A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups.
Seagrasses are not true grasses but are flowering plants that carry out their entire lifecycles underwater. Like all plants, seagrasses rely on sunlight to convert carbon dioxide into food/energy (via a process called photosynthesis). Therefore, they only succeed in clear, shallow waters. When the conditions are just right, seagrasses can densely cover the sea floor, creating an ecosystem known as the seagrass bed or seagrass meadow.
A biodiversity hotspot is a biogeographic region with significant levels of biodiversity that is threatened with destruction. For example, forests are considered as biodiversity hotspots.