Evolution of real glowfish

Bioluminescence, the production and emission of light from a living organism, is a fascinating phenomenon that is documented in over 700 genera of metazoans across the tree of life, with the vast majority living in the ocean. Among vertebrates, bioluminescence has evolved in cartilaginous (Chondrichthyes) and ray-finned fishes (Actinopterygii), and it is not observed in any lobe-finned fishes or tetrapods.

Bioluminescence is  a form of chemiluminescence produced in living organisms. The substrate luciferin reacts with oxygen and the enzyme luciferase and the reaction releases a photon:

A new study aimed to determine the number of independent evolutionary origins of bioluminescence in ray-finned fishes. The authors discovered that bioluminescence has evolved many more times than previously hypothesized across fishes and the tree of life. Actually,  the colleagues showed that it has evolved independently 27 times in 14 major fish clades on of which includes the most abundant vertrebrate on Earth, the bristlemouth, numbering in the hundreds of trillions to quadrillions. 

There is a variety of ways bony fish can deploy bioluminescence, such as leveraging bioluminescent bacteria, channeling light though fiber-optic-like systems, or using specialized light-producing organs.  This shows how important bioluminescence is to vertebrate fish that life in the world's deep seas.

You have this whole habitat where everything that's not living at the top or bottom of the ocean or along the edges - nearly every vertebrate living in the open water - around 80 percent of those fish species are bioluminescent. So this tells us bioluminescence is almost a requirement for fishes to be successful.

The study also shows that once an evolutionary line of fish developed the ability to produce light, it tended soon thereafter to branch into many new species. The process is not fully understood and will require further study but there are some ideas:

Our exploration of the macroevolutionary patterns of bioluminescent lineages indicates that the present day diversity of some inshore and deep-sea bioluminescent fish lineages that use bioluminescence for communication, feeding, and reproduction exhibit exceptional species richness given clade age. We show that exceptional species richness occurs particularly in deep-sea fishes with intrinsic bioluminescent systems and both shallow water and deep-sea lineages with luminescent systems used for communication.