The diversity of reef life that can be found on the most diverse Pacific coral reefs is truly staggering, with perhaps as many as 700 or so species of corals present, along with perhaps 3,000 kinds of coral reef fishes.
However, all these species are not present throughout the entire region. Rather, most species are restricted (endemic) to particular (and sometimes quite small) portions of the region. For example, about 25% of all the inshore fishes found in Hawaii are endemic (found nowhere else).
Thus, although there is a common thread of like species throughout most of the region, many distinctive sub regions exist. Because of its sheer size and remoteness, much of the Pacific Ocean even today remains largely unexplored, and new species of fishes and other reef life are continually being discovered.
The center of biodiversity for the Indo-Pacific region as a whole is a rough triangle encompassing an area extending from the southern Philippines to eastern Indonesia and western New Guinea. The farther a reef is located from this center, the correspondingly fewer species of fishes, coral and other reef life will be found.
The Pacific Ocean proper includes most of this area, except for most of the reefs around the islands of Indonesia, which are located within the Indian Ocean.
There are two likely reasons for the far higher biodiversity found on Pacific coral reefs and their Caribbean counterparts.
First, the sheer size of the region occupied by Pacific coral reefs, as well as the length of coastline where corals can successfully grow is much greater than in the Caribbean, allowing more room and opportunities for species diversification.
Secondly, most Pacific coral reefs did not experience the severe temperature changes the Caribbean region suffered during the Pleistocene ice ages. This likely resulted in more Indo-Pacific species of coral, fishes, and other types of reef life surviving into the current post-glacial epoch.