Coral = animal + plant!

Most of us fortunate enough to have had exposure to coral reefs, have come to recognise that a special relationship is involved.

Corals are essentially anemone-like animals that secrete a skeleton whilst living in a symbiotic relationship with algae. A single unit is called a polyp; collectively they form a coral colony. Most corals that make up coral reefs, such as the Ningaloo Reef in W.A., are colonial.

A schematic representation of general structure of a polyp and its underlying skeleton.

Painting: Geoff Kelly – Australian Institute of Marine Science, 2013.

The coral provides the algae with a protected environment and compounds they need for photosynthesis. In return, zooxanthellae supply the coral with the products of photosynthesis and help it to remove wastes. The mutualistic relationship between the algae and coral polyp facilitates a tight recycling of nutrients in the fairly nutrient-poor waters of the tropics.

The success of the symbiotic relationship between these ‘hard’ corals (an animal) with zooxanthellae algae (a plant) to harness the energy of the sun, powers the process of reef-building.

Why do these organisms invest so much energy into building something that is dead? That is, the calcium carbonate skeleton of the polyp which makes up the reef foundation. - One aspect is that these skeletons provide the necessary structure that is needed to form colonies, which are required to build large wave-resistant reefs.

Colony formation and algal symbiosis are two evolutionary innovations that clearly go together and have very likely evolved in conjunction. The importance of this is verified by the fact that most living Cnidaria (the Phylum which includes corals) that reap the advantages of reef-building, are both colonial and symbiotic.

It is not necessarily what species of plant and animal, but the relationship between them is what is critical to what are the most prominent modern reef-builders.

This specific symbiotic relationship allows the limitless resources of seawater carbonates and sunlight to be harnessed and harvested to construct an ecosystem.

In effect, this special relationship allows reefs to be built by animals because it gives them the energy-generating capacity of plants.

Undoubtedly, the evolutionary advantage of this symbiosis is prominent, but the cost is also great. These zooxanthellate corals are limited to the most hostile of marine environments, the ocean’s surface, both physically and biologically. This symbiosis also constrains corals to live in habitats in high competition with macroalgae. Coral-algae symbiosis is therefore ultimately responsible for the geographic constraints of coral reefs, as well as their construction.

This short video gives a brief introduction to coral anatomy and the importance of coral reefs in simple terms. With its cute cartoon animations, it is a cry out for coral reef peril and sends a great message advocating for coral reef conservation.

 BIBLIOGRAPHY:

·         Baird, A., Cumbo, V., Leggat, W., & Rodriguez-Lanetty, M. (2007) Fidelity and flexibility in coral symbioses. Marine Ecology Progress Series, 347, 307-309.

·         Marine Pollution Bulletin (2004) Coral symbiosis under the spotlight. Marine Pollution Bulletin, 49(7-8), 529-529.

·         Rowan, R., & Knowlton, N. (1995) Intraspecific diversity and ecological zonation in coral-algal symbiosis. Proceedings of the National Academy of Sciences of the United States of America, 92(7), 2850-2853.

·         Veron, J.E.N. (2013) coral info sheets, Australian Institute of Marine Science, viewed April 2013, <http://coral.aims.gov.au/info/structure.jsp >.