Rejuvenating reefs

THE waters off the Hawaiian island of Oahu are visited each winter by migrating marine mammals such as humpback whales. All year round they are home to much smaller animals that form vast reefs: corals. Intricate pink structures stand out amid contortions of vegetable-green ones; dark-striped fish flit among them and turtles hover above. Corals lay down limestone skeletons of different shapes and sizes: branching types like small trees; ground-huggers spreading squat.

The colours that lure snorkelling and scuba-driving tourists are produced bysingle-celled algae that grow symbiotically in corals’ tissue. These use carbon dioxide respired by their host to make oxygen and carbohydrates through photosynthesis, giving corals most of the energy they need to form their skeletons. But this delicate balance is threatened by humans, both in the short term and over the coming years.