What is a coral?
Some people imagine corals as colorful rocks, sitting on the bottom of the ocean. Corals actually do build a limestone skeleton as they grow – but corals are much more than this.
So, if corals can grow… does that mean they are animals? Yes! Corals are Cnidarians and are closely related to jellyfish. But corals are even more than that…
Corals are miniature ecosystems. The coral animal partners with tiny plants (single-celled algae) that live inside the coral’s tissues. These tiny plants act like solar panels. They absorb light from the sun and photosynthesize (just like your house plants), and in return for a safe place to live they feed their coral host. Corals also host a myriad of other microbes, which help the coral live and grow. These other microbes work similarly to the human gut bacteria that help you digest the food you eat. Some combinations of corals, algae, and microbes are deviant combinations – they grow faster and survive better.
What happens to these miniature ecosystems when El Niño turns up the heat? A water temperature increase of just a few degrees can be devastating to coral reefs. When a large environmental disturbance like El Niño occurs, the coral ecosystem can start to break down because for many corals the water is simply too hot to survive. Algae and good microbes get kicked out of the coral, and the coral suffers (see NOAA's website for more info). ‘Bad’ microbes can enter into the ecosystem and wreak havoc on the coral. However, there are some deviant corals – miniature coral ecosystems that have the ideal combination of animal, algae, and microbes. These deviant corals can survive, and even thrive when water temperatures are too hot for most other corals to handle.
Photo taken on Kiritimati during the El Niño bleaching event (Baum Lab, July 2015)