Vampire squid and bioluminescence
Glowing Water
The 'Milky Seas' phenomenon was recounted by many mariners, but was thought to be a legend created by hallucinating sailors. It was also mentioned in Jules Verne's 'A Thousand Leagues Under the Sea'.
If you have seen the popular Pixar movie Finding Nemo, you must have seen the scene where Dory and Marlin encounter a fish that attracts prey with an appendage attached to its body. That appendage glows, and Dory and Marlin almost become a snack for the predator. The predator is called an anglerfish, and the orb attached to its body glows due to the phenomenon called bioluminescence. Anglerfish use it for the purpose of predation, although it is used for many more purposes by different creatures.

Bioluminescence is caused by a reaction between two chemicals inside the creature's body. Thus, it is a type of chemiluminescence. It should not be confused with radiation, incandescence, fluorescence, and phosphorescence. Incandescence is where light is produced due to heat. This is very similar to the incandescent bulbs we used, and still use. The temperature of the filament that glows in the bulb reaches a temperature of up to 700°C, and requires 100 watts of power to keep it on; a feat a simple creature like a firefly cannot achieve. Fluorescence is where light is first absorbed and then released simultaneously. Fluorescence needs a light source, and cannot happen in completely dark places. Phosphorescence is a subset of fluorescence, but the absorbed light is released for an extended duration of time, and hence, can show in the dark. But it does need a light source at the start to absorb the light. This phenomenon is shown by marine creatures, and hence, is easily confused for bioluminescence.
A sudden burst of luciferase...
Eastern firefly
...and there's light!
Bioluminescence properties
♦ Bioluminescence does NOT need a light source.
♦ Bioluminescence is possible in complete darkness.
♦ Bioluminescence does not generate heat while generating light.
Bioluminescence phenomenon
The phenomenon is usually achieved by mixing of two chemicals, namely luciferin and luciferase. Luciferin is the chemical that undergoes oxidation to give off light. Luciferase is the enzyme or catalyst that is used to accelerate the reaction. Although these are the main chemicals used to generate light, there are other chemicals secreted too.

So, if I had a test tube of luciferin and oxygen, the test tube would glow faintly.
Bioluminescent Sea Squirts
Fluorescent blue tunicates
If I added a bit of luciferase to that test tube, the glow would increase. If I added a lot of it, the brightness of the glow will increase even more, because it is speeding up the reaction by which light is generated. But this also means that the bright light will glow for a shorter period of time.

In an organism like the firefly, these two compounds are naturally made in the body. When it wants to emit a glow, the firefly will let in a flush of luciferase, and we see the glow emitted. This is how we see the lights blinking from its body, because the fly is controlling the bursts of luciferase.

The phenomenon of bioluminescence is due to evolution. It is present more in deep sea creatures than on land-based ones. In the deep sea, the visibility of light reduces as we go deeper. After a particular point, it is extremely dark down there, even during the day. Even if we do not know, there is a thriving ecosystem, indigenous to the deep oceans.

Here, for various reasons such as evasion, prey luring, and navigation, many creatures developed bioluminescence as a characteristic for sustenance and survival. Some developed phosphorescence and some developed fluorescence. The reasons for this evolution will become clearer as we see the reasons for the phenomenon with examples below.
Luring a Prey, by Anglerfish.
Predator and prey
♦ Anglerfish are very efficient predators in the deep sea. They have a fishing rod-like dangling aperture attached to the top of their body.
♦ The tip of this fishing rod has a bulbous swell that glows. Small fish are attracted to this light and swim towards it.
♦ The luminous bulb is right in front of the anglerfish's mouth. This makes it easier for an efficient ambush.
♦ Close research was conducted on this fish, and it was revealed that, by itself, it cannot produce all the chemicals required for luminescence.

A special class of bacteria has a symbiotic relationship with the fish, and lives inside the bulb to produce the enzyme for the luminescence. In turn, it gets appropriate nutrition for survival.
Stun, Startle, and Confuse, by Squid.
Dorling kindersley
♦ We are very familiar with the concept of an octopus spraying ink to confuse its predator, and then escaping ninja style.
♦ A similar thing happens with vampire squid, that spew bioluminescent mucus for as long as an astonishing 10 minutes. This is in contrast to the octopus, that expels the ink and escapes all of a sudden.
♦ The main advantage of this evasion tactic is that the spew is relatively brighter than its surroundings, and hence, the predator is more attracted to the brighter surrounding, while the squid can escape into the darkness.

This maneuver, however, is kept as a last resort by the squid, because to reproduce all that ink takes a huge amount of energy.
Milky Seas, by Bacteria
Red tide
♦ The strain of bacteria named Vibrio fischeri, demonstrates microbial bioluminescence. It is the same bacteria present in the bulb of anglerfish and many other bioluminescent creatures.
♦ The bacteria is present in abundance in the ocean, but is very scattered.
♦ On special occasions, there have been cases where a great span of the ocean was covered by these bacteria, and they, all at once, emit a glow. The cumulative glow was good enough to be picked up by a satellite.
♦ This could have been due to the fact that the expanse the bacteria covered was 6,000 square miles (15,500 square kilometers)!

This strain of bacteria uses bioluminescence for mating, communication, and warding off predators. It is thought that when glowing in a large group, it gives the illusion to the predator that it is facing a creature far bigger than itself.
Burglar Alarm, by Jellyfish
Jelly fish
♦ Jellyfish usually swim in swarms, sometimes in very large ones.
♦ If a predator tries to infiltrate the swarm, the jellyfish at the most exterior will illuminate, which will alert the jellyfish in the vicinity.
♦ This spreads like a chain reaction, which in turn alerts the whole group.
♦ This tactic can also be used for predation, although it is rare. The tactic, however, is popularly used to deter settlers, such as another school of fish. When the invading school sees the illuminated jellyfish, it will take evasive action.
Calling for Love, Tough Love, by Fireflies
♦ Fireflies have the classic reaction in their bodies that produces natural luminescence. Their bodies have perfected this phenomenon.
♦ This is because the energy required to create light is extremely efficiently used. The efficiency is very close to 100%. This is in stark contrast to the electronic LEDs we use, of which, the best have an efficiency of 22%, and incandescent bulbs which have an efficiency of 10%.
♦ The purpose of this luminosity was thought to be a warning, but later studies related it to mating calls.
♦ Mating calls range from simple to complex. A simple one would be a random sequence of pulses of light.
♦ A more complex one will have a specific pattern, which in turn, is returned by the female if she wants to mate.
♦ When there are multiple males present, the pulses synchronize, and the pulses of light are extremely coordinated, leaving onlookers in awe. This happens every July in the Great Smoky Mountains, Tennessee.
♦ A hungry female can also use her luminescence to attract a male. After they copulate, the female kills the male and eats it, thus satisfying her hunger and the pressure to propagate.
Blending into the Light, by Cookiecutter Shark
♦ 95% of the fish hide in the depths known as the twilight zone. This depth ranges from 200 to 1,000 meters under the surface of the sea. The light from the surface is seen as a faint glow at this depth.
♦ For the predators at this depth, an efficient technique of predation is that of looking for a shadow or a silhouette of a fish.
♦ Fish such as the cookiecutter sharks have a network of light-producing layer on their underbelly/ventral surface.
♦ This technique is known as counter-illumination.
♦ Due to this, the predator who is swimming underneath the cookiecutter will not be able to notice the shadow as efficiently, thus letting the shark evade predation.
♦ If the cookiecutter is below the level of the predator, the light that reaches that depth is very faint, and the cookiecutter can just get away in low light.

This camouflage technique is shown by certain species of squid, and some varieties of hatchetfish too.

The two cases, namely that of using bioluminescence for predation and counter-illumination reveals even more interesting details. The anglerfish and cookiecutter shark can control the illumination levels with respect to their depths. For example, during daylight, the anglerfish has to hunt at greater depths, since the light will penetrate deeper. As the day progresses and night falls, the anglerfish moves towards the surface to hunt.

Fish that show counter-illumination have biological equivalents of photodiodes, called photoreceptor cells, that gauge the intensity or brightness of light. If the light is low, the luminosity on their ventral side will be low too. This ensures that there is no difference in the intensity of luminosity between the background light and the light the fish are emitting. Any significant difference can make a predator suspicious. and lure it in. For example, if the cookiecutter shark illuminates its belly more than the background light, it will attract predators, foiling its camouflage bid.
A Disadvantage
Tree frog after firefly
♦ A creature like the firefly uses bioluminescence for attracting mates, and sometimes for communication purposes.
♦ If there is a predator around, like a tree frog, the firefly is doomed. The blinking lights will attract the frog's attention.
♦ The firefly can switch the light off, but it will mean a lesser chance of finding a mate. Although tree frogs are not exclusive predators of fireflies, the fireflies can risk capture for finding a mate.

Bioluminescence is very interesting to study, and new breakthroughs are happening every day. The study on this subject led to the invention of glow sticks. Many navies used the counter-illumination technique to evade enemies. Bioluminescent imaging is used to study the targeting capabilities of anti-cancer drugs. It is also used in bioluminescence imaging, a much cheaper way to diagnose. New projects are being forayed into, like the Glowing Plant Project, where scientists intend to make plants bioluminescent, so that there will be no need of street lamps.

The phenomenon has many other benefits, because of its efficiency seen in animals. Thus, it makes it not only beautiful, but useful too.