How Do Starfish Eat?

Starfish or sea stars are interesting marine animals with a very unusual way of eating. If you wonder how they eat, in this blog post, we’ll talk all about that. However, let’s begin with a quick answer:

Starfish eat by extending their stomach out of their mouths, wrapping it around their prey, and digesting food directly. They hold their prey, such as mussels, clams, or oysters, with their arms and tube feet so the caught animal can’t escape.

However, that certainly doesn’t tell the whole story. Below I’ll explain more about how starfish eat and kind of food they consume. Furthermore, I’ll explain how often and how much they eat. Read on!

How do starfish eat? (Explained)

Starfish have a very unique way of eating. First, they use their feet with suction cups and arms to hold the prey. They hold it on the oral side of their bodies, where their feet and mouth are located. Next, if the prey has a shell, such as mussels or oysters, they open it with their feet.

Following, sea stars extend their stomach out of their mouths over the digestible parts of their prey. What happens next is that they digest the food directly. When the dinner is finished, they pull their stomach back into their bodies.

They do not have teeth, so they don’t bite or chew their food. Because of this feeding strategy and because starfish are generally slow movers, they eat slow-moving small animals that are nearby. See the video below to see how the sunflower starfish eat its dinner.

Starfish digestive system

Let’s now look at what’s happening inside the starfish body while eating. As I mentioned previously, sea stars have mouths located at the center of their oral side (underside). Their anus is on their aboral side (upper surface). It means that starfish do have a complete digestive system.

The mouth connects with a short oesophagus, leading to the stomach. Interestingly, seastars have two separate stomachs: the outer muscular, cardiac stomach, and inner pyloric stomach. They play different roles in the digestive system.

The larger stomach that a sea star can push out of its body is called the cardiac stomach. It’s supported by ligaments attached to ossicles in the arms so it can be pulled back inside. When the seastar decides to eat, it pulls the stomach out, waps it around the prey, and secretes digestive enzymes to break down the prey.

Next, when it pulls the stomach back, the food goes to the second, smaller pyloric stomach, where digestion is complete. The pyloric stomach has two extensions into each arm, called pyloric caeca. These branched hollow tubes produce digestive enzymes and absorb nutrients from the food.

Next, the intestine and rectum run from the stomach and open out through the anus located on the starfish’s aboral surface. Some seastars lack an anus (for example, Ctenodiscus), and solid waste materials are ejected through the mouth. 

What do starfish eat?

Starfish eat other animals, which makes them carnivores. They eat slow-moving small animals that are nearby but also eat fish that are injured and unable to move away. Interestingly, starfish can prey on animals much larger than their mouths simply because of their ability to digest food outside of their bodies.

Their favorite food is mollusks such as oysters, clams, and mussels. As I mentioned previously, they can open up the shells of their prey by using their sticky tube feet. You can read more about how they use their feet and water vascular system in this blog post. 

Starfish also feed on algae, snails marine worms, sea urchins, sea cucumbers, other small starfish, or organic detritus, which is a dead particulate organic material on the bottom of the ocean where sea stars live. In this blog post, you can read more about where starfish live.

Other starfish species, like crown-of-thorns starfish, are corallivores, which means they feed on corals. They prefer hard corals like table and branching corals, but they can also eat sponges, soft corals, algae, and encrusting organisms. The crown-of-thorns starfish outbreak is a massive problem in the Great Barrier Reef. You can read more about it in my other blog post: “What Do Crown Of Thorns Starfish Eat?”.

What do baby starfish eat?

Baby starfish are tiny and barely invisible to the naked eye. They are about 1 millimeter when in the larval stage. They flow alongside plankton and algae, and during this time, they feed on minuscule plants and animals that float nearby.

When they’re juvenile, they feed on microalgae, and when they’re grown enough to catch food, their diet becomes the same as the adult starfish diet.

How much do starfish eat?

Because there are many different species of starfish (about 2,000), it’s very difficult to say the average amount of food they consume. In addition to that, many of them adapted to very different environments such as sandy sea floor, rocky shores, kelp forests, and more, and have very diverse diets.

Everything depending on a lot of on different factors such as food availability, kind of species, their size, and size of their prey because the time of digestion will be longer if the prey is large.

Some sources say that some starfish can consume about 50 small clams a week in the wild. This research says that A. indicus sea star can consume about 27 mussels (M. senhousia) at 24 hours but others consumed six or twelve during that time.

This research says about A. rubens sea star consuming about 3 scallops per day. However, the experiment was conducted in the tank so scallops could not escape.

Interestingly, there is a lot of research on the crown-of-thorn starfish diet since its problematic feeding habits. Each crown-of-thorns starfish individual can consume 6-10 square meters (65 – 107 sq ft) of living coral per year. Feeding rates may vary with the changing temperature, but the central GBR studies noticed that large adults consume as much as 478 cm2 (0.5 sq ft) of coral per day in summer.

What’s interesting about these species is that they can survive without food for nine months!

How often do starfish eat?

Again, because there are many different species of starfish, it’s challenging to say how often they eat on average. In order to survive, grow and reproduce, they need a positive balance between their daily energetic requirements and consumption rate. Therefore, the consumption rate can also be dependent on how active the sea star is.

Sea stars are moving all the time and can be more active at night, but again, depending on the species. Most of the time, starfish eat during the day and the night, and their consumption rate depends on food availability, conditions, species, season, and more.

Resources

• Loh, K & Todd, Peter. (2011). Diet and feeding in the sea star Astropecten indicus (Döderlein, 1888). Raffles Bulletin of Zoology. 59.
• Magnesen, Thorolf & Redmond, Kirsten. (2012). Potential predation rates by the sea stars Asterias rubens and Marthasterias glacialis , on juvenile scallops, Pecten maximus, ready for sea ranching. Aquaculture International – AQUACULT INT. 20. 10.1007/s10499-011-9451-y.