How do Starfish eat? – (Nutrition and Feeding in Starfish)

Starfish belong to the Phylum Echinodermata of the Animal Kingdom. They are all invertebrates that is without the vertebral column or backbone.

Starfish are a part of the most beautiful animals of the Animal Kingdom. Some of the well-known and familiar starfishe belong to the Class Asteroidea.

Here, in this post we will talk about the nutrition of starfish from the zoological point of view.

There are approximately 2,000 species of starfish, all of which live in marine waters of the ocean.

The following description of the nutrition in Starfish is generalized and based mainly on the starfish Asterias rubens.

The common starfish, common sea star, or sugar starfish (Asterias rubens) is the most common and familiar starfish in the north-east Atlantic ocean.

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Introduction to Nutrition in Starfish

The common starfish (Asterias rubens) is one of the most commonly known, seen, and well-studied starfish till date.

The common starfish aren’t choosy eaters and are predatory in nature. They literally eat anything they can get their arms on.

They feed on a wide variety of organisms, but the most common are molluscs, aquatic annelids, small crustaceans, dead fish, and other echinoderms.

Notable and well-known predatory species include Asterias forbesi, Asterias vulgaris, and Asterias rubens in the Atlantic, and Asterias amurensis and Coscinasterias sp. in the Pacific ocean.

They can hunt prey much larger than their mouth. Some can also eat algae or organic detritus if the need arises.

As the starfish are slow moving, the same way the food that they prey upon are usually attached to rocks or otherwise unable to escape quickly.

They are aggressive hunters, not in the sense that they are aggressive to humans. They are so aggressive towards their prey like the shellfish that some can eject their stomachs out of their body to capture their prey.

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How do Starfish eat? Nutrition in Starfish takes place by 5 steps

1. Selection of Food: In this step, the starfish will first learn and recognize its food that it will be going to eat. After it has selected its food it will perform the below mentioned further steps.
2. Ingestion of Food: In this step, the starfish will extend its stomach out of its oral cavity to capture and devour its prey.
3. Digestion of Food: In this step, the starfish will perform the partial digestion of food outside its body, and the full digestion of the food inside its body.
4. Absorption & Assimilation of Food: It is the process through which the Starfish will absorb the various nutrients from the food and will utilize it for its various cellular metabolic uses.
5. Egestion of Food: It is the process through which the Starfish will egest or take out the undigested parts of the food and then will eliminate it completely out of its body through the anus.

Let’s understand each of these steps in detail…

1. Selection of Food: How do Starfish select the food to eat?

As already said, starfish are voracious carnivores meaning that they can eat anything they can get their arms on.

They are not pure carnivores as well so, supplementing their diets with algae or organic detritus too.

Mostly, it prefers to eat sedentary marine animals like clams, oysters, mussels, snails, crabs, barnacles, worms, sea urchins, and even other small starfish, whatever they do get.

At times it can also feed on small fishes and injured and dead marine animals. They are also known to act as marine decomposers in many of the cases.

You will usually find them single and free-living in the sandy and muddy bottoms in the ocean bed crawling over rocks and shells.

They usually move extremely slow on hard substratum or adhere firmly to it with the help of their tube feet.

Starfish have eyes on their arms and they use them to see their beautiful and food-rich coral reef environment.

And, after finding their food they slowly move near to their prey using their suction type tube feet.

They also use their tube feet, which have suction-cups on the bottom, to help them hold on to their prey.

2. Ingestion of Food: How do Starfish ingest the food?

The mouth of the starfish is located on the oral side in the centre of the central disc that is in the ventral part of the starfish’s body that is attached to the substratum.

Starfish shows a peculiar style of ingestion of food. It can extend its cardiac stomach out of its mouth and over the digestible parts of its prey.

It’s stomach is like a large sac having five lobes along the five radii.

Scientists have discovered a neuropeptide molecule that carries signals between neurons called NGFFYamide, which triggers the stomach to contract and retract back into the starfish.

Before extending its stomach out the starfish, it will fast go near to its prey (most probably a clam, oyster, etc.), and hold on to it by its arms and tube feet.

To hold on to its prey it will conveniently creep over the clam, place its whole body over it, and then it will firmly attach its tube feet to the two shell valves of the clam in such a way that the ventral margin of the clam will lie in front of its mouth.

It will then pull apart the two valves held tightly by the powerful adductor muscles and also with the pull action of its tube feet.

These adductor muscles are the very powerful muscles of the starfish.

When the two valves of the clam get opened, the starfish will now instantly extend its cardiac stomach out of its mouth and get it over the digestible parts of its prey that is in the mantle cavity of the calm to quickly devour it.

Sooner or later ingestion of the prey occurs, with the retraction of the cardiac stomach, but before that retraction, partial digestion of the food has to take place.

Let’s understand the digestion process in the next section of this article.

3. Digestion of Food: How do Starfish digest the food?

The digestion of the prey starts to happen outside of the body of the starfish.

This digestion starts when the cardiac stomach is everted and have got covered over the captured prey (here clam for example). The digestive secretions of the stomach and the pyloric caeca are poured over it, then.

Various digestive secretions like the enzymes proteases, amylases, lipases, acts on the food there itself and starts to digest the proteins, starch contents, fats respectively.

Digestion thus takes place outside of the body of the starfish and this type of digestion that occurs outside is better to be known as partial digestion.

This partial digestion turns that part of the prey into loose and liquid type of food.

These partially digested materials are then carried into the alimentary canal by the retraction of the cardiac stomach back inside the oral cavity of the starfish.

The further digestion occurs inside the starfish body inside the stomach and the pyloric caeca.

In the latter course of digestion, some intracellular digestion is also believed to occur within the cells of the stomach and the pyloric caeca.

4. Absorption & Assimilation of Food: How absorption and assimilation occur in Starfish?

Next after digestion, the absorption and assimilation of the fully digested food occurs. These are quick processes and nothing so elaborate have been known about these so far.

The digested food is absorbed mainly by the pyloric caeca and distributed to various parts of the body as nutrients by the coelomic fluid.

Any excess food will be stored in the storage cells of the pyloric caeca as fat molecules for future use of energy.

The intestine of starfish is really very short. And, as the absorption of food is more or less completed in the pyloric caeca, starfish doesn’t require a long intestine.

In very simple words, once the prey is sipped in through the cardiac stomach, it passes into the pyloric stomach.

From there, the nutrients travel down the pyloric ducts to the pyloric caeca at the end of each arm where nutrients are absorbed and stored.

Next, assimilation occurs. Assimilation is the movement of digested food molecules into the cells of the body where they are used.

For example: Glucose is used in respiration to provide energy, Amino acids are used to build new proteins, fats used as the future stock of energy.

5. Egestion of Food: How egestion occurs in Starfish?

After the digestion has completed in the stomach and pyloric caeca, and then sooner or later the absorption and assimilation of the nutrients have occurred, then by that time most of the undigested food is sent out through the mouth.

It is also to be noted that, in starfish very small amount of undigested food will be sent out through the anus, hence the intestine and rectum are very small.

The mouth and the anus are located opposite to each other in the body of the starfish.

Its anus is located on top that is on the dorsal side of the body, precisely in the middle. And its mouth is exactly the opposite, that is on the underside of the body.

The anus is a small, rounded aperture, situated somewhat eccentrically on the aboral surface of the central disc.

So, in very simple words, it can be concluded that as the starfish ingests partially digested food, it has a little undigested material which is mainly eliminated out through the mouth itself.

Little if any that remains is egested through the hole of the anus.