Coral Reef Ecology

Before we look at climate change's effects on coral health, let's first gather a basic understanding of corals. Doing so, we can appreciate their intricacies, diversity, and importance.


What is a Coral?

Corals are animals, they are part of the phylum Cnidaria. As Cnidarians they have nematocysts (stinging cells) allowing for prey capture, defense against predation, and competition.

In detail, we will focus on hermatypic corals. Hermatypic corals are colonial, with tentacled polyps, and can house zooxanthellae. Most importantly, they are today's reef builders by accreting a CaCO3 skeleton

Fig.1: Nematocysts at microscopic level (Coralhub, 2012)

What are Zooxanthellae?

Zooxanthellae are primarily dinoflagellates, a type of protist, which can fix carbon through photosynthesis. They form a symbiosis with corals by localizing transfer of nutrients and give the corals color

Thus, corals are carbon autotrophic and this relationship also helps increase number of corals and their calcite skeleton production. In turn, the zooxanthellae have a home and protection to safely photosynthesize and thrive.

In addition, the coral's excrement serve as nutrients for the zooxanthellae. However, these zooxanthellae are particularly sensitive to temperature and light. Too high temperatures can cause zooxanthellae to be expelled, thus bleaching the coral reef. Most corals will not be found in deep water since zooxanthellae light capture for photosynthesis can happen only to a certain depth.
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Fig. 2: Zooxanthellae on coral (Coralhub, 2012)

Coral Reef Traits

Coral comes in diverse morphology as some can have shapes similar to branches, disks, and even brain-shaped. Similarly, the coral polyps come in varying size, from tiny to large.

Coral reefs can only grow in water that is warm, nutrient poor, clear, normal salinity, and shallow.Most Coral reefs are located in the Indian and Pacific Ocean. Coral reefs types are atolls, fringe, barrier, and patched. For example, an atoll reef forms when a volcano sinks and simultaneously the coral reef area rises across the lagoon, just as Darwin hypothesized.

Looking at a coral system, one would assume that the picturesque scene means that all the corals are "living in harmony." But that is far from the truth! In fact, corals compete not only with other organisms but also with each other for resources and space. Consequently, we can also see distinct zonation patterns in a reef system.

Fig.3: Coral reef populated around equator, most notably in Indo-Pacific (NOAA, 2008)

Fig.4: Fan shaped
(Seasky, 1993)

Fig. 5: Brain shaped
(Seasky, 1993)

Fig. 6: Competition for space
(Seasky, 1993)

Fig.7: Polyp size
(Seasky, 1993)

Coral Reefs and Biotic Interactions

Reefs serve as a habitat for numerous organisms and show complex interactions with other organisms. Macroalgae can overgrow on coral and suffocates it but overgrowth is hindered by grazers like parrotfish and sea urchins.


There are predators like the Crown-of-thorns seastar, its invasion can drastically reduce corals. There are not that many large predatory fish, mammals, or reptiles like turtles found


Other organisms like crustaceans, such as crabs, and diverse number of fishes are also present.

Fig.8: Macroalgae overrunning coral
(Ozcoasts, 2012)

Fig. 9: Sea Urchin attached to coral
(Stephens, 2012)

Fig. 10: Parrotfish moving across reef
(Stephens, 2012)

Fig.11: Crown-of-thorns sea star eating coral
(Hanson, 2008)



Now lets see what is going on in the ocean environment

Text information compiled from McCauley, 2012