It may come as a surprise, but glow sticks from store shelves and glow worms are not the only things that light up after dark. Many species use bioluminescence for a variety of reasons, including luring prey or mates and warding off predators. This tactic is seen primarily in deep-sea waters where little to no light can reach from the surface.
Bioluminescence is the result of a chemical reaction involving light produced from within an organism. In order for a reaction to take place, the creature must have or come in direct contact with the chemical luciferin. This molecule produces light when exposed to oxygen. A catalyst called luciferase can speed up the reaction and can be produced by many of these light-up creatures. Some combine luciferase and oxygen to form a photoprotein, a pre-made cocktail that is ready to illuminate when a certain ion is added such as calcium. Some creatures take in other bioluminescent creatures in order to produce the glow. For instance, the bobtail squid of Hawaii is born with a light organ that is filled with bioluminescent bacteria right after it is born. As a whole, bioluminescence is a form of chemiluminescence (meaning any chemical reaction that produces light) and bioluminescence designates such a reaction that takes place inside of a living being. The term fluorescence may be seen incorrectly used interchangeably with bioluminescence. Florescence is in fact not produced by a chemical reaction but by absorbing a stimulating light and then reemitting. This process is impossible without the presence of the stimulating light and does not involve the chemicals necessary for bioluminescence.
Deep-sea animals are the most common to possess bioluminescent capabilities, however bioluminescent creatures can be found all over the globe except in almost all natural freshwater habitats. On land, they come in the forms of fireflies, worms, and fungi. Such organisms can be found in just about any region of the oceans ranging from surface to the Hadalpelagic zone, the deepest parts of the oceans. It is more common in the deep-sea since visible light rarely travels down so far because light with longer wavelengths cannot reach. Blue-green light is the most common color as it has shorter wavelengths and can travel, and in turn be seen where other colors die out fast or simply do not exist at all. Violet, red, and orange are examples of long wavelength lights that cannot reach such depths. Many species use this to their advantage as some creatures are red in color (not glowing), allowing them to become effectively invisible to predators as most deep-sea creatures do not have the ability to see these colors. The dragonfish is an extraordinary exception. It is bioluminescent and produces red light, revealing red-colored prey and communicating to other dragonfish. Since most other creatures cannot see these signals, they are unaware of this predator’s presence until it is too late. Bioluminescence is also noted as a “cold light” because less than 20% of the light radiates heat, leaving creatures that hunt with thermal radiation in the dark. Most organisms that live in the deepest waters are able to glow through bioluminescence, but this phenomenon is not strictly for deep-sea creatures. Small plankton organisms close to the surface can also light up. Bioluminescent dinoflagellates bloom in dense layers on the surface of the water under particular conditions, making the ocean appear rust-red in color during the day and at night they appear to sparkle as the waves displace them. Some dinoflagellates are poisonous and these create harmful algae blooms, which can cause severe sickness and even death for other ocean creatures as well as humans who digest the algae itself or a creature that has.
There are several reasons why creatures light up in the sea but the most common reasons are to lure prey, to ward of dangerous predators, or to find a mate. Humans can sometimes see bioluminescent animals when a physical disturbance (such as waves or the rocking of a boat) trigger the animals to glow, but this is just a fraction of the bioluminescent creatures in the sea. Many bioluminescent organisms have complete or partial control of their lighting abilities and can communicate through light signals by varying the brightness or the color. Creatures use this ability as an offensive tactic to hunt prey in the murky depths. The most infamous of these creatures is the anglerfish that uses a bioluminescent orb (lit by glowing bacteria inside) attached to its head to attract unsuspecting fish. Like the anglerfish, the Stauroteuthis octopus brings dinner straight to its mouth, but this octopus uses bioluminescence around its beak to attract tiny plankton. A species of dragonfish called “loosejaws” emits light when hunting for prey as well. These fish can give off red light, illuminating areas of the ocean so that they are the only ones who can see the prey and the prey cannot see them. Cookie-cutter sharks attract food by releasing a glow from their underside, luring in larger whales and squid so they can snatch a bite when the victim is within reach of its wicked-sharp teeth. Worms and small crustaceans are commonly seen using flashes of light to attract mates. One of these crustaceans, the male Caribbean ostracod, has a bioluminescent upper lip that is used to entice females. In open water, the female Odontosyllis enopla lights up for males while swimming in a circular motion. Fish including the anglerfish, pony fish, and flashlight fish are all believed to light up to distinguish males from females and to find mating partners.
Many creatures utilize bioluminescence as a defense mechanism in several different ways. Squids living closer to the surface where light is visible tend to use dark ink to confuse predators and cloak their escape, but this would not be useful to deep-sea squids of the dark. Some squids, such as the large vampire squid, release bioluminescent mucus instead. This mucus is both bright and sticky, producing the same effect as the squid’s ink closer to the surface, and allowing the squid ample time to escape. The light created by creatures of the deep can scare off even the toughest of predators. Swima bombiviridis, better known as the green bomber worm, is one of a family of worms that releases a bioluminescent cloud when danger is near. This particular worm was discovered in 2009 as it lives close to the bottom of the sea. One trick some organisms use is the art of distraction. The deep-sea squid Octopoteuthis deletron, brittle stars, and some types of sea cucumbers break off pieces of bioluminescent body parts to attach to other fish. Brittle stars do this often since, like all sea stars, they can regrow limbs when necessary. These creatures trick the predator and are able to inch away to safety at the expense of another, unsuspecting organism. Camouflage is important to many animals from the deepest of seas to the canopies of the rainforest, to woodland animals. Bioluminescent marine animals have manipulated camouflage to their advantage through a technique called counterillumination. Predators that hunt from below, like sharks, watch for the shadows of creatures that swim through the light. Some organisms have photopores along their belly-side that mimic the light of the surface above, effectively blending in and tricking the predators below. Hatchet fish are a species that practices counterillumination through a bioluminescent organ on their undersides that the fish can manipulate, making them practically invisible if a predator is beneath them.
Many organisms are considered bioluminescent. Bacteria, sea stars, fish, sharks, crustaceans, worms, jellyfish, and algae all have species that produce light. A phenomenon known as “milky sea” is caused by bioluminescence as well. Unlike algae that flash periodically, the bacteria that creates this effect glows continuously. Millions of these tiny bacteria are necessary to produce the glow under particular water conditions. But, when it does occur, milky seas are bright and vast enough to be seen from satellites orbiting Earth, namely the warm waters of the Indian Ocean.