Morphology and Structure

Corals are bi-radially symmetrical, adults are composed of a central mouth with several rings of tentacles surrounding the top of the polyp. The polyp attaches to the substratum with a basyl plate from which it secrets calcium carbonate allowing the structure to grow (Kaiser et al. 2005). (Figure 3)   

Figure 3: Diagramatic representation of cross-section through a coral polyp. (URL 4)

The tentacles posses stinging cells called cnidocytes lined with nematocysts for defence and prey capture (Roberts et al. 2009). Leading from the mouth is the Actinopharynx this follows through to the main chamber of the polyps gastrovascular cavity. Lamellar tissue sheets longitudinally divide the polyp, these tissue sheets are called mesenteries and carry digestive cells, spermatocytes and oocytes (Roberts et al. 2009).   

Living at depth is a challenge for any organism, they are required to make form and function modifications to adapt to the extreme environmental pressures in order to survive. It can be argued that because of the adaptions made to survive in cold dark depths the environment is no longer extreme to them, and is only extreme to an organism not adapted to tolerate such conditions.   

Scleractinian corals from the family Micrabaciidae are adapted to live at depths as deep as 5000 metres. At depth calcification is assumed to be harder in terms of decreasing the weight and density of the polyp structure (Squires 1967). It has been suggested that numerous adaptions are made due to food scarcity in the deep (Roberts et al. 2009).
In general there is an increase in coral porosity, a decrease in trabeculae and thinner corallum elements (Roberts et al. 2009). The following table (Table 1) will describe adaptions made by deep water coral and the advantages it gives them, it is adapted from Roberts et al. (2009) and Squires (1976).    

Table 1: Summary of adaptions made to the morpholgy of deep water corals   

Adaption Advantage Example
Flattened discoidal corallum Increase surface area for feeding Stophanocythus
Exsert septa Maximise food capture Ulocyathus
Smaller overall size Decrease need for food Turbinoliids
Thinner skeleton Less effort exerted into calcium carbonate production, light weight Deltocyathus

Leptopenus hypocoelus is the deepest found species of coral in the scleractinian family. Its skeleton only contributes to a tiny proportion of the corals overall weight due to it’s delicate lattice structure (Figure 4).   

Figure 4: Skeleton of Leptopenus hypocoelus (Replicated from Roberts et al. 2009)

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