From https://en.wikipedia.org/wiki/Coral_reef_fish
Coral reef fish are fish which live amongst or in close relation to coral reefs. Coral reefs form complex ecosystems with tremendous biodiversity. Among the myriad inhabitants, the fish stand out as colorful and interesting to watch. Hundreds of species can exist in a small area of a healthy reef, many of them hidden or well camouflaged. Reef fish have developed many ingenious specializations adapted to survival on the reefs.
Coral reefs occupy less than one percent of the surface area of the world oceans, but still they provide a home for 25 percent of all marine fish species. Reef habitats are a sharp contrast to the open water habitats that make up the other 99% of the world oceans.
However, loss and degradation of coral reef habitat, increasing pollution, and over-fishing including the use of destructive fishing practices, are threatening the survival of the coral reefs and the associated reef fish.
Coral reefs are the result of millions of years of co-evolution among algae, invertebrates and fish. They have become crowded and complex environments, and the fish have evolved many ingenious ways of surviving. Most fishes found on coral reefs are ray-finned fishes, known for the characteristic sharp, bony rays and spines in their fins.These spines provide formidable defenses, and when erected they can usually be locked in place or are venomous. Many reef fish have also evolved cryptic coloration to confuse predators.[2]
Reef fish have also evolved complex adaptive behaviors. Small reef fish get protection from predators by hiding in reef crevices or by shoaling and schooling. Many reef fish confine themselves to one small neighborhood where every hiding place is known and can be immediately accessed. Others cruise the reefs for food in shoals, but return to a known area to hide when they are inactive. Resting small fish are still vulnerable to attack by crevice predators, so many fish, such as trigger-fish, squeeze into a small hiding place and wedge themselves by erecting their spines.[2]
As an example of the adaptations made by reef fish, the yellow tang is a herbivore which feeds on benthic turf algae. They also provide cleaner services to marine turtles, by removing algal growth from their shells. They do not tolerate other fish with the same colour or shape. When alarmed, the usually placid yellow tang can erect spines in its tail and slash at its opponent with rapid sideways movements.
Coral reefs contain the most diverse fish assemblages to be found anywhere on earth, with perhaps as many as 6,000–8,000 species that can be found dwelling within coral reef ecosystems of the world’s oceans.
The mechanisms that first led to, and continue to maintain, such concentrations of fish species on coral reefs has been widely debated over the last 50 years. While many reasons have been proposed, there is no general scientific consensus on which of these is the most influential, but it seems likely that a number of factors contribute. These include the rich habitat complexity and diversity inherent in coral reef ecosystems, the wide variety and temporal availability of food resources available to coral reef fishes, a host of pre and post larval settlement processes, and as yet unresolved interactions between all these factors.
There are two major regions of coral reef development recognized; the Indo-Pacific (which includes the Pacific and Indian Oceans as well as the Red Sea), and the tropical western Atlantic (also known as the “wider” or “greater” Caribbean). Each of these two regions contains its own unique coral reef fish fauna with no natural overlap in species. Of the two regions, the richest by far in terms of reef fish diversity is the Indo-Pacific where there are an estimated 4,000–5,000 species of fishes associated with coral reef habitats. Another 500–700 species can be found in the greater Caribbean region.