Coral reefs provide a home for at least 25% of all marine species. Although reefs cover less than 1% of the ocean floor, they support exceptionally diverse communities of fish, mollusks, crustaceans, worms, sponges, algae, and other organisms.
That richness comes from the reef’s three-dimensional structure. Coral colonies build hard skeletons that create ledges, crevices, caves, and sheltered lagoons. Different animals occupy different spaces, while complex food webs connect algae, plankton, herbivores, predators, and scavengers.
The figure is often reported as “over one-quarter” rather than as an exact census. Scientists continue to discover and classify reef species, and estimates vary depending on whether the count includes species that use reefs temporarily or depend on nearby reef habitats.
A common mix-up is confusing coral reefs with the open ocean. Reefs thrive in nutrient-poor tropical waters partly because nutrients are recycled efficiently within the ecosystem, allowing a small area to support a remarkable concentration of life.