Anatomy & Physiology

Polyps of scleractinian corals have three cell layers. The epidermis is the outside layer, the gastrodermis is the inside layer and the middle layer is called the mesoglea. The polyps are supported by a calcium carbonate skeleton called a calyx. The basal plate is the base of the calyx and the side walls of the calyx are called the theca (Earths Labs2008). The theca is covered by the soft parts of the coral, including the mouth surrounded by tentacles. Stinging cells, or nematocysts, lie on the tentacles and are used for feeding. Digestive filaments allow for captured food to be digested (Earths Labs 2008). The mouth is also used for waste disposal. In a coral colony each individual polyp is connected by the coenosarc. All of this information is demonstrated in figure 2 below.  

Figure 2:The structure of a coral polyp (Earths Labs 2008). This image can be found at http://serc.carleton.edu/eslabs/corals/2b.html

M. digitata transmit their algal endosymbionts, xoozanthallae, directly to their eggs. Other species do not do this and have to gain their endosymbionts from the surrounding environment (Van Oppen 2004). A study by Harpeni and David 2011, explored this through the use of microscopes.  They found xoozanthallae inside the gastrodermal layer and inside the oocytes as depicted in the image below (figure 4).

Figure 4: The cells of M. digitata depicting zooxanthallae. Seen in the photo are zooxanthalle (z), oocytes (o), gastrodermis (g), nucleus (n), mesoglea (m), and the epidermis (ep). This image was acquired from Harpeni and David 2011. For more information see the reference list.