The colours of a healthy tropical coral reef along a steep drop off. A good indication of a healthy reef is a range of different colours and textures of corals. This indicates a high competition for space between different coral species as well as a variety of different types of symbiotic algae which live within the corals. Coral reefs are the 2nd most biodiverse ecosystem in the world. Most corals are Phototrophic meaning they require sunlight so their photosynthetic algae can produce food in which they rely on to survive. As depth increases sunlight availability decreases and as a result optimal conditions for coral growth is reduced. This means there is intense competition for space in the top 30m where sunlight is optimal. Some species of coral will even secrete a mucous to kill surrounding different species of coral to give them more space to grow.