The Reef / The fascinating complexity of coral colonies


Like landscapes, organisms are the sum of complex networks of interacting elements from many different groups of smaller organisms. Coral colonies are a dramatic example of this idea, with animal, plant, bacterial and other actors contributing to the whole that we call a “coral”.

⟼ Corals rely on plantlike algae to produce their energy, without these partners they could not form reefs.

⟼ Animal partners help to fight disease and repel predators.

⟼ Bacterial populations on and in the coral tissues are involved in many critical functions of the colony.

Next time you find yourself next to a piece of live coral, stop and spend a minute looking closely between the coral fingers and amongst the polyps. 

Upon the surface and within the mucous layer, in the flesh and in the matrix of the coral skeleton, reside a fantastic collection of highly adapted, mutually interdependent symbiotic partners.

These symbionts include representatives of all of the major groupings of the tree of life. There are bacterial, archean, protozoan, fungal, plant and animal communities occurring throughout all levels of the coral colony. 

These genetically diverse populations themselves are intricately integrated with each other and with their animal hosts, into stable structures which we call coral colonies.

 A wide assortment of animals inhabits the complex microhabitat of the coral plate. These residents are often sculpted and coloured in ways that provide almost complete camouflage against the background of their particular host coral.

These residents have complicated relationships with their host corals:

They clean debris from their home which ensures vital sunlight is not blocked and helps prevent infections.

They actively repel coral carnivores like the Crown of Thorns Starfish, nipping and stinging the thousands of starfish feet that fill the spaces between the coral fingers as the animal settles down to eat. 

They eat away at infection fronts, slowing the spreading disease and removing infectious material before it can slough away and spread the disease to neighbouring colonies, as exemplified in the service provided by the Hairy Black Fingered Crab in the slowing of White Syndrome.

The algae within the coral feeds the colony, accelerating its growth and modifying the chemical conditions within the coral tissues in ways that aid the development of the skeleton. The algae also emit a gas under certain conditions which becomes a mist of microscopic particles above the sea with the ability to modify the weather itself in ways we are only just beginning to grasp.

The bacterial communities help to modify the chemical emissions of the coral and algae and may even help to enable the weather engineering superpower mentioned above. They also have roles in the coral immune system, protecting the host tissues from pathological invaders.

The list goes on…

Each without the other would fail to thrive. In each other’s company the whole becomes more than the sum of its parts and long term survival becomes a possibility. 

This feature of interdependence is not limited to coral.

 Your own body is as dramatic an example of this most fundamental characteristic of life on Earth as is anything on the reef.

 There are ten times more non-human cells in your body than there are human ones and at the genetic level, your 25 thousand genes are seriously outnumbered by the literal millions of non-human genes that are in operation in the colony that you refer to in the first person.


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