Coral Polyp Anatomy

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Corals, often mistaken for plants due to their flower-like appearance, are animals that generally exist in the form of colonies. These colonies attach to oceanic substrate and when numerous colonies form a consortium, a coral reef arises. Corals are members of the Animal Kingdom with classification in the Phylum Cnidaria; further classification places them in the class Anthozoa that is closely related to the class Scyphyzoa (e.g. jelly fishes) and the class Hydrozoa (e.g. Hydra).

The polyp is the basic living unit of Anthozoa (further subdivided into subclass Hexacorallia, which includes stony corals, and subclass Octocorallia, which includes soft corals). Polyps can occur either singularly, as in the case of anemones and some stony corals, or can form colonies, as they do in most stony and all soft corals. A key feature of Cnidaria is the presence of cnidocytes, or stinging cells, which the polyp uses for prey capture and defense.

Polyp Anatomy

The fundamental structure of a coral polyp is a cylindrical sac 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 portion of the polyp that is able to extend beyond the skeleton is called the column. Located at the distal end of this region is the polyp’s mouth, oral disk, and ring of tentacles. The mouth is surrounded by the peristome, which is an elevated region surrounding the mouth opening, and the oral disk, which extends from the peristome to a ring of tentacles around its perimeter. Tentacles are studded with stinging cells called nematocysts, and in some cases, can be topped with a bulbous structure called an acrosphere.

Tentacles are used by the coral polyp to capture and ingest particulate and planktonic food from the environment. For corals that live within the photic zone (the depths where light penetrates into the ocean) the nutrients that the coral acquires through this heterotrophic method supplements those derived from the algal symbionts that live within its internal tissue layer, known as the gastrodermis. The symbionts that live within corals are microscopic dinoflagellates of the family Symbiodinacea, which undergo photosynthesis and exchange nutrients and waste molecules with the coral host cells. This consortium of algae within coral tissues contributes to their color and drives the coral’s need for light in shallow reef habitats.

A defining feature of anthozoans is the actinopharynx, which is an invagination of the epidermis that forms a short muscular passageway from the mouth to the gastric cavity. The actinopharynx controls the opening and closing of the polyp mouth by using a ring of muscle fibers called the oral sphincter that is located on its distal edge. The lumen of the actinopharynx is lined with epithelial cells with elongated cilia to move material in and out of the polyp mouth.

Extending down from the actinopharynx into the interior cavity of the polyp are tissue partitions known as mesenteries. Mesenteries provide structural support to the polyp column and are also important for digestion, as they increase the surface area of the interior epithelia for gas exchange and nutrient absorption. Retractor muscles on mesenteries near the body wall allow the polyp to contract and extend. Mesenteries that are attached to the actinopharynx are known as “complete” or “perfect” mesenteries, those that are not connected are known as “incomplete” or “imperfect.” The open space within the polyp between mesenteries is known as the gastrovascular cavity.

Mesenterial filaments are the thickened and elongated free edges of mesenteries below the actinopharynx. These can be stretched out of the mouth to help capture and digest food outside and inside of the polyp using their high abundance of granulated gland cells, which secrete digestive enzymes. Bulbous lobes at the end of mesenterial filaments are called cnidoglandular bands. These structures contain a battery of nematocysts for catching prey and for defense, and of granular gland cells to aid in digestion.

In coral colonies, the common tissue that surrounds and links polyps is known as the coenenchyme. Within the coenenchyme, tubes called gastrovascular canals link the gastrovascular cavities of polyps throughout the colony. This network of tubes is what allows individual polyps to act as a unified organism by shuttling resources and cellular signals throughout the colony. In imperforate corals, polyps of a colony are separated by solid skeleton (known as the coenosteum), so this network of canals runs just under the surface of the coenenchyme above the skeleton. In perforate corals, numerous gastrovascular canals pierce the more porous skeleton between polyps

Mind map of coral polyp anatomy terms. Click to enlarge. Credit: Murphy McDonald, NOAA

Learn More about the Essentials of Coral Biology:

Microscopic Anatomy

Learn more about the tissues that make up 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

Publications Relevant to Coral Anatomy

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