What Does Coral Eat? How Corals Farm Sunlight and Hunt Plankton

Short answer

• Corals eat in two ways at once. Most reef building corals get much of their energy from microscopic algae called zooxanthellae that live inside their tissues. These algae use sunlight to make food through photosynthesis and pass much of that food to the coral. At the same time, the coral animal itself catches food from the water. Each coral polyp extends tentacles armed with stinging cells, traps tiny drifting animals called zooplankton, along with bacterioplankton and other small particles, and pulls the catch into its mouth. Corals that live in deep water without sunlight have no zooxanthellae and must catch all of their food. When a coral bleaches, it loses its algae and therefore loses its main food supply, so it must rely on catching food alone until the algae return.

Corals Are Animals That Both Farm and Hunt

A coral looks like a rock or a plant, but it is an animal. The US National Oceanic and Atmospheric Administration (NOAA) places corals in the phylum Cnidaria and the class Anthozoa, making them relatives of jellyfish and sea anemones. A coral colony is made up of many small animals called polyps, each with a mouth ringed by tentacles. What makes coral feeding unusual is that the animal does not rely on hunting alone. It also hosts a partner that farms sunlight on its behalf.

Scientists describe this double strategy with the word mixotrophy, which means getting nutrition in more than one way. In the case of a reef building coral, the two ways are photosynthesis carried out by symbiotic algae inside the coral, and active capture of food from the water by the coral polyp. Neither way alone tells the whole story. The balance between them changes with species, depth, light, water movement, and season. A coral in bright, shallow water may lean heavily on its algae, while a coral in dim or deep water leans more on hunting, or hunts entirely.

This double diet also explains a famous puzzle of coral reefs. Reefs grow in clear, warm, tropical water that is poor in nutrients, yet they support an extraordinary abundance of life. NOAA notes this paradox directly: reef building corals need clear water so sunlight can reach their algae, but reefs are among the most productive and diverse marine environments on Earth. The partnership inside the coral is a large part of the answer. The algae turn sunlight into food, and the coral and algae recycle nutrients between each other so tightly that little is wasted.

This article explains coral feeding itself: what a coral eats, how it catches food, and what changes when light, depth, or stress alters the balance. It is about the coral animal and its algae, not about the wider reef community. For the animals that live among corals, see animals on the coral reef. For how energy moves from one reef creature to another, see the coral reef food chain.

The Polyp: A Small Animal Built for Catching Food

To understand what coral eats, start with the polyp. A polyp is a soft, tube shaped animal with a mouth at the top, surrounded by a ring of tentacles. In a hard coral, the polyp sits in a cup of calcium carbonate that it builds itself, and over time those cups pile up to form the reef framework. In a soft coral, the polyps join to form flexible, tree like or fan like colonies without a rigid skeleton. NOAA describes soft corals as mostly colonial animals whose colonies can resemble trees, bushes, fans, whips, and grasses.

The tentacles are the coral’s hunting tools. They carry stinging cells called nematocysts, the same kind of cells that jellyfish use. When a small drifting animal brushes a tentacle, the stinging cells fire and help hold the prey. The tentacles then bend toward the mouth and pass the food inside, where it is digested in the polyp’s central cavity. NOAA’s Coral Reef Conservation Program describes this plainly: during feeding, a coral polyp extends its tentacles out from its body and waves them in the water current, where they encounter zooplankton, bacterioplankton, or other food particles.

A coral cannot chase prey. It is fixed in place, so its hunting depends on water movement bringing food within reach. Currents carry plankton past the colony, and the extended tentacles act like a living net. This is why many corals feed best when there is a gentle current, and why a coral with its tentacles withdrawn is usually not hunting at that moment. The polyp can also pull its tentacles back into its body for protection during the day or when disturbed, then extend them again when conditions favor feeding.

Size matters in coral hunting. A coral’s prey is typically microscopic zooplankton, NOAA states, not large animals. The exact size a coral can catch depends on the size of its polyps and the spacing of its tentacles. Small polyp corals catch small particles, while larger polyp corals can handle larger items. This simple rule explains much of the variation in coral diets: different species are built to catch different sizes of drifting food.

Food Source One: Zooxanthellae and Photosynthesis

For most reef building corals, the largest food source is not something the coral catches at all. It is food made inside the coral by algae. Zooxanthellae are microscopic, single celled algae that live within the cells of the coral’s inner tissue layer. NOAA describes the relationship as mutually beneficial. The coral gives the algae a protected place to live, plus carbon dioxide and water produced by the coral’s own cells. The algae use sunlight to turn carbon dioxide and water into oxygen and the building blocks of sugars and proteins. The coral then uses those products to make proteins, fats, and carbohydrates, and to produce calcium carbonate for growth.

The scale of this transfer is large. NOAA reports that as much as 90 percent of the organic material the algae manufacture through photosynthesis is transferred to the host coral tissue. Because photosynthesis needs light, this energy production happens during daylight hours. That is why reef building corals are generally found in clear, shallow water: sunlight must reach the algae inside them. NOAA notes that reef building corals generally grow best at depths shallower than 70 meters, and that clear water with little suspended material is essential for their algae to photosynthesize.

The algae also give many stony corals their color. The Smithsonian Institution explains that the brownish green specks visible in a coral polyp are the zooxanthellae, and that the pink color of some coral colonies comes from the algae living inside. A healthy coral’s color is therefore, in part, a sign of a well populated algal partnership, not a color produced by the coral animal alone.

It is important to be precise about who does what. The coral animal itself does not photosynthesize. Its algae do. The coral benefits from photosynthesis without being a plant, and that is why older descriptions that call corals plants are wrong. Corals are animals that host photosynthetic partners. When people say a coral “eats sunlight,” they mean this partnership: sunlight powers the algae, and the algae feed the coral.

The partnership also recycles nutrients. Tropical reef waters are poor in nutrients, but the coral and its algae pass nitrogen and other compounds back and forth, so the same nutrients get used again and again. NOAA calls this a tight recycling of nutrients in nutrient poor tropical waters, and identifies it as the driving force behind the growth and productivity of coral reefs. In effect, the coral and its algae form a small, efficient food system inside the animal itself.

Food Source Two: Catching Plankton and Particles

The second food source is the one most people picture when they ask what coral eats: the coral catches living prey. The main catch is zooplankton, the tiny drifting animals of the sea. These include copepods, small crustaceans, invertebrate larvae, and other microscopic animals carried by currents. NOAA states that a coral’s prey is typically microscopic zooplankton, and that polyps also encounter bacterioplankton, which are drifting bacteria, and other food particles.

Corals also capture phytoplankton, the microscopic plant like plankton of the sea, and detritus, which is fine organic debris. A NOAA supported study of mesophotic corals, corals living in middle light depths, found diets that included zooplankton, phytoplankton, and other microscopic organisms, with the mix changing by season. In that study, corals appeared to eat more phytoplankton in spring, following a spring phytoplankton bloom, and more zooplankton in fall, when zooplankton peaked. The exact menu varies by species and place, but the pattern is consistent: corals eat what the water brings them, in the sizes their polyps can handle.

Hunting also supplies nutrients that photosynthesis alone cannot provide. Sugars from algae are rich in energy but poor in some nutrients, while animal prey supplies proteins and other compounds the coral needs to build tissue. Researchers studying coral nutrition have found that the contribution of hunted food is often larger than older methods suggested, because much of the carbon from prey is quickly respired or released rather than stored in tissue, while nitrogen from prey is taken up into tissue more strongly. In other words, counting only what stays in the coral can undercount how much hunting matters. The practical point for readers is simple: both food sources matter, and they complement each other. Algae supply energy, and prey supplies building materials.

Corals also feed on particles that are not alive in the usual sense. Marine snow, the slow rain of organic particles sinking from surface waters, is an important food source for deep water corals in particular. The Smithsonian Institution describes deep sea corals catching drifting particles called marine snow as they fall from productive surface waters. Even in shallow reefs, fine particles and organic debris contribute to the diet, especially for corals with small polyps that specialize in small particle sizes.

Food Source Three: Dissolved Nutrients

A third, smaller part of coral nutrition comes from dissolved substances in seawater. Corals and their algae can take up dissolved inorganic nitrogen, in forms such as ammonium and nitrate, from the surrounding water. Research on mesophotic corals notes that corals obtain nitrogen through uptake of dissolved inorganic nitrogen, with ammonium generally preferred. Dissolved organic matter, which includes sugars, amino acids, and other dissolved compounds released by other organisms, can also be absorbed, though the size of this contribution to a coral’s overall diet is less well established than photosynthesis and plankton capture, and researchers still debate how large it is.

Readers should treat dissolved nutrients as a supplement, not as a main course. The well supported picture is that photosynthesis and particle capture provide the bulk of coral nutrition, while dissolved uptake helps meet specific nutrient needs, especially nitrogen for the algae. Claims that corals live mainly on dissolved food would go beyond what the institutional sources used here support, so this article does not make that claim.

Hard Corals and Soft Corals Eat Differently

Hard corals and soft corals share the same basic feeding toolkit, polyps, tentacles, and stinging cells, but they use it differently. Hard corals, also called stony or scleractinian corals, build a rigid calcium carbonate skeleton and are the primary reef builders. Most reef building hard corals host zooxanthellae and depend heavily on photosynthesis. Their feeding is therefore strongly tied to light and clear water. NOAA notes that hard corals that form reefs are called hermatypic corals, and that they are the primary reef building corals.

Soft corals, also called Alcyonacea, do not produce a rigid skeleton and do not form reefs, though they live in reef ecosystems. Many soft corals also host zooxanthellae, but as a group they tend to rely more on capturing food from the water than hard reef builders do, and some soft corals have no zooxanthellae at all. Their flexible, branching or fan like shapes help them face currents and expose many polyps to passing food. A soft coral colony swaying in a current is, in feeding terms, a large net held open in moving water.

This difference is a matter of emphasis, not a strict rule. There are hard corals that hunt heavily and soft corals that depend strongly on algae, and individual species vary. The useful general point is that hard reef builders are usually the most dependent on sunlight and their algae, while soft corals as a group show a wider range of feeding strategies, including a stronger reliance on captured food. Any statement stronger than that, such as claiming all soft corals eat one way, would oversimplify a diverse group.

When Corals Feed: Day and Night

Coral feeding follows a daily rhythm, because its two food sources operate on different schedules. Photosynthesis happens during the day, when sunlight reaches the algae. Hunting happens most actively at night, when many corals extend their tentacles and when zooplankton rise toward the surface. By day, many corals keep their tentacles partly withdrawn and let their algae do the feeding work. After dark, the same colonies open up and hunt.

This rhythm is not absolute. Some corals feed during the day as well, especially when currents bring food past them, and some keep their tentacles extended around the clock. Water movement, light, and the local plankton cycle all influence the pattern. But the general day and night division is real and useful: day is mainly for farming sunlight, and night is mainly for hunting plankton.

The night plankton rise is part of a daily migration in the sea. Many small animals spend the day in deeper, darker water and rise toward the surface at night to feed, then sink again before dawn. Corals sitting on the reef intercept this traffic. A coral extending its tentacles at night is fishing in a passing stream of food that was not there during the day. This is one reason a reef can look quiet and closed by day and busy and open at night, even though the same animals are present throughout.

Feeding also varies with season and tide. Plankton abundance changes through the year, and the NOAA supported mesophotic study described above found seasonal shifts in coral diets linked to plankton blooms. Currents driven by tides can also change how much food passes a colony. A coral cannot move to find food, so its feeding success depends on what the water delivers, when it delivers it, and whether the polyp is open to catch it.

Shallow, Middle Light, and Deep Water: Three Different Diets

Depth changes coral feeding more than almost any other factor, because depth controls light. Three depth zones illustrate the range.

Shallow water corals live where sunlight is strong. These are the classic reef building corals, and they depend most on zooxanthellae. Their algae can supply a large share of their energy, with NOAA reporting that as much as 90 percent of the organic material made by the algae is transferred to the coral. Hunting still matters for nutrients, but sunlight is the main engine.

Mesophotic corals live in middle light, roughly from 30 meters to more than 150 meters deep in tropical and subtropical regions, according to NOAA. These corals still host zooxanthellae and still need light, but light is weaker, so they often grow in shapes that catch as much light as possible, such as flattened plates. Research on mesophotic corals shows a mixed strategy: some species remain strongly dependent on their algae, while others rely more on captured food to make up for lower photosynthesis. The balance varies by species and by local food availability, which is why mesophotic coral diets are an active area of research rather than a settled single answer.

Deep sea corals live where there is effectively no light, often defined as below about 200 meters. These corals have no zooxanthellae, NOAA states, because algae cannot survive where there is no light. Deep sea corals therefore gain all of their nutrition by filter feeding organic matter from the water. The Smithsonian Institution describes the same point: in deeper waters, corals have no zooxanthellae at all and must catch all their own food, using tentacles armed with stinging cells to catch marine snow and small drifting animals. Deep sea corals often grow slowly, which fits a life where food arrives only as particles drifting past in cold, dark water.

The lesson is that there is no single coral diet. Asking what coral eats has a different answer at 5 meters, at 60 meters, and at 1,000 meters. The shallow answer is mainly algae plus plankton. The middle light answer is a shifting mix. The deep answer is hunting alone.

What Bleaching Does to Coral Feeding

Bleaching is, at its core, a feeding crisis. When corals are stressed, most often by water that is too warm, they expel the zooxanthellae living in their tissues. NOAA’s tutorial explains that the colony then takes on a stark white appearance, because the coral tissue itself is mostly clear and the white skeleton shows through. The color loss is the visible sign, but the nutritional loss is the serious problem: the coral has lost its main food source.

NOAA states the consequence plainly. If corals go for too long without the nutrients that zooxanthellae provide, bleaching can result in the coral’s eventual starvation and death. A bleached coral is not dead, and NOAA’s bleaching guidance stresses that corals can survive a bleaching event if the stress is not severe and does not last too long. During bleaching, the coral falls back on hunting alone. The Smithsonian Institution makes the same point: coral animals can survive for a short time without their main food source by catching particles from the water with their tentacles, but they are more susceptible to disease and other disturbances.

Whether a bleached coral recovers depends on whether conditions improve and the algae return, and on how much food the coral can catch in the meantime. Research on bleached corals has found that richer, more varied hunted food can partly compensate for the lost algae, helping bleached corals grow better than those given only simple dissolved food. That finding matters, but it should not be overstated: partial compensation is not a replacement. A bleached coral running on hunting alone is living on a reduced and less reliable food supply, and the longer bleaching lasts, the harder recovery becomes.

Bleaching also shows why the two food sources cannot be treated as interchangeable. Algae provide a steady daytime supply of energy in shallow water. Hunting provides a nightly and variable supply that depends on currents and plankton abundance. Losing the steady supply and keeping only the variable one leaves the coral vulnerable, even if it catches food every night. Recovery, when it happens, means the algae return and the steady supply resumes.

What Corals Do Not Eat

Clearing up common misunderstandings helps sharpen the answer. Corals do not eat seaweed or large fish as a regular diet. A coral polyp is small, fixed in place, and built to catch microscopic prey, not to pursue or swallow large animals. Reports of corals catching small fish or larger invertebrates refer to occasional, small items caught by larger polyp species, not to a general coral diet. The typical prey, as NOAA puts it, is microscopic zooplankton.

Corals also do not eat sunlight directly. The algae inside them use sunlight, and the coral eats the food the algae make. This distinction matters because it explains why cloudy water, sediment, or shading can starve a coral even when plankton is present: the problem is not a lack of food in the water, but a lack of light for the algae.

Corals do not graze on algae the way parrotfish or sea urchins do. The algae that feed a coral live inside it, in a partnership, rather than being eaten from the reef surface. Grazing animals keep algae from overgrowing corals, which is a separate and valuable role on a reef, but it is not how corals feed. For the wider cast of reef feeders, from grazers to predators, the coral reef food chain gives the fuller picture, and the living factors that shape reef life, including the coral algae partnership itself, are covered in biotic factors for coral reefs.

Why Coral Feeding Matters Beyond the Coral

Coral feeding shapes the whole reef. When algae feed corals efficiently, corals grow, build calcium carbonate skeletons, and create the three dimensional reef structure that shelters countless other animals. When feeding fails, through bleaching or through loss of light and clear water, growth slows, skeletons weaken, and the habitat that other animals depend on declines with it.

Feeding also links corals to the wider ocean. Plankton caught by corals comes from surface waters and currents far beyond the reef. Marine snow falling to deep sea corals connects deep reefs to surface productivity. Nutrients recycled inside the coral algae partnership reduce the reef’s dependence on nutrient rich water, which is one reason reefs can thrive in nutrient poor seas. In each case, what a coral eats is not a private matter for the coral alone: it is a connection between sunlight, plankton, currents, and the reef structure.

Understanding coral feeding also clarifies what threatens reefs. Warm water that causes bleaching removes the main food source. Sediment and pollution that cloud the water reduce light for the algae. Changes in plankton abundance or currents change the hunting supply. A reef under stress is often, in feeding terms, a reef whose corals are getting less food from one or both of their sources. Protecting coral feeding therefore means protecting clear water, stable temperatures, and healthy plankton supplies, not just protecting the coral animals themselves.

Frequently Asked Questions

Do corals eat plankton?

Yes. Corals catch zooplankton, which are tiny drifting animals, along with bacterioplankton and other small particles, using tentacles armed with stinging cells. NOAA describes a coral’s prey as typically microscopic zooplankton. Plankton capture is the hunting half of the coral diet, alongside food supplied by symbiotic algae.

Do corals eat fish?

Not as a regular diet. Corals are small, fixed animals that catch microscopic prey. Larger polyp species may occasionally catch very small items such as tiny larvae or small invertebrates, but fish are not a normal part of what corals eat. Anyone describing corals as fish eaters is overstating an occasional event into a diet.

How do corals eat at night?

Most reef building corals extend their tentacles at night and catch zooplankton rising toward the surface, while their algae rest without sunlight. By day, the same corals often keep their tentacles partly withdrawn and rely on photosynthesis by their algae. Some corals also feed during the day when currents bring food past them, but night is the main hunting period for many species.

What do hard corals eat?

Hard corals eat the same two ways as other reef building corals: food made by zooxanthellae through photosynthesis, plus zooplankton and particles caught by their polyps. Because most reef building hard corals depend heavily on their algae, they need clear, shallow, sunlit water. Hunting supplies additional nutrients, especially proteins, that algae alone do not provide.

What do soft corals eat?

Soft corals eat zooplankton and particles caught by their polyps, and many also host zooxanthellae that supply food through photosynthesis. As a group, soft corals tend to rely more on captured food than hard reef builders do, and some soft corals have no zooxanthellae at all. Their branching or fan like shapes help expose many polyps to currents carrying food.

Do deep sea corals eat the same food as reef corals?

No. Deep sea corals live without light and have no zooxanthellae, so they cannot use photosynthesis. They get all of their nutrition by catching organic matter from the water, including zooplankton, small crustaceans, and marine snow falling from surface waters. Their diet is hunting alone, while shallow reef corals combine hunting with algal food.

What happens to a coral’s food supply when it bleaches?

Bleaching removes the coral’s main food supply. When stressed, usually by warm water, corals expel their zooxanthellae and lose the food those algae made. The coral can survive for a short time by catching particles with its tentacles, but it is under stress and more vulnerable to disease. If bleaching lasts too long, NOAA warns, the coral can eventually starve and die. Recovery depends on conditions improving and the algae returning.

Do corals make their own food?

The coral animal does not make its own food through photosynthesis. Its symbiotic algae do, and they pass much of that food to the coral. The coral itself is an animal that catches prey. Together, the coral and its algae form a partnership in which the animal hunts and the algae farm sunlight, which is why corals are described as mixotrophs rather than as plants or as pure hunters.

Sources & references

• National Oceanic and Atmospheric Administration, Coral Reef Conservation Program. Coral Facts. https://coralreef.noaa.gov/education/coralfacts.html
• National Oceanic and Atmospheric Administration, National Ocean Service. Corals Tutorial: What is Zooxanthellae? https://oceanservice.noaa.gov/education/tutorial_corals/coral02_zooxanthellae.html
• National Oceanic and Atmospheric Administration, National Ocean Service. What is coral bleaching? https://oceanservice.noaa.gov/facts/coral_bleach.html
• Smithsonian Institution, Smithsonian Ocean. Zooxanthellae and Coral Bleaching. https://ocean.si.edu/ocean-life/invertebrates/zooxanthellae-and-coral-bleaching
• Smithsonian Institution, Smithsonian Ocean. Deep-Sea Corals. http://ocean.si.edu/ecosystems/coral-reefs/deep-sea-corals
• National Oceanic and Atmospheric Administration, National Ocean Service. Corals Tutorial: Deep-sea corals. https://oceanservice.noaa.gov/education/tutorial_corals/media/supp_coral05b.html
• National Oceanic and Atmospheric Administration, Coastal Science. What's for Dinner? New Study Unlocks Mesophotic Coral Diets. https://coastalscience.noaa.gov/news/whats-for-dinner-new-study-unlocks-mesophotic-coral-diets/