Coral is often mistakenly identified as a rock or a plant, but it is actually a marine animal that lives in colonies and acts as a foundational engineer for vast underwater ecosystems. The massive structures known as coral reefs are created by the accumulated skeletons of countless generations of these organisms. Because the term “coral” is used broadly to describe thousands of different species, there is no single scientific name that applies to all of them. Instead, various groups of coral species are organized into multiple taxonomic categories based on their biological structure and evolutionary relationships.
The Broad Taxonomic Home of Coral
All corals belong to the phylum Cnidaria, a diverse group of aquatic invertebrates that also includes jellyfish and sea anemones. Cnidarians are defined by specialized stinging cells called nematocysts, which are used to capture prey. This phylum is further categorized into the class Anthozoa, which translates to “flower animals.” Anthozoa encompasses all true corals, sea anemones, and sea pens, establishing the broadest scientific boundary for these organisms.
Within the Anthozoa class, all species are characterized by a cylindrical, sac-like body plan and lack the free-swimming medusa stage seen in other Cnidarians. Anthozoa is a large group containing multiple orders and subclasses, reflecting the complexity of coral diversity. Moving beyond this general classification is necessary to understand the different types of coral.
Differentiating Hard and Soft Corals
The significant structural differences among corals lead to a split into two major subclasses, which are commonly known as hard and soft corals. Hard corals, which are the primary architects of coral reefs, fall under the order Scleractinia, a group within the subclass Hexacorallia. These corals secrete a rigid, external skeleton composed of calcium carbonate, specifically the crystalline form called aragonite, which forms the massive, stony framework of the reef structure. The polyps of these stony corals typically exhibit a six-fold symmetry in their internal structures and tentacle arrangement.
Soft corals belong to the subclass Octocorallia, which includes orders like Alcyonacea. They do not produce the massive external skeleton that builds reefs. Instead, they rely on a flexible, jelly-like tissue called mesoglea for their structure, often supported by microscopic calcium carbonate elements known as sclerites. Their polyps are distinguished by their strict eight-fold symmetry, meaning each animal possesses exactly eight feathery tentacles.
The Essential Biology of the Coral Polyp
The coral polyp is the individual animal that makes up the colonial structure, and its biology defines how the entire reef functions. Each polyp is a small, soft-bodied organism with a simple sac-like structure, featuring a mouth surrounded by tentacles. These polyps are connected by a thin layer of living tissue called the coenosarc, allowing the entire colony to function as a single unit. The individual polyp is responsible for both capturing food and the construction of the skeletal material.
The defining biological feature of most reef-building corals is the mutualistic relationship they share with single-celled algae known as zooxanthellae (family Symbiodiniaceae). These microscopic algae live within the coral polyp’s tissues in a protected environment. The zooxanthellae use photosynthesis to convert sunlight into energy, transferring up to 90% of the organic compounds they produce, such as sugars and proteins, directly to the coral host.
In return for this energy, the coral provides the algae with essential compounds, including carbon dioxide and nitrogenous waste products. This nutrient recycling is important in the clear, nutrient-poor waters where reefs thrive. The energy provided by the zooxanthellae is necessary to fuel the process of calcification.
Calcification is the mechanism by which hard corals build their skeletons, involving the active secretion of calcium carbonate beneath the polyp. The polyp manipulates the chemical environment in a space between its tissue and the existing skeleton, called the extracellular calcifying fluid. By concentrating calcium and carbonate ions in this fluid, the coral promotes the precipitation of aragonite crystals. The photosynthetic activity of the zooxanthellae enhances this process by removing carbon dioxide, which increases the availability of carbonate ions and accelerates the growth of the stony structure.