Why and how do some animals give light?

Answer

That’s a very nice question. In fact, there are an amazing number of animals that give off light. It is especially common in the deep ocean: as many as 9 out of 10 animal species would give light there.
There is also not one reason why they give light, for different animals there can be different reasons. For some animals it is a way of attracting prey, or they use the lighting to find prey. For others, it’s just a way to scare off enemies. Or they use the light to find their way, or as a way to exchange signals with peers.
To understand how they produce that light, you should actually know something about molecules. All living beings are made up of many cells, and those cells are made up of many molecules. A certain type of molecules, the proteins, play a very important role in the functioning of the cells: they ensure, among other things, that chemical reactions take place that are necessary for the cells to work properly. One of those proteins is found in luminous animals, and it has been given a very difficult name: luciferase. The cells also need another molecule to make light, called luciferin. The luciferase causes a chemical change in the luciferin, and the result is that the luciferin starts to emit light – indeed, as if the cell were striking a very small match.