Flooded World

Flooded World is set in shallow seas of the world, 100,000,000 years into the future. The episode focusses on three species which all live together.
Ocean Phantom - a type of Siphonophora that can grow to ten metres in diameter.
Reef Glider - a descendant of the sea slug which hunts ocean phantoms. The young, however, are prey to the ocean phantom.
Spindle Trooper - spider-like creatures that live in chambers on the ocean phantom and protect it from danger. The ocean phantom in return feeds it.
This episode focusses on the extinction of the coral reefs and their replacement with red algae reefs. It shows how this food chain works and how the animals have evolved together. It also shows how ocean phantoms can be ripped apart by a severe storm but still carry on living.