The vibrant structures known as coral reefs are built by millions of tiny animals called coral polyps. Each polyp is a small, sac-like invertebrate belonging to the phylum Cnidaria, related to sea anemones and jellyfish. A coral polyp’s nutritional strategy is complex, relying on diverse mechanisms that allow it to thrive in the nutrient-poor waters of tropical oceans. This unique combination of feeding behaviors enables reef-building corals to sustain the growth of the largest biological structures on Earth.
The Symbiotic Foundation: Zooxanthellae
The primary source of energy for reef-building corals comes from a partnership with single-celled algae called Zooxanthellae. These microscopic organisms live within the coral polyp’s tissues in a mutually beneficial arrangement. The coral provides the algae with a protected environment and access to the carbon dioxide and nitrogenous waste products necessary for photosynthesis.
The Zooxanthellae use sunlight to convert these waste products into energy-rich organic compounds, just like plants on land. The most significant products of this process are carbohydrates, such as sugars and glycerol, which are then transferred directly to the host coral’s tissues. This transfer is highly efficient, supplying the coral with up to 90% of its total energy requirements.
This reliance on photosynthesis explains why coral reefs are generally limited to clear, shallow tropical waters. The water must be transparent enough for sunlight to penetrate to the Zooxanthellae housed inside the polyps. This constant, internal source of nutrition is the driving force behind the rapid growth and productivity of coral reefs in an otherwise nutrient-sparse marine environment.
The relationship allows for an efficient recycling of nutrients, which permits corals to flourish in what marine biologists sometimes refer to as oceanic deserts. Without this symbiotic energy source, the growth rate of the coral skeleton would be significantly reduced. When corals experience stress, such as from high water temperatures, they may expel these algae, a phenomenon known as coral bleaching.
Active Capture Using Tentacles and Nematocysts
While the symbiotic algae provide most of the energy, coral polyps are also active, carnivorous predators that hunt for external food sources. Corals are generally nocturnal feeders, extending their ring of tentacles into the water column when the sun sets. This hunting behavior allows them to capture small prey, primarily microscopic animals known as zooplankton.
The tentacles are armed with specialized stinging cells called nematocysts, which are unique to all cnidarians. Each nematocyst contains a miniature, coiled, barbed harpoon housed under high pressure. When a zooplankter brushes against the tentacle, the nematocyst is triggered, rapidly launching the barb and injecting a paralyzing or lethal venom.
The venom immobilizes or kills the prey, which is then held fast by the discharged barbs and the sticky surface of the tentacle. The tentacles contract, drawing the captured food toward the polyp’s central mouth opening. This active predation ensures the coral receives external sources of protein and lipids, supplementing the carbohydrates provided by the Zooxanthellae.
This capture method is particularly important for acquiring nitrogen and phosphorus, which are often limited in tropical waters but are concentrated in plankton. Corals that do not host Zooxanthellae rely entirely on this type of active capture for their survival. The ability to switch between internally-produced and externally-captured nutrients makes the coral a highly adaptable feeder.
Passive Collection and Absorption
Beyond the two main feeding methods, corals employ passive strategies to gather nutrients from the surrounding seawater. One significant passive method involves the secretion of a sticky layer of mucus that coats the polyp’s exterior. This mucus acts as a net, passively trapping fine debris, bacteria, and microscopic food particles.
The trapped material is collectively known as Particulate Organic Matter (POM), including detritus and small aggregates of organic material. Cilia, which are tiny, hair-like structures covering the coral’s surface, then sweep the mucus sheet and its trapped cargo toward the mouth. The mucus itself is rich in organic carbon and nitrogen, effectively recycling nutrients back into the coral’s diet.
Another non-predatory feeding behavior is the direct absorption of Dissolved Organic Matter (DOM) from the water column. DOM consists of organic molecules, such as amino acids and simple sugars, that are dissolved in the seawater. The coral polyp’s surface tissues can actively take up these molecules, essentially drinking nutrients directly from the ocean.
These passive methods become more important for the coral when light levels are low, such as throughout the night when the Zooxanthellae cannot photosynthesize. By combining active predation, passive collection, and the energy from their internal algae, coral animals maintain a complex and highly effective nutritional strategy. The ability to utilize multiple food sources is a key factor in the long-term survival of coral colonies and the stability of the entire reef ecosystem.