The weird and wonderful underwater world of coral reefs

Often mistaken for rocks or plants, corals are actually invertebrate animals. Found in tropical and subtropical waters, from the Caribbean to Hong Kong (pictured), these creatures are under threat from climate change. Scroll through to learn more about corals and their underwater world.

Almost all corals belong to an animal group called Anthozoa, which includes hard corals, soft corals, sea anemones and sea fans.

Hard corals, or stony corals, have a rock-like appearance and are known as reef builders. Hard corals create a hard limestone exoskeleton which form reefs, like this honeycomb coral. They rely on tiny algae to survive, which also give the corals much of their color.

Platygyra, known as "brain coral," is another of these hard corals. Its maze-like valleys attract marine life that can nest or hide from predators in its ridges.

Soft corals look more like plants than rocks; this gorgonian coral in the Philippines' UNESCO World Heritage site of Tubbataha has a tree-like appearance, and marine life often lives in its branches.

Both hard and soft corals are made up of hundreds or thousands of individual organisms called "polyps." Related to the jellyfish family, these polyps have tentacles -- multiples of six for hard corals, and eight for soft (pictured).

The Coral Triangle -- encompassing 2.3 million square miles which includes the oceans surrounding the Philippines, Malaysia, Timor-Leste, Papua New Guinea, the Solomon Islands and Indonesia (pictured) -- has the most coral diversity in the world, with over 600 species.

Coral reefs cover less than 1% of the ocean floor, but more than 25% of the world's known marine life live there -- including more than 2,200 fish species in the Coral Triangle alone.

Most coral reefs are found between 30 degrees north and south of the equator in warm tropical waters. This includes the Caribbean (pictured) and the Great Barrier Reef in Australia, as well as South East Asia's Coral Triangle.

While reef-building corals only live in shallow water, where the sunlight can reach the algae they coexist with, some corals like the spiral whip can thrive in deep water. Some corals have even been found 20,000 feet below the surface.

But coral are under threat. Scientists predict that increasing ocean temperatures and water acidity may destroy up to 90% of coral reefs around the world over the next two decades. The increased temperatures lead to coral "bleaching", when the corals reject the algae they rely on to live, as seen at this reef in French Polynesia.

Coral reefs support underwater ecosystems, such as seagrass meadows, which are important carbon stores.

Coral reefs provide economic benefits, too. Coral reefs support around 1 billion people through coastal industries including tourism and fishing, as well as providing coastal defense from storms.