There are few terrestrial organisms that emit light, only a few species such as fireflies, while almost all categories of marine organisms, from bacteria to fish, have luminous species. So why do marine organisms glow?
Let's start with unicellular algae. There is a famous scenic spot "fluorescence bay" on Puerto Rico island in Latin America. At night, you take a boat through the mangroves and enter the sea. As long as you stir it by hand, the dark fluorescence will be emitted on the water surface. The faster you stir it, the more fluorescence will be. Even where the boat passes, there is also a luminous ship trail, which is the light emitted by the single-cell plant dinoflagellate. Why does dinoflagellate glow? Scientists speculate that it is for defense: the natural enemy of dinoflagellates is copepod arthropods, that is, fish, insects or water fleas. Sudden luminescence can scare them off, or use light to attract larger animals and eat copepods first.
This "light-emitting enemy retreat method" is a common trick of marine organisms, not the patent of dinoflagellate. The same is true of echinoderms: when you touch it suddenly, its wrists will glow to scare off incoming enemies. Another function of luminescence is to mislead the enemy. For example, the luminescence of the abdomen of some squids is a kind of camouflage, which is similar to the light of the surrounding seawater to avoid being found by the enemy; Some squids emit light at the end of their feet. In an emergency, they can give up the luminous touch, which is enough to mislead the enemy and escape by "breaking their wrists".
Of course, marine bioluminescence is not only for protection, some use luminescence as bait to attract prey, which is a common "technology" of many carnivorous fish. In addition, in the dark deep sea, luminescence is also an important intraspecific communication tool. Some fish, octopus and ostracods have this function and rely on luminescence to "date" during the mating season.
In fact, we don't know much about the truth of marine bioluminescence. For example, both types of single celled plankton have proteinaceous skeletons, in which radiolarians can emit light, while diatoms do not. Besides, the use of light is also controversial. Some people say that some marine bioluminescence is not for use, but a residual habit of early evolution. For example, dinoflagellates evolved very early. At that time, the earth was still in a reducing environment, free oxygen was harmful, and luminescence was an oxidation, so it was a way to consume oxygen and avoid damage. Now the earth's atmosphere has been oxidized, but the ancient performance has been preserved. On the contrary, diatoms evolved much later. When they appeared, the atmosphere had been oxidized and this method was no longer needed, so diatoms would not emit light. Of course, this is just a conjecture, which needs to be verified in the future.