Coral is not a plant; it is an animal. Many people confuse coral for a plant because of its stationary nature and appearance on the seafloor, often resembling underwater gardens. However, its biological characteristics distinctly classify it within the animal kingdom. This common misunderstanding stems from how corals obtain some of their sustenance, which involves a process similar to how plants derive energy.
Why Coral Isn’t a Plant
Plants are organisms that produce their own food through a process called photosynthesis, making them autotrophs. This process relies on specialized organelles called chloroplasts, which are present in plant cells, and a rigid cell wall providing structural support. These features are fundamental to how plants generate energy and maintain their form.
Corals, in contrast, do not possess chloroplasts in their cells and lack the rigid cell walls characteristic of plant cells. As animals, corals are heterotrophs, meaning they must obtain nutrients by consuming other organisms. While some animals, like corals, are sessile and remain fixed, their cellular structure and nutritional methods align with animal biology, distinguishing them from plants.
What Coral Actually Is
Coral is an invertebrate animal, specifically a polyp, belonging to the phylum Cnidaria, which also includes jellyfish and sea anemones. Each coral polyp is a small, soft-bodied organism, cylindrical with a mouth surrounded by tentacles. These tentacles have specialized stinging cells for defense and prey capture.
These polyps secrete a hard, cup-shaped external skeleton, composed of calcium carbonate, providing protection and structural support. Over time, successive generations of polyps build upon these skeletal foundations, forming the complex, three-dimensional structures known as coral reefs. Most corals are colonial, meaning numerous identical polyps live interconnectedly, sharing tissues and a common skeletal framework.
Within the tissues of many reef-building corals live microscopic, single-celled algae known as zooxanthellae. This symbiosis is a significant source of the misconception that corals are plants. The algae reside within the coral’s gastrodermal cells, receiving a protected environment and photosynthetic compounds from the coral. In return, these algae provide the coral with energy-rich organic compounds.
How Corals Sustain Themselves
Corals sustain themselves through two distinct methods, showcasing their animal nature and the benefits of their symbiosis. Like other animals, coral polyps capture food from the water column. They extend their tentacles, often at night when zooplankton are abundant, to ensnare tiny organisms like copepods and other small invertebrates. These prey items are moved into the polyp’s mouth for digestion.
Beyond capturing macroscopic prey, many corals utilize a mucus layer to trap smaller particles, including dissolved organic matter and sediment. This feeding mechanism provides the coral with nutrients like nitrogen and phosphorus, often scarce in clear, tropical habitats.
The second, and often more substantial, energy source comes from zooxanthellae within the coral’s tissues. These algae perform photosynthesis, converting sunlight into sugars, glycerol, and amino acids. A significant portion of these organic compounds (up to 90%) is transferred directly to the coral, providing energy for growth, reproduction, and metabolic processes.
This arrangement allows corals to thrive in nutrient-poor, sunlit waters. While this process provides a plant-like energy source, it is the algae, not the coral, that performs photosynthesis. The coral provides a protected environment and carbon dioxide for the algae, completing this mutually beneficial exchange.