Oceans and seas covers most of the Earth’s surface, performing many important functions for our planet, such as regulating our climate and weather. Our oceans also host a great diversity of living organisms, around 80 percent of our planet’s diversity, and offer unique habitats for marine life. The mysterious and deepest area of the sea—where the light is rare—is an example of challenging places for organisms to survive.
In the ocean, ambient light provides either an extended scene of visual features or point-like flashes coming from bioluminescent organisms (Warrant & Locket, 2004). In shallower depths, the extended type of light allows organisms to see their surroundings in all direction, find prey, or avoid predators.
As the oceans go deeper, this extended light wanes. The underwater surroundings become darker and the point-like light sources start to appear. In this deep sea (greater than 200 meters deep), some of the most remarkable adaptations exist, such as bioluminescence and ultra-black fish.
Bioluminescence of Marine Organisms
Bioluminescence is a chemical reaction by a living organism that produces light. Around summer time, you often see the light-producing chemical reaction from fireflies (Photinus pyralis). However, this reaction is more common in marine life than terrestrial habitats.
How Does Bioluminescence Work?
To make light, organisms needs an enzyme, either a luciferase or a photoprotein. With the presence of a substrate (for example coelenterazine), this enzyme catalyzes a chemical reaction with oxygen, producing a product and light energy. This chemical reaction often requires other ingredients, such as adenosine triphosphates and calcium.
Who Makes Bioluminescence?
Large varieties of organisms living in marine habitats produce bioluminescence. This huge list of bioluminescent sources includes bacteria, planktonic algae (for example dinoflagellates), jellyfish, cnidarians, crustaceans, sea stars, and fish. Although many bioluminescent organisms make their own enzyme and luciferin, some of them absorb a substrate from their food.
Below are some examples of different substrate groups:
- Coelenterazine: Coelenterazine is the most common bioluminescent molecule in marine life. This name came from a group of organisms, cnidarians (including the jellyfish Aequorea and the sea pansy Renilla). However, other organisms including squid Watasenia and the shrimp Oplophorus also produce coelenterazine (Haddock et al. 2010). Coelenterazine is a substrate for Renilla luciferase and a calcium-binding apoaequorin photoprotein.
- Ostracod luciferin: this type of luciferin (or vargulin), found in a small ostracod crustacean called Vargula (formerly Cypridina), consists of three amino acids: tryptophan, isoleucine and arginine (Haddock et al., 2010).
Glowing shrimp. Bioluminescent shrimp or sea fireflies Vargula leave spectacular blue light trails on the beach in Okayama, Japan.
- Dinoflagellate luciferin: the structure of dinoflagellate luciferin (a tetrapyrrole) is almost similar to chlorophyll, but it has metal ions (Haddock et al., 2010).
- Bacterial luciferin: Some bacteria use FMNH2 as a luciferin substrate for bioluminescence (Wilson & Hastings, 1998). The chemical reaction requires FMNH2, a luciferase, and an aldehyde.
- Finding food: by either built-in headlights, or the use of glowing lures to attract its prey.
- Communication: using species-specific spatial and temporal patterns of light emission.
- Defense against predators: by distracting or blinding predators; signaling predators that it tastes bad; warning others about the presence of predators; or marking its predators with a luminescent slime, so secondary predators can see and capture them.
What Color is Bioluminescence?
Many organisms actively use the colors of bioluminescence as an adaptation in the deep sea. Light at shorter wavelength, such as green and blue, travels farther down in the ocean, even to the zone deeper than 100 meters. This is probably why most marine organisms emit green or blue (Hastings, 1996) to hide themselves in their surroundings.
In coastal areas, marine organisms typically produce light in the green (490-520 nm), whereas the majority of organisms in the deep sea emit blue light (at 450-490 nm) (Hastings, 1996).
Light at longer wavelengths travels on the shallower sea, but fails to reach the deep-sea zone. Therefore, some deep-sea organisms use red pigmentations on their skin to make them invisible (Johnsen et al., 2005). Although rare, three species of deep-sea dragonfish see and emit red light (with peak emission around 705 nm) from organs below their eyes, in addition to blue light emission (Herring & Cope, 2005; Widder et al., 1984).
Why Bioluminescence is Used in the Deep Sea?
Marine organisms use bioluminescence to help them in many ways (Widder, 2010):
Bioluminescence in Jellyfish. Glowing jellyfish use bioluminescence to startle their predators as a defense strategy.
4. Camouflage (counter illumination): by concealing its silhouette with bioluminescence to match the brightness of its surroundings and resemble its environment.
Bioluminescence is not the only strategy used by marine organisms to survive in the deep sea. Another way fish blend into their surroundings is by using dark coloration to avoid predators.
Ultra-Black Fish of the Deep-Sea
In the press release, Dr. Karen Osborn from Smithsonian’s National Museum of Natural History explained after taking those samples from the deep sea, she could not get a detailed image of some extraordinary black fish caught in the trawl nets. No matter how she set up the camera or lighting, those fish sucked up all the light.
Ultra-black Pacific blackdragon fish, Idiacanthus antrostomus. This fish has a bioluminescent lure and light-producing organs below their eyes as a headlight to find a prey. Its ultra-black skin conceals their presence from their prey from their own light (Photo used with permission |image credit: Karen Osborn, Smithsonian Institution).
To investigate this phenomenon, Dr. Osborn and her colleague used a special black-reflectance light probe. This probe measured the angles and the amount of light absorbed from 18 ultra-black fish from the Gulf of Mexico and Monterey Bay. They found skin of 16 species reflected less than 0.5% of light, whereas two fish reflected less than 0.6% at 480 nm (Davis et al., 2020). This is the peak wavelength of deep-sea ambient sunlight and most bioluminescence in the ocean.
Ultra-black Fangtooth fish, Anoplogaster carnuta. This specimen was one of the 18 ultra-black fish that was caught in a trawl net (Photo used with permission |image credit: Karen Osborn, Smithsonian Institution).
The researchers found that skin from nine of those fish have a continuous and thin layer of melanosomes or organelles containing melanin (the same pigment that gives dark color to our skin). This structure is responsible for producing ultra-black coloration and scattering light sideways within the layer to increase light absorption (Davis et al., 2020). To imagine how black this color actually is, you can imagine the skin of ultra-black fish is blacker than electrical tape or a brand-new tire.
Ultra-black fish absorb more than 99.5% of the light landing on their skin, making them almost invisible to others. In the darkness of the deep ocean, a single ray of light will attract attention of many preys or predators. Therefore, the blacker the organism, the higher chance of survival it has.
While having ultra-black skin is a good strategy to camouflage, researchers thought this finding could inspire people to develop a better technology for us, such as telescopes, cameras, or camouflage materials.
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