Imagine yourself visiting a tide pool. What do you see? Perhaps your eye catches sight of a few crabs, anemones and mussels. But one inhabitant that you are sure to find is the starfish. Did you know that these creatures live both in the intertidal regions and farther out in the deep ocean? But, what exactly are they able to find in deep waters, and especially in a small tide pool?
What Do Starfish Eat?
Regardless of how far out from the ocean they are, a starfish is considered to be an omnivore. This means that they sustain themselves off of both plant material and other animals. It’s hard to think of a starfish as being carnivorous, but they do consume underwater sea creatures such as mollusks, mussels, and clams. In fact, they will even feed on other starfish in the area.
What a Star
Studying the starfish and its physical morphology allows us to see how unique they truly are. The unique characteristics can be used to identify their phylum, class and family and genus.
These critters can be found in the phylum with a hard exterior that you may have felt in a tidal pool. The class Asteroidea refers to their star-shaped appendages.
There are approximately 1,600 species of starfish that exist and thrive in one of the world’s oceans, covering a large distribution between sea beds and intertidal zones.
With such a large diversity, the starfish has become able to adapt to their surroundings. The starfish has evolved to become either omnivorous or carnivorous depending on what the situation needs. These separate species do not always share the same preferences when it comes to their diet.
What Do They Eat in the Ocean?
Living out in the ocean means that you have to make the most of what prey comes along. Because of their expansive meal choices, a starfish is considered a generalist predator. This is extremely beneficial as they are slow-moving creatures.
In fact, the fastest that a starfish can travel is at about 30 centimeters per minute, which equates to roughly 60 feet per hour. Compared to other deep sea creatures, stealth is not the strongest advantage. Instead, they must find food that is just as, if not slower than themselves.
Starfish are considered invertebrates, meaning that they do not have a backbone. But did you know that they eat other invertebrates? This technically classifies them as either carnivorous or omnivorous depending on if they include plants into their diets.
Meals that contain fellow invertebrates include clams, mollusks, oysters, mussels, barnacles, marine snails, and sand dollars.
Many of these animals can be found in tidal pools, making those areas a great hunting ground for a hungry starfish.
Another way in which a starfish could be classified as a carnivore is if it consumed vertebrates, or animals that have a backbone. For some echinoderms, a fish can make a worthy meal option. With the speed of a fish, however, it is only possible when their prey is injured.
What’s one possibility when it comes to food that won’t move? Well, the easiest of these choices are plants. Although some starfish prefer to eat other animals, they still rely on plants for additional nutrients.
In the deep ocean, starfish will find decomposed organic plants known as detrivores that are easy to digest. A few species even consume coral polyps as a food source.
Unknown to most is the process in which a starfish consumes its prey. They don’t have the typical mouth that would be found in fish, dolphins, or even your housecat. So then how exactly do they get those nutrients?
How Do Starfish Eat?
Before going into how a starfish eats, first think about where the mouth is located.
Some believe that they don’t even have a mouth to begin with. And as a matter of fact, it is not the typical mouth found on a mammal like yourself.
Instead, it is located on the bottom. If you were to carefully turn a starfish over on to its back, you would see an opening in the center as well as small, spindly projections covering each leg. All of these are known as “tube feet” by scientists. One of their primary functions is to aid in feeding.
When a starfish comes along a dead fish, it needs to get it near the mouth on the bottom. The arms are not prehensile, meaning that they can’t grab the food item and feed themselves.
What happens is the starfish will move – at a slow pace – to the prey until it is close enough for touch. The tube feet on the bottom will then move the food along to the mouth, allowing the starfish to begin digestion.
With two rows of tube feet that have suckers, each arm is equipped and ready for any oncoming meals.
Now even though the tube feet help bring the food closer to the mouth, this does not mean that it can be pushed inside like a conveyer belt. The starfish has evolved one more adaptation that enables them to get the nutrients inside their body at a quicker pace.
The Use of a Starfish’s Stomach
Without the help of their stomach, a starfish would not be able to eat.
One of their most advantageous features is their cardiac stomachs. This specialized organ is unique in the regard that it can actually be extended outside of the mouth.
The stomach actually slowly covers the prey itself and begins the process of digestion. At this time, enzymes also commence to break down the prey item, partially digesting it before it even enters the animal’s mouth.
With the mouth, tube feet and cardiac stomach all working together, a starfish can eat a variety of prey whether it is omnivorous or a strict carnivore.
Depending on the species and where it resides, a starfish can either be considered omnivorous or carnivorous. To understand the meal preferences of a starfish, you must dive into a few of the most common species present in the world’s oceans.
Red knob starfish
The Red knob starfish (Protoreaster linkchii), is commonly referred to as the African sea star. True to the nature of the class, this echinoderm has five arms with a central mouth. One difference that is easily noticed are the red knobs, or tubercles, found on the top of the body.
These are used for a number of reasons, one of which being defense. This species is found to exist in the Indo-Pacific ocean regions, where it is heavily fished. In particular, their exoskeletons are desired in the curio trade, which are seaside décor sold worldwide.
This species of starfish is most active during the day, where it typically preys upon invertebrates. In fact, this individual is one of the predatory starfish individuals. Among the animals it consumes, the Red knob starfish eats soft corals, tube worms, clams, sponges and other starfish.
As you would assume, the Blue starfish (Linckia laevigata) can be identified by its blue coloration. Some individuals can be either a light hue while others take on a bold blue. This color can also include yellows, greens and pinks depending on the specimen.
The Blue starfish is found in the tropical waters of the Indian and Pacific Oceans. This also includes bodies of saltwater from western India to southeastern Polynesia. They seem to prefer reefs with water temperatures ranging from 22 to 26 degrees Celsius. This is due to the fact that they are extremely sensitive to changes in the water surrounding them.
Such as other starfish, this species is quite the opportunistic generalist. This species is omnivorous, generally feeding off of algae and marine worms. They will also scavenge for detritus, which is defined as an organic matter that exists when an organism begins decomposing.
Different Class, Same Phylum
Those belonging to the phylum Echinodermata include starfish because of their several arms, central body and bilateral symmetry, which is when the animal is not a mirror image on either side.
Although some species do not belong to the true starfish class, Asteroidea, it is still important to study those that are identical to the unknowing observer.
The Serpent starfish (Ophiolepsis superba) is known by many as the Fancy tiger striped starfish because of its remarkable stripes located on the “appendages”.
Unlike the starfish, this sea creature also has spindly arms to help with locomotion and feeding. They are found nearly in every ocean water around the world, including tropical, temperate and polar waters. They are not incredibly tolerant to changes in their water when it comes to pH level, salinity, or oxygen levels.
As for the diet of the Serpent starfish, they are strictly carnivorous. They typically forage for animals such as shrimp, mussel and fish to digest. When given the opportunity, they will even consume another starfish.
The Brittle starfish (Ophiothrix speculata), also known under the name Brittlestar belongs to the class Ophioroidea. Contrary to the common name, this echinoderm is not a member of the starfish family. Instead, it is closely related, yet shares many of the same adaptations.
The Brittle starfish is characterized by its long, spindly arms that can even reach up to 60 centimeters, or 24 inches in length. They are found in both deep and shallow waters around the world with a wide variety of habitats. This allows them to have a versatile diet.
The way in which a Brittle starfish consumes its prey is identical to that of a true starfish. The stomach is also able to extend past the mouth, yet one difference lies in the five teeth surrounding this body part. This is because they usually eat hardier invertebrates such as shrimp, squid and plankton. Being omnivores, they also consume decaying plant matter.
What Will a Starfish Eat in an Aquarium?
Because of the way that these animals forage, they need to wait until the food hits the bottom of the aquarium. Starfish in captivity are given a variety of options that are scattered along the exhibit’s sand bed. This typically includes leftover fish food that the other tank inhabitants were unable to eat.
It is also not unusual to see a starfish glued to a piece of coral or rock. This is because of the fact that they will gain nutrients from algae and sponges. Usually a captive starfish will be fed every 2 or 3 days for an adequate diet.
How Many Stomachs Does a Starfish Have?
Earlier on in the article, we discussed the cardiac system, which helps the starfish to digest the prey item before it fully enters the individual. But did you know that these animals actually have more than one stomach? In total, the starfish is equipped with two separate stomachs.
The second stomach included in the anatomy of a starfish is referred to as the pyloric stomach. As you know now, the cardiac stomach can be extended outside of the body. This is achieved through ligaments that are attached to the arms. It is considerably more plushy than the second stomach.
The pyloric stomach is known by fewer people than the cardiac stomach. In contrast, it is more stream-lined, only having two extensions from each of the arms. These hollow tubes are designed to take the somewhat digested prey, secrete digestive enzymes, and pass it along to the excretory gland, or the caeca. Without both of these stomachs working in unison, the starfish would not be able to both eat and get rid of its prey.
How Big Can They Get?
Starfish, in general, can range in size drastically based on the species. The current biggest individual in the world is the Sun starfish (Pycnopedia helianthoides). They are found in the northeast Pacific. When measuring the length of the arms, each appendage reaches about 40 inches. The heaviest specimen has weighed in at around 11 pounds. An interesting fact about the Sunflower starfish is that it also has the highest number of arms for Echinodermata. This would make more sense as a larger body mass would require additional ligaments to move around on the ocean floor.
With over 1,600 species in the phylum Asteroidea, or the true starfish, it is fascinating to compare the world’s smallest individuals as well. This title belongs to the Paddle-spined starfish (Patiriella parvivipara). In fact, it is so small that it requires a microscope to clearly see them. They typically measure to about 5 millimeters, which is roughly the size of a human’s fingernail. Unlike the world’s largest starfish, they do not have the fewest number of arms. Instead, they have the usual 5 appendages.
What Eats a Starfish?
These animals are protected by rough exoskeletons and suction cups for gripping onto rocks, making any predators consider hunting them down. Despite their defenses, there are a number of predators in the ocean. These can include sharks, manta rays, crabs, other starfish and even humans. People have overfished starfish species for nautical decoration, making us a large predator.
Can a Starfish Kill You?
Starfish are not a marine animal that will go after people in order to hurt them. But, they can inflict pain. Some species are actually venomous such as the Crown-of-thorns starfish (Acanthaster planci) where it releases venom when provoked. This can only happen if a human were to accidentally step on or handle the starfish. When looking through tidal pools, always be careful and look up the existing species in your neck of the woods.
Why Are They Called Starfish?
There is no historical proof as to why these creatures were named starfish. The closest documentation is the word “starfyshe” in the Oxford English Dictionary that dates back to 1538. It has been proposed that explorers during this time called most beings from the sea “fish” and the first part described the appearance. They also may have thought that they essentially were fish.
Since the early 1990’s, scientists have been trying to change the common name to “seastar”. When taking the characteristics of a fish into consideration, it is easy to tell that a starfish and a gill-equipped fish are two separate animals. The choice is up to you. Both are acceptable in many countries. It just depends on your own personal preference.
The starfish is an incredible creature, known for its resemblance to a star with 5 appendages. Little do people know that they actually use their tube feet to travel and feed with two separate stomachs aiding in digestion. Being a slow mover on the ocean floor, they are adapted to finding prey items that are even slower than them. This often makes for a omnivorous creature that will eat mussels, clams, dead fish and organic plant matter. Regardless if you call them by their existing name or the newly-recommended “seastar”, they are a unique and fascinating discovery for ocean lovers.
Hello everybody! This is French, the author behind the animal article you have just stumbled upon. Writing about critters of various sizes and shapes has been a wonderful experience so far! With a Bachelor’s of Science in Wildlife: Conservation and Management from Humboldt State University, I have been passionate about using my degree to teach others about animals. In fact, education is among the most important ways that we can save future wildlife. These articles are a way to help others relate to these animals, thus raising awareness. If you have any questions about biology, wildlife, botany, or any other science, feel free to ask!