Anatomy & Physiology
Each zooid (individual polyp in a colony) has a gut-like cavity surrounded by a body wall. Its mouth is at the top of this, facing away from the colony (see fig. 1). The tentacles surrounding the mouth have many functions, such as feeding and gas exchange (see Life History and Behaviour). As a rule, Cnidarians all have 3 body tissues – the epidermis, gastrodermis and the mesoglea. The epidermis is the outer layer and acts as the skin, it is the majority of the tissue found in coral. Sensory cells, as well as digestion, movement and communication are all associated with this outer layer. The gastrodermis lines the the gut-like cavity and joins with the epidermis at the mouth (see figure 1). It also is responsible for the production of gametes (the eggs and sperm). Sandwiched between these two layers is the mesoglea which is a gelatinous extracellular matrix (it is a jelly like substance keeping the two layers together). Being a connective tissue is its primary function, unlike the mesoglea associated with other phyla.
The symbiosis with the bacterial zooxanthellae is a delicate relationship. It can be upset by a rise in temperature only a few degrees above the regular local maximum. This leads to bleaching and usually death of the coral as the reliance on these tiny cells is so strong. The zooxanthellae are photosynthetic and provide extra energy to the coral that cannot be gained by heterotrophic mechanisms (Hamada et al 2012). The coral provides the bacteria with excess nitrogen and other nutrients necessary to sustain life (Hamada et al 2012).
Cnidaria have a basic nerve system that is known as a nerve net (Chen, 2008). It means they can control movement at a cellular level and allows them to understand what is happening in the world around them (Chen, 2008).
Digestion occurs within each individual polyp, through the blind gut (Ruppert et al, 2004). A blind gut also means that waste exits through the same area that food enters. To avoid sucking in water that they have already processed, the polyp creates its own current, that brings water in from one way and pushes it away in the opposite direction (Ruppert et al, 2004).
Figure 1: Polyp zooid showing different cell types. Adapted from Ruppert et al, 2004.
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