Coral reefs are home to a quarter of all marine species. Survival in these undersea mega-cities is a challenge with many different solutions.

A turtle heads to the reef's equivalent of a health spa - but she must use trickery to avoid the queue. A remarkable Grouper uses the fish equivalent of sign language to collaborate with an octopus, flushing their prey out of hiding holes.

A metre-long, ferocious-jawed Bobbit Worm hides in its tunnel. Monocle Bream retaliate by squirting water to expose its sandy lair.

In the undersea forests of kelp, algae and sea-grass, there is competition between species and sea otters are seen helping Garibaldi fish protect their seaweed garden from sea urchins while the common octopus uses shells to create a coat of armour against sharks.