Corals form a close association with single-celled algae, called zooxanthellae. There are many species and types of zooxanthellae and AIMS research has shown that many coral colonies harbour more than one type of zooxanthellae simultaneously.

Growth rates of corals as well as their upper temperature tolerance limits have been shown by AIMS scientists and their external colleagues to be partly dependent on the type of zooxanthella that dominates the coral tissues.

Early on in life, most corals take up several kinds of zooxanthellae. Later, though, one type usually comes to dominate the tissues while the other types persist at very low densities. However, after a coral has bleached, some of the low density zooxanthella types - in particular the ones that have a higher bleaching tolerance threshold - are still healthy and may be able to increase their densities, a process called ‘symbiont shuffling'.

As a consequence of symbiont shuffling, the coral can undergo a change in its physiological tolerances, including its upper temperature tolerance limit.

Return to:
Bleaching tolerance and adaptation