A coral reef is a system made of colonies of polyps that share an exoeskeleton made of secreted calcium carbonate. Usually, they live in waters that contains few nutrients, so they need to be in a symbiotic relationship with a green algae called zooxanthellae. The zooxanthella provide the coral with calcium carbonate as a product of photosynthesis, a process by which the algae uses light from the sun and carbon dioxide to feed itself. So they need to be in shallow and warm waters in order to get some light and do the photosynthesis.
Last January, researchers from the University of Queensland, with a remotely operated submersible, found the deepest living corals in the Great Barrier Reef. They were living at a depth of 410 feet (125m)! It was unexpected because the reef ended at 330 feet (100m) depth. The corals were Leptoseris specimens, which usually live up to 40m below the surface. It’s an amazing discovery because there’s almost no light, so it remains unclear as to how these species of corals are able to live as the zooxanthellae may not be able to under photosynthesis for survival.
The researchers also found Acropora specimens at a depth of 240 feet (73m). The team from Queensland brought one specimen of Leptoseris and another one of Acropora to land, in order to investigate the difference between their symbiotic algae and the one from the surface corals from the same species.
It would be interesting to find out what gives them ability to survive in such extreme conditions with little to no light.
But this is not the deepest reef in the world. Other scientist have found corals as deep as 500 feet (150m) in Puerto Rico, and in the Gulf of Mexico they have corals at 2,620 feet (799m), but those corals don’t need sunlight to survive.
A possible explanation as to why are they able to live without light is that agricultural fertilizers are increasing nutrients so they don’t depend on zooxanthellae as much. This increase in nutrient is also benefiting the starfish populations, including the crown-of-thorns starfish, which eats coral. And as the water is getting warmer, there is a better chance they can grow in areas where it was traditionally colder.
Could be this an adaptation to climate change? Or are corals more resilient than we thought?