Scleractinian corals are in the phylum Cnidaria along with sea anemones, hydroids, gorgonians and jelly fish (Pechenik 2010). Scleractinian corals are in the class Anthozoa, the subclass Hexacorallia (meaning”six and coral animal” in Latin) and are in the order Scleractinia which translates to”hard and ray” in Latin (Pechenik 2010). Scleractinian coral polyps have a smooth, tubular shape with stinging tentacles that are arranged in multiples of six surrounding their mouth (Pechenik 2010).Scleractinian corals or “stony corals” come in a variety of different morphologies or forms and can be either colonial or solitary (NOAA 2005; Morrissey et al. 2018).
Reef-building scleractinian corals or “hermatypic corals” are the organisms primarily responsible for creating the framework fo rand building up coral reefs (Buddemeier and Kinzie 1976; Schuhmacher and Zibrowius 1985; Lalli and Parsons 1997; Humann 2002; NOAA 2005; Pechenik 2010; Jones et al. 2015; Morrissey et al. 2018). This is because these colonial animals produce large calcium carbonate (CaCO3) skeletons (Lalli and Parsons 1997; Pechenik 2010). Over time, coral reefs are formed by the accumulation of hermatypic corals, their skeletons, and other calcareous organisms (Spalding et al. 2001; Pechenik 2010; 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).
It is because of this mutualistic relationship that coral reefs are such productive ecosystems even though they reside in nutrient poor environments (Lalli and Parsons 1997; Pechenik 2010; Morrissey et al. 2018).Since zooxanthellae require sunlight for photosynthesis, corals must live in shallow, clear waters(Barnes 1987; Morrissey et al. 2018). This is one of the reasons why turbid waters with high levels of sedimentation and high nutrient concentrations are detrimental to coral health(Barnes 1987;Risk et al. 2011).Reef-building corals also require warm waters to function properly, therefore, coral reefs are restricted to tropical and subtropical waters around the world(Barnes 1987; NOAA 2005; Morrissey et al. 2018).
1.Barnes, R.D. 1987. Corals and Coral Reefs. In Invertebrate Zoology;Fifth Edition (pp. 92-96). Fort Worth, TX: Harcourt Brace JovanovichCollege Publishers.