How Do Starfish Eat? Stomachs That Turn Inside Out

Where is a starfish’s mouth?

A sea star’s mouth sits in the center of its underside, facing the rock or seabed. From the mouth, a short tube leads to a stomach with two parts. The lower part, called the cardiac stomach, is large, folded and can be pushed out through the mouth. The upper part, the pyloric stomach, connects to a pair of long digestive glands in each arm, called pyloric caeca, which finish digestion and absorb the food.

There are no teeth and no jaws. Instead, sea stars use the pulling power of hundreds of tube feet, small water-filled tubes on the underside of each arm, and strong digestive enzymes. Many species have a tiny anus on the upper side of the disc, but a lot of undigested material, such as bits of shell, simply leaves through the mouth.

How do starfish open mussels and clams?

Opening a mussel looks impossible for a soft-bodied animal with no claws, but sea stars do it every day. The ochre sea star of North America’s Pacific coast is the classic example.

The sea star climbs onto the mussel and humps its body over it, attaching the tube feet of several arms to the two halves of the shell. Then it pulls, steadily and for a long time. The mussel closes its shell with a strong muscle, but it cannot hold it shut forever, and it also needs to open slightly to breathe and feed.

The sea star does not need the shell to open wide. Its stomach can slip through a gap only a fraction of a millimeter across, and mussels have a natural narrow opening where the threads that anchor them to rock come out of the shell. Once part of the stomach is inside, digestive enzymes start dissolving the mussel’s tissues, the shell muscle weakens and the shell falls open.

The stomach that turns inside out

Pushing the stomach out is called eversion. Muscles and fluid pressure squeeze the cardiac stomach out through the mouth, and it spreads over or into the prey like a thin, wet sheet. Enzymes released by the stomach and the glands in the arms digest the prey into a soupy mix, which is drawn back into the body.

When the meal is finished, ligaments attached to the skeleton pull the stomach back inside. Scientists have identified a natural signaling molecule, a neuropeptide, that triggers the stomach to contract and retract.

Eating this way has a big advantage: a sea star can eat prey far larger than its own mouth. A large mussel or clam would never fit through the mouth whole, but its tissues can be digested inside its own shell.

What do starfish eat?

Most sea stars are predators of animals that move slowly or not at all, but diets vary a great deal between species.

Many sea stars are opportunists and will take whatever suitable prey they find, including dead animals. Some can also absorb dissolved nutrients directly from seawater through their skin, which may add a useful extra to their diet.

Young sea stars eat differently again. The larvae of most species drift in the plankton and eat microscopic algae, sweeping them toward their mouth with bands of tiny beating hairs called cilia.

Starfish that swallow prey whole

Not all sea stars evert their stomach. Older, more primitive groups such as the sand stars, including Astropecten and Luidia, have no suckers on their tube feet and live on soft sand or mud. They swallow clams, snails and small sea urchins whole, begin digesting them inside the stomach and later spit out the empty shells.

The sunflower sea star of the North Pacific, one of the largest and fastest sea stars, also swallows many prey whole, including sea urchins, and pushes the hard leftovers back out. Its appetite for urchins helps keep kelp forests healthy, which is why its collapse during the sea star wasting disease outbreak worried scientists so much.

How do starfish find their food?

Sea stars have no nose, but they smell very well. The tube feet, especially those at the tips of the arms, are sensitive to chemicals in the water, and a sea star can follow the scent of prey or a dead animal. When one arm detects something attractive, it takes the lead and the others follow.

The simple eyespots at the arm tips help some species find their way to the right habitat, but sight plays only a small role in finding food.

How often do starfish eat?

Sea stars are slow, patient hunters. A meal of one large mussel or clam can take hours to finish. How often they feed depends on temperature, season and how much prey is around.

Intertidal species such as the ochre sea star feed mainly while the tide is in and stop when they are exposed to air. Experiments show that the ochre sea star eats and grows less when its body temperature rises above about 23 °C (73 °F), so hot weather at low tide can slow it down. Many sea stars also eat less in winter, and they can survive long periods without food by living off reserves stored in the digestive glands of their arms.

Why starfish feeding matters

The way sea stars eat has shaped whole coastlines. On the rocky shores of Washington, the ecologist Robert T. Paine showed in the 1960s that when ochre sea stars were removed, mussels took over almost all the space and many other species disappeared. That work gave ecology the idea of a keystone species, a predator with an outsized effect on its community. Sea otters, which also eat sea urchins, play a similar role in kelp forests, as explained in Why are sea otters a keystone species?.

Sea star feeding can also cause harm. Outbreaks of the crown-of-thorns starfish have stripped large areas of living coral from reefs in the Indo-Pacific, including Australia’s Great Barrier Reef.

For more about what makes sea stars such unusual animals, see Is a starfish a fish? and the purple sea urchin, one of the favorite foods of the sunflower sea star.