Coral and Cnidarian Anatomy

Corals are colonies of simple polyps with stinging cells and symbiotic algae. Learn the polyp body, the calcium-carbonate skeleton, nematocysts and zooxanthellae.

Corals are cnidarians, the same phylum as anemones and jellyfish. Despite looking like rock or plants, a coral is an animal, or rather a colony of many tiny animals. Understanding its simple body plan explains its light needs, its skeleton, and its capacity to sting neighbours.

The cnidarian body plan

Cnidarians are radially symmetrical and built from just two cell layers, an outer epidermis and an inner gastrodermis, with a jelly-like mesoglea between them. They have a single body opening (the mouth) leading to a gastrovascular cavity that handles both digestion and circulation; there is no separate anus. Instead of a brain they have a decentralised nerve net. Cnidarians take two forms, the sessile polyp and the free-swimming medusa; corals are the polyp form.

The polyp

The basic unit of a coral is the polyp: a sac-shaped body whose wall has those two cell layers, with a single mouth at the top ringed by tentacles. Most stony corals are colonies of many genetically identical polyps connected together, so what looks like one coral is really a cooperative of individuals.

Skeleton

Stony (scleractinian) corals secrete a hard skeleton of calcium carbonate in the form of aragonite, and this accumulated skeleton is what builds reefs over time. Soft corals do not lay down a solid skeleton; instead their tissue is stiffened by small calcium-carbonate elements called sclerites.

Nematocysts

The tentacles carry nematocysts, the stinging cells (cnidocytes) that define the phylum. These are harpoon-like organelles that fire to pierce, poison and hold prey, and they also serve in defence. In the reef tank this is the basis of coral aggression: polyps can sting and damage neighbouring corals that grow too close, so placement and spacing matter.

Zooxanthellae and bleaching

Most reef-building corals host symbiotic single-celled algae (zooxanthellae, dinoflagellates of the genus Symbiodinium) inside their tissue. These algae photosynthesise and supply the coral with most of its energy, which is why such corals need appropriate light. When corals are stressed (for example by heat) they expel their zooxanthellae and turn white, a process called bleaching; a bleached coral has lost its main food source and may die if the stress continues.