What Do Starfish Do?

Starfish, or sea stars, have been studied for decades, and scientists have learned a lot about their behavior in their natural habitat. Have you wondered what starfish do during their day? In this post, we’ll talk all about that but let’s begin with a quick answer:

Starfish spend their time doing various activities, including eating, breathing, moving, protecting themselves from predators, and reproducing. They also play an essential role in our ecosystem as keystone species.

However, this certainly doesn’t tell the whole story and because there are about 2,000 different species of starfish, some of them may differ a lot. Below I’ll explain every activity starfish do, and what they do to our ecosystem. Read on!


You may be surprised, but starfish are active predators. Their favorite food is mollusks such as oysters, clams, and mussels, but they also will eat any small-moving animals nearby or eat fish that are injured and unable to move away.

Starfish also feed on algae, snails, marine worms, sea urchins, sea cucumbers, other small starfish, or organic detritus – a dead particulate organic material on the bottom of the ocean where sea stars live. Their diet may depend on what’s currently available and on the species. For example, Crown-of-thorns starfish feeds on coral polyps.

Interestingly, starfish have a very unique way of eating. They feed by extending their stomach out of their mouths and wrapping it around their prey. Then, they digest the food directly, and when the dinner is finished, they pull their stomach back into their bodies.

starfish eating fish
You can see the starfish stomach wrapped around the fish

Read more about how starfish eat in my other blog post: “How Do Starfish Eat?”


Obviously, starfish, in order to survive, need to breathe. This is definitely their top priority activity, together with eating. But the way they breathe might not be that obvious.

Starfish don’t have lungs or gills like fish do, so they use their papulae (skin gills) and tube feet for breathing. They absorb the oxygen directly from the seawater and exchange gases through diffusion. You can read more about this process in my other post: “How Do Starfish Breathe?”.


Another starfish activity is moving, and because a lot of people haven’t seen starfish moving before, you may wonder how and why starfish move. Interestingly, starfish cannot swim, but they walk using their tube feet underneath their bodies.

Starfish have hundreds or thousands of tube feet, and at the end of each, they have a small suction cup, which can attach to objects like rocks or corals. Thanks to the suction motion, starfish can also open up the shell of their prey.

Interestingly, they can move their tube feet thanks to a system of canals in their bodies called the water vascular system. They pump the seawater inside these canals and create contractions. Read more about the system and how starfish move in my other post.

However, why do starfish move, and why it’s so rare to see them moving? First of all, starfish are considered to be one of the slowest animals on the planet, right after a three-toed sloth and a garden snail. Therefore, if you saw a starfish not moving, maybe it was moving extremely slowly, and you haven’t noticed?

The main reason starfish move is to find food or shelter far from their predators. Nevertheless, starfish don’t move a lot and aren’t in a continuous state of motion. They prefer to attach themselves to the rocks, which also protects them from predators.

Protecting themselves from predators

Marine animals are constantly exposed to danger, and most of the time, they hide from predators. Depending on starfish sea habitat, they have developed different defense strategies. The first strategy is hiding under large rock structures, like coral reefs, home to 25% of the marine animals. They can spot them with their eyes.

Some starfish don’t hide, but thanks to their tube feet, they attach firmly to the rocks. Their firm attachment and rough outside skin make challenging for predators to bite. Some starfish are also covered with sharp venomous spines like Crown-of-thorn starfish.

Another defense strategy – or I would even call it a superpower – is releasing and regenerating their arms. When starfish are attacked, they can either drop off their arm or, if the predator has already taken a bite, they can regrow the lost part.

Many starfish can regenerate their body if they were bitten in half, but some can regenerate even from a single arm. Starfish larva can even clone itself when exposed to danger but we’ll talk more about it below.


When starfish reach two years old, they start reproducing. Most of them reproduce in a discrete and predictable annual cycle, but many complex environmental factors control their reproduction, such as water temperature, light, or the phase of the moon.

Starfish can reproduce sexually and asexually. 


Most starfish reproduce sexually through broadcast spawning, which means they release their eggs into the seawater. During this process, several females release their eggs, and several males release their sperm into the water column, all at the same time.

Once eggs are fertilized, they develop into a larva that floats freely in the ocean. Next, it settles on the seafloor and develops into the juvenile and, later – adult starfish. Interestingly, they are some brooding species that lay eggs under their bodies or on the rocks instead of releasing them into the water. 

Asexual reproduction 

Some starfish species can reproduce asexually by fission of their bodies and others by autotomy of one or more of their arms.

Fission occurs when the animal splits its body into two parts, and each half regenerates into a new and complete individual. The division takes place either across the sea stars’ central disc or at the base of the arm.

Some species, such as Linckia multifora, have even more remarkable regenerative capabilities and can grow a complete individual from just a single arm. Interestingly, asexual reproduction is often a response to predators, so they are a less obvious target.

What do starfish do for the ecosystem?

Starfish are important members of the marine environment and essential keystone species for the ecosystem. They’re referred to as keystone species because their feeding habits affect the entire ecosystem.

Sea stars feed on mussels and barnacles, keeping their population in check. If starfish were removed, the mussel population would expand rapidly, covering rocky intertidal shores and driving out other species.

Interestingly, Ochre starfish were the first animals to be identified as keystone species. In 1966, ecologist Robert Treat Paine conducted an experiment during which he removed starfish from the shoreline of the Pacific Northwestern region. He observed dramatic changes in the ecosystem due to the increased population of mussels, barnacles, snails, etc.

This population increase destroyed the majority of seaweed, causing a decrease in the population of many species that used to feed on the seaweed. This disturbed the entire food chain balance in that area and proved that a single predator could control the diversity and distribution of other organisms sharing the ecosystem.

Another reason why starfish are important to our ecosystem is that they consume dead organic matter and carry out the decomposition process. This means they help to recycle nutrients back into the environment, and the cycle continues with other animals using the nutrients.

Through decomposition, starfish play a significant role in the food chain. Without them, the organic matter would pile up, its nutrients would go to waste, and organisms at the beginning of the food chain wouldn’t survive, destroying the entire chain balance.

Starfish as decomposers are considered to be the “cleaning crew” of the ecosystem.


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