Tropical reefs might look like inanimate rock, but these colorful seascapes are built by tiny jellyfish-like animals called corals. While adult corals build solid structures that are firmly attached to the sea floor, baby corals are not confined to their reefs. They can drift with ocean currents over great distances to new locations that might give them a better chance of survival.
The underwater cities that corals construct are home to about a quarter of all known marine species. They are incredibly important for humans, too, contributing at least a trillion dollars per year in ecosystem services, such as protecting coastlines from wave damage and supporting fisheries and tourism.
