Brain coral is a collective name for various massive, stony corals in the order Scleractinia, characterized by grooved, dome-shaped colonies that resemble the human brain. These organisms are foundational builders of coral reefs, forming large, durable structures in warm, shallow ocean waters. A colony is composed of thousands of individual polyps, which are soft-bodied animals. The survival and growth of the entire colony depend on a dual feeding strategy combining energy harvested from light and physical capture of food from the water column.
Primary Energy: Symbiotic Algae
The vast majority of a brain coral’s energy comes from a symbiotic relationship with microscopic algae called zooxanthellae. These single-celled dinoflagellates live within the tissues of the coral polyps, providing the host with a steady supply of nutrition.
The zooxanthellae perform photosynthesis, using sunlight to convert carbon dioxide and water into organic compounds. The coral receives a substantial portion of these products, primarily simple sugars and lipids. Up to 90% of the organic carbon produced by the algae is transferred to the coral host, fueling its metabolism and growth.
This nutrient transfer supports the energy demands required for building its calcium carbonate skeleton. In return, the polyps provide the algae with a protected habitat and access to compounds like nitrogen and phosphorus, which are byproducts of the coral’s waste. This reliance means the coral must live in clear, shallow waters where sunlight can easily penetrate.
External Diet: Capturing Particulate Food
While symbiotic algae provide the bulk of the coral’s energy, brain corals are also heterotrophs, actively feeding on particulate matter suspended in the water. This external diet is necessary for obtaining specific elements and complex molecules. Food captured from the water column supplies essential nutrients like nitrogen and phosphorus, which are often scarce in nutrient-poor reef environments and are not provided sufficiently by the algae.
Brain corals function primarily as suspension feeders, consuming a variety of small organisms and organic debris. Their diet includes zooplankton, such as copepods and larval stages of other marine creatures. They also capture phytoplankton and particulate organic matter (POM), which includes marine snow and general detritus floating through the water. Regular feeding with this particulate matter can significantly increase the coral’s growth rate and overall health.
The Physical Mechanics of Ingestion
Brain coral polyps use specialized structures to physically capture and ingest external food sources. The primary tools for securing prey are the polyps’ tentacles, which are typically extended to feed at night when zooplankton are more active. These tentacles are armed with microscopic stinging cells called cnidocytes, which contain a harpoon-like structure known as a nematocyst. When prey brushes against a tentacle, the nematocyst rapidly fires, injecting venom to stun or secure the organism. Once immobilized, the tentacles contract and move the prey toward the central mouth, or oral pore, of the polyp.
The mouth leads directly into the gastrovascular cavity, also known as the coelenteron, where digestion begins. For smaller particles, the coral utilizes a layer of mucus that covers its surface. Tiny hair-like projections called cilia, located on the polyps’ outer layer, create currents that sweep the mucus and trapped food particles toward the oral discs. Once the food enters the gastrovascular cavity, enzymes are secreted to break down the material in a process called extracellular digestion. The resulting nutrients are then absorbed by the cells lining the cavity.