1. Stinging Tentacles Catch Prey and Pass It to the Mouth
Sea anemones belong to a group of animals called cnidarians, which also includes corals and jellyfish. What they share is a set of stinging cells called nematocysts. When one fires, it shoots out a barbed thread tipped with poison [1].
On top of an anemone's body is a flat disc. The mouth sits in the middle of it, and the tentacles ring the edge [2]. A small fish or shrimp that brushes the tentacles is stung, goes limp, and is swallowed whole through the mouth [2]. The anemone has no jaws or teeth, so this is the whole job of eating: sting, hold, swallow.
That already answers the title for one source of food. But it raises an odd question about what happens after the meal.
2. A Body With One Opening: Leftovers Leave the Way Food Came In
Cnidarians are described as essentially bags made of two layers of cells, with a gut inside [3]. An anemone has no separate anus. Food that cannot be digested is pushed back out through the mouth [2].
Many people call that "pooping," but that word can mislead, because the leftovers do not travel through a long tube and come out a different end. A more careful way to say it is that the anemone spits them out. Sources I could read do not explain why this body plan exists, so I will not guess.
With one opening for both taking in food and expelling leftovers, it may seem as if an anemone has few ways to get food. Yet anemones can live in places where prey is scarce. Part of the reason is that the mouth is not their only source of food.
3. Algae Inside the Anemone Make Food From Sunlight
Many anthozoans, the group that includes anemones and corals, add to their diet by growing algae inside their own tissues. These algae use sunlight, and in reef-building corals they may supply most of the coral's food [4]. Many shallow-water sea anemones keep similar partner algae, called zooxanthellae, and use what the algae make [2].
Two cautions. Not every anemone has these algae, so this does not describe the whole group [2]. And how much of an anemone's daily food comes from them differs from species to species, so no single percentage fits. A fair summary is that an anemone is a meat-eater and also a light-powered one, depending on where and how it lives.
4. A Clownfish's Waste Can Feed Its Anemone
Clownfish, also called anemonefish, live among the tentacles of certain anemones. Fish give off ammonia, a nitrogen-containing waste, and nitrogen is something living things need in order to build their bodies. Could the anemone use it?
In a laboratory study of two Red Sea species, the clownfish Amphiprion bicinctus and the anemone Entacmaea quadricolor, researchers measured ammonia going in and out. The amount of ammonia the fish released was highest about two hours after a meal. The anemone took ammonia up during the day but almost stopped at night, which the authors read as a sign that its algae were doing the absorbing. When fish and anemone were kept together, almost no ammonia was left in the water (the authors report the net release as virtually zero), so the anemone had taken up most of what the fish made [5].
A graduate thesis by the same first author estimated that the fish's nitrogen output could in principle supply more than 100% of the anemone's nitrogen needs. In a starvation experiment, anemones living with a fish or given added ammonia had a higher density of algae than those with neither. These details come from the thesis abstract only, so treat them as an estimate and an early result, not a settled number [6].
The limits matter. These were lab tests on two species. They show a path by which waste becomes food, not that every anemone and fish pair works this way.
5. Some Clownfish Also Bring Food to Their Anemone
In 2025, a team at Osaka Metropolitan University in Japan reported something more active. In the field, they offered food of various sizes and types to a clownfish, Amphiprion clarkii, living with the anemone Entacmaea quadricolor. The fish ate small pieces of animal food (krill, clams, squid and fish) and small green seaweed itself, but gave larger pieces of animal food to the anemone. It avoided eating or giving brown seaweed and sponges [7].
When the team repeatedly offered small pieces, the fish first ate them, then increasingly gave them to the anemone once it was full. Anemones that received food grew faster. The authors concluded that clownfish provide food to their host actively and according to the situation [7].
Read this with care too. It is one study of one clownfish species and its host, and I read the abstract and introduction, not every experimental detail. It makes the idea of a fish feeding its host a real possibility, not a rule for all clownfish.
6. How to Watch Anemone Feeding Without Touching
At an aquarium, or in a tide pool with an adult, you can look for these things without touching anything. The tentacles can sting, so keep your hands out of the water and stay on dry, safe footing.
- Where is the mouth, and what does it look like when the anemone is open?
- If a clownfish is present, does it ever carry something to the tentacles?
- Do the tentacles seem to move differently when something touches them?
If what you see differs from this page, that is not a mistake. Species differ in whether they have algae and in which fish live with them, so a careful observation can be the start of a new question. If you want to dig deeper, the 2008 paper and the 2025 paper in the sources are good places to start, and both have short abstracts.