A calm clear sky is the perfect opportunity to go camping or stargazing, but if you’re out near the ocean - you may see another natural wonder. A special show where the water glows in a bright blue and green light and flashes with the rippling movements of the waves. Just as nuclear reactions deep within stars generates the bright glow we see from earth, there too exists a natural phenomenon where chemical reactions within living organisms create a twinkling light within.
Once considered a mysterious phenomenon called “sea fire” or “sea twinkle” by coastal dwellers and sailors, we now know this process to be bioluminescence.  The meaning of this term comes from the root words in Greek and Latin: “bios” is life in Greek and “lumin” is the Latin for light. 
Bioluminescence is a very simple chemical reaction. When oxygen and a compound called luciferin, interact with an enzyme known as luciferase, an energy is created which is then released as light. 
© Vincent Hyland, Wild Derrynane
A light in the dark
We often ask, why do marine animals create light? Bioluminescence is present in many animals on land too. The firefly bug is a very well-known example of a land animal which produces light to attract mates or prey. 
In fact, many species have the ability to either create light or reflect it. Certain animals cannot produce their own light and therefore utilize bioluminescent bacteria or plankton to emit light on their behalf. This is known as a symbiotic relationship, as both the animals and the plankton or bacteria gain something from the encounter. Often the plankton or bacteria gain shelter or nutrients from the host animal.
However, it is true that most of the species on the planet which manipulate and create light, are found in the ocean. That’s because in the depths of the ocean there is an absence of light from the sun. Due to this, organisms create their own light to adapt to the dark. Examples of marine organisms which are bioluminescent are bacteria, algae, jellyfish, worms, crustaceans, sea stars and fish. 
There are many reasons why marine organisms use light in the dark abyss of the ocean. It can be a way for organisms to locate and communicate with one another in a colourless world. They can use a flashing light to attract and find a mate.  Light can also be a shocking defence mechanism against predators which can startle them away from attacking. The atolla jellyfish use flashes of red light as a warning signal in response to being attacked. This warning signal will attract other organisms which will scare away the predator.  It has even gotten the nickname of “alarm jellyfish”. 
Counter-illumination, which is when the bioluminescent light from an animal reflects the light of the environment, is an optical illusion that many underwater fish, squid, crustaceans and even sharks use to camouflage themselves with the environment.  Light can also be used as a lure for food. Anglerfish use their light to lure in prey. A tiny glowing bacterium lives on the anglerfish’s esca (which is the “lure”). This is a perfect example of how bioluminescence is often a symbiotic process. This bacteria gains protection while the anglerfish gets to use their light to attract food into its grasp. 
What causes a glowing sea?
The bioluminescent blue glow of the waves you see at night-time in late summer and early autumn is actually a plankton defence mechanism! The plankton startle their predators by flashing their light. They also hope the light wards off other potential predators too. As the waves and crashing sea move and mix the plankton it activates their defence mechanism and causes them to emit their bright green and blue glow. This is why you are able to activate the glow by splashing your hand in the water during a bioluminescent event.
Not all plankton use bioluminescence. Dinoflagellates are an algae which are the most common form of marine phytoplankton which create their own light by using luciferin. When water quality and nutrient supply is just right, these conditions allow the colonies to expand to 100,000 single cells in one litre of water. 
With the sun growing strong throughout the day, these dinoflagellates can be charged, almost like a solar lamp in your back garden. The luciferin-luciferase reaction lets them sparkle and glow throughout the night. Noctiluca scintillans is one of the most common bioluminescent dinoflagellates across the world and the one you are most likely to see lighting up the Iveragh peninsula. It is often fondly referred to as “sea sparkle”. It is found in coastal waters. The name comes from Latin; Noctiluca means “light, light at night” and scintillans means “shining, throwing out flashes of light”. 
When and where to see bioluminescence?
Bioluminescent plankton ecosystems are rare, mostly forming in warm-water lagoons with narrow openings to the open sea. Bioluminescent dinoflagellates get swept in these lagoons or bays, and the narrow opening prevents them from escaping. A whole lagoon can be illuminated at night.
The Kerry International Dark Sky Reserve creates the perfect conditions and visibility for observing bioluminescence. The lack of light pollution allows the glow to be seen in its full intensity. Bioluminescence may be seen at many beaches along the Iveragh peninsula, however Derrynane beach provides a perfect embayed area for bioluminescent plankton to gather in the bay and bloom.
It is the calm, warm seas during summer months which allow for the perfect conditions for the phytoplankton populations to grow from the end of July up to early October. Although the light from the bioluminescence is quite vivid and bright, the light of a full moon might distract from the twinkling seas. It’s best to try go three days before or five days after the new moon. 
A calm, clear night is when to best expect to see bioluminescence. And it’s always worth giving the sea a little nudge yourself and splashing the water in your hands to activate it!
A singular Noctiluca scintillans dinoflagellate seen under the stereomicroscope.
Image of the dinoflagellate Noctiluca scintillans (phytoplankton) under the stereoscope ©Hannah Brownlow, The Plankton Club UCC
This video shows Notciluca scintillans under a microscope. It is a circular single celled organism. You can see the blue glow just around the edges of it’s body. We see here how light can directly effect the ability to see the planktonic glow, as the amount of light in the microscope is increased and decreased. Noctiluca is usually between 400 to 1500 μm (micrometer) in length. The white shape in the middle of the body is the central nucleus, and sometimes if you look closely you can see little white lines or striae radiating out from the nucleus to the edge of the body shape.