Coral, with its vibrant colors and intricate structures, often resembles an underwater garden. This plant-like appearance, coupled with its stationary nature, frequently leads to the common misconception that coral is a type of marine plant. However, despite its rooted presence on the seafloor, coral is unequivocally an animal, possessing distinct biological characteristics that set it apart from the plant kingdom.
What Coral Is Not
Coral’s resemblance to plants is superficial. Unlike plants, which are autotrophs capable of producing their own food through photosynthesis, corals themselves do not perform this process. While many corals derive significant energy from photosynthetic organisms living within them, the coral animal itself lacks the cellular machinery for photosynthesis. Furthermore, plant cells possess rigid cell walls, a structural component absent in animal cells, including those of coral polyps.
Plants are largely immobile, and while adult corals are sessile, attaching permanently to the ocean floor, this immobility does not classify them as plants. The branching or fan-like shapes and vibrant hues of coral colonies are a result of their unique animal biology. Their fundamental biological processes, from feeding to reproduction, align entirely with animal characteristics.
What Coral Is
Coral is an animal, specifically an invertebrate belonging to the phylum Cnidaria, which also includes jellyfish and sea anemones. The seemingly single coral structure is actually a colony composed of thousands of tiny, individual animals called polyps. Each polyp is a sac-like creature with a mouth surrounded by tentacles, which it uses to capture small organisms like zooplankton and fish larvae from the water using stinging cells called nematocysts. These tentacles sweep prey into the polyp’s mouth for digestion, demonstrating a heterotrophic feeding strategy.
A defining feature of many corals is their symbiotic relationship with microscopic algae called zooxanthellae, which live within the coral’s tissues. These algae perform photosynthesis, converting sunlight into energy-rich compounds such as sugars and amino acids, providing up to 90% of the coral’s energy requirements. In return, the coral provides the algae with a protected environment and essential compounds like carbon dioxide. This mutualistic partnership is responsible for the vibrant colors of many corals and enables reef-building corals to grow their calcium carbonate skeletons. Corals reproduce both asexually, by budding new polyps to expand the colony, and sexually, by releasing eggs and sperm into the water.
Corals are broadly categorized into hard (stony) and soft corals. Hard corals are the primary reef-builders, secreting rigid calcium carbonate skeletons that accumulate over time to form the foundation of coral reefs. Soft corals, in contrast, do not produce a rigid external skeleton; instead, they are supported by a jelly-like tissue and small, spiny structures called sclerites. Hard coral polyps have tentacles in multiples of six, while soft coral polyps have eight feathery tentacles.
Coral’s Vital Role in Marine Ecosystems
Coral colonies collectively create coral reefs, diverse underwater ecosystems frequently referred to as the “rainforests of the sea.” These reefs provide essential habitat, food, and shelter for an estimated 25% of all marine species, including thousands of fish and invertebrate species. They function as nurseries and breeding grounds, supporting a vast array of life that contributes to overall ocean biodiversity.
Beyond their ecological significance, coral reefs offer substantial benefits to human populations. They serve as natural barriers, protecting coastlines from erosion, storms, and floods by significantly reducing wave energy. This coastal protection helps safeguard communities and infrastructure worldwide. Economically, coral reefs support millions of jobs globally through tourism, recreational activities, and fisheries. The annual economic value derived from reefs, including through tourism and seafood production, is estimated to be in the tens of billions of dollars.