Ask a Nerd

AskANerd: Bioluminescence

Donna from Albuquerque, NM asks (in verse):

"I've always wanted to know, what makes a glow worm glow?"

The Nerd Responds:

Bioluminescence is the simple answer.  Bioluminescence is the ability of a living organism to produce light.  The key word for bioluminescence is produce.   For example, I'm sure you have seen a cat's eyes glowing at you in very dim light.   A cat's eyes can glow, but the cat is not producing this light.  The back of a cat's eyes are made of a highly reflective, almost mirror-like substance which a cat uses to gather up what little light there may be.  This gives cats excellent night vision and causes their eyes to flash or glow when the eyes catch any light.  However, in complete darkness a cat's eyes don't glow because their eyes do not produce any light. 

fireflya.gif (25866 bytes) To see this, look at the two pictures of a firefly and a cat in dark rooms.  The firefly is busy making its own light, but if you want to see the light from the cat's eyes, you must turn on the flashlight. eyes2.jpg (2801 bytes)

Click the button to turn on the flashlight

Bioluminescent creatures are different.  They do not depend on any external energy or light sources to make their light.  They produce it in their own bodies.  These creatures produce a chemical known as luciferin which, when combined with a few other catalyst enzymes and oxygen, produces visible light.  

A very interesting feature about bioluminescent light is that it is cold light.  Think of the last time you held a firefly in your hand and watched it give off its glow.  Did you ever notice that as bright as its light is, you don't feel any heat coming from the insect?  Now think of the last time you lit a match.  The light given off is comparable to that of a firefly, but the match also gives off an incredible amount of heat.  Why the difference?

All light is made the same way, namely by injecting an atom with extra energy so that it gives off a particle known as a photon.  The light we most commonly experience results from a thermal process.  That is, we keep heating an object until it finally has enough extra energy to release the desired photon.  The heat friction from the match, the electricity to a light bulb, and the nuclear fusion in the Sun are all examples of thermal processes that produce visible light, but also result in a lot of heat.

The cold light of the bioluminescent creatures results from the chemical process of luciferin, enzymes, and oxygen.  In this case, the chemical energy rather than thermal energy excites the atoms into giving up photons.  Since there is no thermal energy involved, no heat is associated with the light. 

On land we don't see much bioluminescence in life forms, but its actually very common and even comes in varying colors from dark blue, to green, to yellow, and even red.   Most bioluminescenct life exists in the oceans.  In fact, once you go below 600 or so feet, most of the animals in the ocean are bioluminescent.  These include creatures such as shrimp, squid, bacteria, jellyfish, mollusks, and fish.  On land this trait is limited to some algae and fungi, and a few worms and beetles.

Species typically use bioluminescence for defense or mating.  In Japan, for instance, Virgule crustaceans produce the luciferin chemical and then spit it into the water when stressed.  The chemicals are then oxidize by the water and begin glowing, providing a sort of smoke-screen to confuse their predators. 

On land, the light displays of the firefly are entirely for mating purposes.  The male (called a firefly or sometimes a lightening bug) uses the flashing light to alert females.  The female is a flightless insect (known often as a glow worm) that, instead of blinking, produces a long steady stream of soft light.  The male and female each seem to recognize the other's signal.  The light organs of these insects are located between the 6th and 7th abdomens and are composed of fat-bodied cells with many, many nerves which deliver the oxygen required to oxidize the chemicals.  Exactly how the male firefly controls his flash is unknown.  Scientists know that when it produces its luminescent chemicals, it also produces a chemical called adenosine triphosphate which is a muscle contraction chemical used by all animals.   Scientists theorize that this chemical opens and closes the flow of the luminescent chemicals.