Reef-building corals were
confined to relatively shallow depths because
many of these corals have microscopic algae
called zooxanthellae living inside their soft tissues.
These algae
are often important for the corals’ nutrition and
growth, but require sunlight for photosynthesis. There is no sunlight deep in the water.
The closest point to the Sun in an orbit is called the perihelion.
The two ways that oceanographers can identify ocean currents are by mapping the path of debris from vessels and by studying the chemical and physical components of the ocean water.