The Hawaiian Islands are actually a chain of undersea volcanoes that rise from the sea floor. The islands that we see are the top portions of these underwater mountains breaking through the surface. They stretch over a distance of 1500 miles, from the Island of Hawai'i in the southeast (formed less than 1 million years ago) to Kure Atoll in the northwest (formed over 26 million years ago). The islands were all formed at a hot spot where the Island of Hawai'i is presently located. The movement of the ocean plate on which they sit (the Pacific plate) has been moving these islands northwest for millions of years. As each island moved away from the hot spot, the source of the flowing lava, it no longer increased in size but instead began to erode. The shear weight of the islands also cause them to slowly sink into the Earth’s crust. Therefore, the farther northwest an island is from the hot spot, the older it is and the more it has eroded away and sunk. As a result, all that is left in the northwestern Hawaiian Islands are atolls formed by millions of years of coral growth.  At the center of each atoll is a lagoon where once the top of a volcanic island was located.

 The Hawai'i Island Chain is also one of the most isolated groups of islands in the world. The nearest island (Johnston Island) is 450 miles to the south, but it is more than a 1000 miles to any other piece of land. This extreme isolation has had a major impact on both the marine and terrestrial species found here. Since each of these islands began as a barren, lifeless volcanic island, all life, including the coral and the reef fish, had to arrive from some distant place. An important part of almost all coral reef fish's life cycle is a pelagic larval stage. This means that after the eggs of coral reef fish hatch they spend their first few days or months floating on the surface currents of the open ocean as members of the zooplankton. Most of these larval fish don't survive. The few that do survive must find a suitable habitat once they reach the juvenile stage. Because the Hawaiian Islands are so far from any other coral reefs only a small percentage of those surviving larval fish ever reach Hawai'i. The result is that there is a much smaller number of fish species found in Hawaiian coral reefs than on reefs in other parts of the tropical Pacific. Hawai'i only has about 680 species, of which about 420 are found in shallow, coastal waters. As a comparison, Micronesia has about 1,400 species and the Philippines has about 2,000 species.

Endemism

Another result of Hawai'i's isolation is the high percentage of fish species that are "endemic" to Hawai'i, which means that they are found nowhere else in the world. Some of the most common fish seen in Hawaiian coral reefs are endemic such as the Saddle Wrasse and the Milletseed Butterflyfish. Island biogeography is the study of why certain species are found here, how they have evolved in this environment and become specialized and how they are distributed throughout the Hawaiian Island Chain.

The Proportion of Endemic Hawaiian Species, by Family: