Corals get most of their energy from tiny algae living inside their own tissue, but they also actively hunt, absorb nutrients from seawater, and scavenge dissolved organic matter. Up to 90 percent of a coral’s energy comes from these internal algae, with the rest supplied by a surprisingly diverse set of feeding strategies.
The Algae Living Inside Coral Tissue
The single biggest food source for most reef-building corals isn’t something they catch. It’s something they host. Microscopic algae called zooxanthellae live embedded in coral tissue and photosynthesize just like plants do. They take in carbon dioxide and water that the coral produces, then use sunlight to generate sugars and proteins. As much as 90 percent of what these algae produce gets transferred directly to the coral, according to NOAA. In return, the coral provides the algae with a protected home and a steady supply of the raw materials they need.
This relationship is why most reef corals grow in shallow, clear, sunlit water. The algae need light to photosynthesize, and if conditions get too deep or too murky, they can’t produce enough to sustain the coral. It’s also why coral bleaching is so dangerous: when water temperatures rise too high, corals expel their zooxanthellae, losing that dominant energy source almost entirely.
Hunting With Stinging Cells
Corals are animals, and they hunt like their relatives, the jellyfish. Each coral polyp (the soft, anemone-like creature that builds the hard skeleton) has tentacles armed with specialized stinging cells called nematocysts. These contain tiny, coiled, venom-filled tubes under high pressure. When a small organism brushes against a tentacle, the tube fires outward, puncturing the prey’s tissue and injecting a paralyzing toxin. The whole process happens in microseconds.
Once the prey is immobilized, the tentacles guide it into the polyp’s mouth. Corals primarily catch zooplankton, the tiny drifting animals that swarm in ocean water at night. This includes copepods, larval fish, small shrimp, and other invertebrate larvae. Some larger polyp species can capture bigger prey, while species with very small polyps rely more heavily on their internal algae and less on active hunting.
When and Why Polyps Open
If you’ve watched a coral reef during the day, you may have noticed the polyps tucked tightly into their skeletons. Many species extend their tentacles primarily at night, when zooplankton migrate up from deeper water and swarm near the reef. But the trigger isn’t just darkness.
Research on passive suspension feeders shows that polyp expansion is driven by a combination of factors. Water current speed and the concentration of zooplankton in the water are the two strongest triggers. When currents pick up and carry more plankton past the coral, polyps respond by rapidly extending. Chemical signals from zooplankton in the water appear to be especially potent stimuli. Even without direct contact, the presence of plankton “smell” in the current prompts corals to open up and start feeding. Temperature also plays a role, potentially linked to internal biological rhythms, but the combination of current flow and zooplankton presence is the primary driver.
How Corals Digest Food
Coral polyps have a simple but effective digestive system: a single internal chamber called the gastrovascular cavity. It functions like a primitive gut, lined with tissue that breaks down prey and absorbs nutrients. Food enters and waste exits through the same opening, the polyp’s mouth. Despite this simplicity, the cavity handles digestion, nutrient circulation, and even reproduction.
In colonial corals, where thousands of polyps share a connected skeleton, the gastrovascular cavities of neighboring polyps are linked. This means nutrients captured by one polyp can flow to others in the colony, allowing the whole structure to share resources. The interior of the cavity tends to be low in oxygen and rich in carbohydrates and metabolic byproducts, creating a unique internal environment with its own microbial community, much like the gut bacteria in more complex animals.
Nutrients Absorbed Directly From Water
Beyond photosynthesis and prey capture, corals pull inorganic nutrients straight from seawater. Nitrogen and phosphorus are the two most important. Nitrogen fuels tissue growth, repair, and the production of mucus and reproductive cells. Uptake of dissolved nitrogen from surrounding water can supply roughly 30 percent of a coral’s daily nitrogen needs. Phosphorus is a building block of DNA, RNA, and the energy molecule ATP, and it also supports the photosynthetic machinery of the zooxanthellae inside the coral.
Dissolved organic matter, the invisible soup of carbon-rich molecules released by all living things on the reef, is another resource. Microbes associated with coral tissue help process this material, and the recycling of dissolved organic matter is considered central to how coral reefs maintain their extraordinary productivity in nutrient-poor tropical waters. Reefs essentially run on tight internal recycling rather than large external nutrient inputs.
What Happens When Algae Are Lost
When corals bleach and lose their zooxanthellae, they lose up to 90 percent of their energy supply. To compensate, bleached corals become more dependent on heterotrophic feeding, meaning they rely more on catching zooplankton and absorbing organic matter from the water. Some species can ramp up their hunting enough to survive a bleaching event, at least temporarily. Others cannot increase their feeding rates sufficiently, especially under the same heat stress that caused the bleaching in the first place.
This variation helps explain why some coral species recover from bleaching events while others die. Corals with larger polyps and more effective prey capture tend to have a better shot at weathering the loss of their algae. Species that depend almost entirely on photosynthesis are far more vulnerable. The ability to switch between energy sources is, in many ways, a survival strategy that determines which reefs persist as ocean temperatures continue to rise.