How Do Sea Anemones Eat? Stinging Tentacles at Work

How an anemone catches a meal, step by step

A sea anemone is a sit-and-wait predator. It cannot chase anything, so it relies on prey bumping into its tentacles.

1. Contact. A shrimp or small fish brushes a tentacle. Stinging cells usually fire only when they sense both touch and certain chemicals from prey, so they do not waste their ammunition on sand or seaweed.
2. Firing. Each stinging cell, or cnidocyte, holds a capsule called a nematocyst with a coiled, hollow thread inside. The capsule bursts open and the thread shoots out, piercing the prey and injecting venom, or wrapping and sticking to it.
3. Holding. Hundreds or thousands of threads fire at once. The venom paralyzes the prey, and the tentacles grip it.
4. Delivery. The tentacles bend toward the center and pass the prey to the mouth. The lips can stretch wide to take surprisingly large items, and the prey is pushed into the throat.

Nematocyst firing is one of the fastest movements known in biology. In a 2006 study on hydra, a freshwater relative of sea anemones, the capsules discharged in about 700 nanoseconds, with accelerations of more than 5 million times the force of gravity. Each capsule can fire only once and then has to be replaced by a new cell.

What do sea anemones eat?

What an anemone eats depends on its size, the length of its tentacles and where it lives:

• Plankton feeders. The plumose anemone (Metridium) of North Atlantic and North Pacific coasts has a feathery crown of many slender tentacles that strain tiny animals from the current.
• General predators. Most rock pool anemones, such as the beadlet anemone of European shores, eat small crustaceans, worms, snails and anything else that drifts or crawls into reach.
• Mussel eaters. On the Pacific coast of North America, the giant green anemone feeds on mussels and other animals knocked loose by waves and dropped onto its tentacles.
• Big-game hunters. Large tropical anemones can catch small fish, and the sun anemone (Stichodactyla helianthus) of the Caribbean can trap sea urchins by folding its carpet-like disc around them.
• Specialists. The walking anemone of South Africa’s Cape Peninsula hunts sea fans, eating them down to their skeletons.

The large magnificent sea anemone of Indo-Pacific reefs catches small fish, fry, shrimp and other invertebrates, but much of its food comes from another source: algae.

Digestion in a single body cavity

Once prey passes through the mouth and a short throat, it enters the gastrovascular cavity, a large chamber that fills most of the body. The cavity is divided by thin walls called mesenteries, and the edges of these walls carry filaments that release digestive enzymes.

The enzymes break the prey down into a soup of small particles, which are absorbed by the cells lining the cavity, where digestion is completed. There is no separate exit. Hard shells, bones and other parts that cannot be digested are pushed back out through the mouth. It is a simple system, but it works well for an animal that eats whole prey at irregular intervals. Sea stars take a very different approach, as described in How do starfish eat?

Help from algae

Many sea anemones, especially in sunny tropical waters, have a second food supply. Single-celled algae called zooxanthellae, and in some cooler-water species green algae called zoochlorellae, live inside the anemone’s cells. Using sunlight, the algae make sugars and other compounds, such as glycerol, glucose and the amino acid alanine, and pass much of them to their host.

For anemones like the magnificent sea anemone, this partnership is a major part of the diet, which is why they live in clear, shallow water and spread their tentacles toward the light. When the sea gets too hot, the partnership breaks down. The anemone expels its algae and turns pale, a process called bleaching that also affects corals and is explained in What is coral bleaching?

Help from clownfish

Clownfish living in a host anemone also contribute to its diet. Their waste is rich in nitrogen and other nutrients that feed both the anemone and its algae. Clownfish may also drop scraps of food among the tentacles, and their fanning fins move water over the anemone.

The fish protect their host too, chasing away fish that bite anemone tentacles. In a study in the Red Sea, host anemones with resident anemonefish grew faster and survived better than anemones without fish. How the clownfish avoid becoming dinner themselves is covered in Why don’t clownfish get stung?

How often do sea anemones eat?

Sea anemones do not have regular mealtimes. They eat whenever suitable prey comes within reach, which may be several times a day or rarely for long periods. They are good at coping with hunger: when food is scarce, many anemones shrink, using up their own tissue, and grow again when food becomes plentiful.

Anemones also defend their feeding space. Some species, such as the beadlet anemone, have special stinging structures below their tentacles that they use in slow-motion fights with neighbors, pushing rivals away from the best spots on the rock.