentence: The evolution of the giraffe’s long neck is thought to be an adaptation for reaching high leaves in trees.
Predators – Animals that hunt and eat other animals for food. – Example sentence: Sharks are top predators in the ocean, playing a crucial role in maintaining the balance of marine ecosystems.
Camouflage – A method of blending with the environment to avoid detection by predators or prey. – Example sentence: The chameleon uses camouflage to hide from predators by changing its skin color to match its surroundings.
Light – Visible energy emitted by the sun or other sources, which is essential for processes like photosynthesis in plants. – Example sentence: Sunlight provides the energy needed for plants to perform photosynthesis, which is crucial for life on Earth.
Marine – Related to the sea or ocean, especially concerning the plants and animals that live there. – Example sentence: Marine biologists study the diverse ecosystems found in the ocean to understand the life forms that inhabit them.
Darkness – The absence of light, which can affect the behavior and survival of organisms, especially in deep-sea environments. – Example sentence: Many deep-sea creatures have adapted to the darkness of the ocean depths by developing enhanced senses or bioluminescence.
