Symbiotic Relations Between Coral and Zooxanthallae and the Bleaching Effect

Coral and Zooxanthallae

Coral (in the class Antrozoa) is a vital part of many aquatic ecosystems, with it’s ability to form large calcium carbonate structures over time which act as a habitat for many different organisms, it plays an important role in it’s ecosystem. A coral reef will consist of many small polyps which surround themselves in a calcium carbonate skeleton in order to protect themselves from predators which will visit the reef. The polyps typically emerge overnight to feed.

Coral polyps feed by catching zooplankton and other microscopic organisms with their tentacles and directing them towards their mouth parts where the zooplankton will be digested in the stomach. They can occasionally also eat slightly larger prey by incapacitating or killing it with their nematocysts before it enters the mouth part. This however is only able to provide around 50% of each coral polyp’s daily requirements the rest it must source from elsewhere.

To ensure their full daily requirements are met coral polyps form a symbiotic relationship with called a group of single celled protozoa called zooxanthallae. The polyp provides the zooxanthallae with a suitable habitat within its soft tissues where it is protected from harsh abiotic factors and filter feeding organisms that would eat the zooxanthallae and the coral itself benefits from oxygen and nutrients expelled by the zooxanthallae.

What is Bleaching?

Bleaching a the process by which coral polyps remove the zooxanthallae when abiotic factors around the polyps are no longer capable of maintaining the populations of zooxanthalae causing the populations to decrease. In response to this the coral polyps expel the zooxanthallae from their soft tissues and into the water column. This is known as bleaching because the individual coral polyps appear white in colour after this as pigments present in their soft tissues where specific to the zooxanthallae and therefore left with the zooxanthallae. The loss of zooxanthallae is detrimental to the health of the coral as it now lacks the means to produce enough nutrients to sustain itself; after a relatively short period of time if the environmental stresses which lead to the expulsion of zooxanthallae do not cease the coral will die.

Why Does Bleaching Occur?

The reason why coral undergoes bleaching when subjected to environmental stresses is not yet discovered, however it was suggested to be a defence mechanism by R. Buddemeier and D. Fautin in 1993 who hypothesised that when coral undergoes bleaching it will enter a state of reduced activity and wait for a new species of zooxanthallae to arrive which is better adapted to live in the new conditions. When the new species of zooxanthallae is found it will be introduced to the soft tissue of the individual polyps and they will continue to function as usual, the benfit of this system to the coral is in future it will be more resistant if conditions were to change further.


Zooxanthallae visible on coral polyps
8 years ago

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