Microscopic Anatomy

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The fundamental structure of a coral polyp is a cylindrical sac lined composed of two layers of epithelia: the surface body wall, which forms the outermost barrier between the organism and its surroundings, and the basal body wall, which anchors the polyp to its skeleton. Water and ingested materials enter the polyp through the mouth at the distal (uppermost) region of the polyp; water and waste products are expelled by the polyp through the same opening. Respiration and excretion occur by direct exchange of molecules through the two layers of epithelia.

The surface body wall is the epithelial tissue that forms the outermost barrier of the organism and is responsible for producing the protective film of mucus that envelops coral colonies. The surface body wall consists of three tissue layers: the outermost layer known as the epidermis, the innermost layer known as the gastrodermis, and a layer of connective tissue between them known as the mesoglea.

The mesoglea supports the two epithelia and the epitheliomuscular cells of retractor muscles. This layer contains fibroblasts that secrete collagen and ground substance, and amoebocytes that serve as the principal cells of the coral’s immune system

The epidermis is a layer comprised of epithelium responsible for secreting the mucus layer that encompasses the organism and aids in protection, sediment removal, and feeding. Specialized cells in the epidermis are ciliated columnar support cells, mucocytes (secrete mucus), sensory bipolar neurons that connect to the subepidermal nerve net, cnidocytes (produce stinging cells called nematocysts or spirocytes), pigment cells, epitheliomuscular cells, and amoebocytes.

The gastrodermis lines the interior of the polyp and serves as its digestive system. Phagocytotic supporting cells contain symbiotic algal cells in vacuoles, which undergo photosynthesis and exchange nutrients and waste molecules with the coral host cells. Other cell types common throughout the gastrodermis are mucocytes, cnidocytes, granular gland cells, pigmented cells, epitheliomuscular cells, and scattered amoebocytes.

The basal body wall is likewise comprised of three tissue layers: the gastrodermis and mesoglea, which are both functionally similar to those layers in the surface body wall, and the calicodermis, which is the tissue layer responsible for creating the coral’s skeleton. Skeleton is formed outside of cells called calicoblasts, which secrete a soluble organic matrix that facilitates calcium carbonate crystal formation. The basal body wall also contains specialized epithelial cells called desmocytes, which interdigitate with the mesoglea and use specialized fibers to anchor the coral polyp to its skeleton.

Learn More about the Essentials of Coral Biology:

Polyp Anatomy

Learn about the gross anatomical features of the coral polyp

Skeletal Anatomy

Learn about all of the components of the coral skeleton and the different morphologies that it can display

Coral Reproduction

Learn about how corals use different reproductive modes to overcome their immobility

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