How Corals & Coral Reefs Grow

To produce a coral skeleton, the individual coral animal or “polyp” secretes a calcium carbonate cup around itself called a calyx. The walls of the calyx surrounding the polyp are called the “theca” and the bottom of the calyx in which the polyp sits is called the “basal plate” (NOAA 2005; Pechenik 2010). Extending upwards and radiating outwards from the basal plate are thin, calcareous septa or”sclerosepta” that provide the polyp with structural support, protection and an increased surface area (NOAA 2005; Pechenik 2010).

From time to time, the polyp will lift of fthe bottom of its calyx to produce a new basal plate (NOAA 2005; Pechenik 2010; Morrissey et al. 2018). This new basal plate is stacked on top of the older one, creating a small chamber within the skeleton that elevates the coral (oceanservice.noaa.gov/; Morrissey et al. 2018). Coral skeletons grow by the accumulation of these calcium carbonate chambers over time (NOAA 2005). Hermatypic corals have extremely slow growth rates, ranging from a few mm per year to a couple of centimetre sper year for the fastest growing corals (Spalding et al. 2001).

Coral reefs are formed over centuries and millions of years by the accumulation and growth of hermatypic corals and other calcium carbonate producing organisms (Lalli and Parsons 1997; Spalding et al. 2001; Pechenik 2010; Latypov 2014; Jones et al. 2015; Morrissey et al. 2018). Over time, coral reefs are damaged by storm events, predation and bio-eroding organisms causing coral rubble and sand to accumulate within the coral reef structure (Lalli and Parsons 1997; Spalding et al. 2001; Pechenik 2010; Jones et al. 2015; Morrissey et al. 2018).

This coral rubble is cemented together by calcium carbonate producing organisms such ashermatypic corals and certain species of algae, creating a more stable structure (Lalli and Parsons 1997; Spalding et al. 2001; Pechenik 2010;Jones et al. 2015; Morrissey et al. 2018). This is how a coral reef is formed.Since hermatypic corals have slow growth rates, and coral reef growth is not constant, it takes approximately10,000 years for a coral reef to form (Spalding et al. 2001; Latypov 2014; Jones et al. 2015).

Themajority of hermatypic corals contain single celled algae, called zooxanthellae, that inhabit cells in their gastrodermis (Lalli and Parsons 1997; NOAA2005; Pechenik 2010; Davyet al. 2012; Morrissey et al.2018). Corals and zooxanthellae have a mutualistic, symbiotic relationship (Lalli and Parsons1997; NOAA 2005; Pechenik 2010; Morrissey et al. 2018). In this relationship, the coral provides the zooxanthellae with a safe place to live(within the corals gastrodermis),and organic and inorganic waste products such as carbon dioxide (CO2) and water that are needed by the zooxanthellae for photosynthesis (Lalli andParsons 1997; Jokiel 2004;NOAA 2005; Pechenik 2010; Morrissey et al. 2018).

In return, the coral receives glucose, glycerol and amino acids that are used to produce carbohydrates, fats, proteins and calcium carbonate(Lalli and Parsons 1997; NOAA 2005; Pechenik 2010; Morrissey et al. 2018). In fact, approximately 20% to 95% of the organic material produced by the zooxanthellae is transferred to the coral(Pechenik 2010; Wooldridge 2010).As a result, corals gain most of their food from zooxanthellae (Lalli and Parsons 1997; Pechenik 2010; Morrissey et al. 2018).