12 Starfish Facts About The Immortal Sea Creature

Starfish, also known as sea stars, is one of the most vibrant marine species. You can see them in tidal pools and reefs. From their characteristics, unique biology, and regeneration abilities, our list of starfish facts might surprise you with quite how much there is to know about these sea creatures.

Commonly recognized for their distinctive shape, sea stars (used interchangeably with starfish and often considered more accurate as starfish are not actually fish) offer more than just an attractive appearance, as they contribute to the diversity and ecological balance of their marine habitats.

An interesting fact about sea stars is that they are not fish but belong to the phylum Echinodermata, which includes sea cucumbers, sea urchins, and sand dollars. Furthermore, they can regenerate lost limbs and even their entire body from a small body fragment. 

In this collection of facts about starfish, we will explore the fascinating world of these marine invertebrates. We will discover their unique body structure, impressive regenerative abilities, and incredible species diversity. 

If you want to learn more about our undersea friends, read more interesting facts about our oceans. You can also enjoy our ocean quotes about the deep depths to inspire your role in its preservation.

12 Surprising Facts About Starfish

1. Starfish are not fish but echinoderms.

Did you know that starfish are not fish? Despite their name, sea stars are echinoderms⁴ and belong to the same family as sea urchins, cucumbers, and sand dollars.

Unlike fish, they don't have any gills, scales, or fins. Instead, they have a unique water vascular system, allowing them to move, breathe, and feed efficiently.

Additionally, sea star bodies exhibit radial symmetry, a common trait in echinoderms, as opposed to the bilateral symmetry of fish. Compared to fish, starfish arms radiate from a central point. Morphologically, starfish range from the iconic five-armed species to those with 40 arms.

2. They do not have blood.

Diverging from the typical circulatory systems found in animals, they possess a unique water vascular system that relies primarily on filtered sea-water to distribute nutrients, gases, and waste products throughout their bodies.

The system starts with a special entrance called the madreporite, which looks like a light-colored mark on the top of the animal. As a gateway and water filter, these sieve-like plates pump nutrients and seawater into the water vascular system while filtering out debris and particles.

The madreporite connects to a short stone canal that leads to a central ring canal. The filtered seawater, filled with nutrients, travels through these canals to different body parts.

This intricate network of canals serves as a circulatory system but also plays a vital role in the movement and feeding of these marine animals.

The seawater-filled canals provide hydraulic pressure to the starfish's tube feet, small flexible appendages on the underside of their arms. These tube feet extend and contract thanks to the hydraulic pressure, allowing the starfish to move around and capture prey gracefully.

3. There are 2,000 recorded sea star species around the globe.

Next on our starfish facts list: scientists have recorded 2,000 species of sea stars worldwide. They are the second most diverse group of the family, Echinodermata. Specifically, their diversity is 40x that of the seahorses!

They thrive in every marine ecosystem across all the world's oceans (except in freshwater ecosystems) - from the tidal pools of intertidal zones to deep waters of trenches. They are like the ultimate adventurers, exploring habitats from bustling shores to the mysterious deep sea.

While most of these species flourish in the warm waters of tropical regions, a remarkable number of sea stars have also acclimated to the harsh climates of even the polar regions, showcasing their exceptional capacity to thrive in even the most unwelcoming environments.

Read more: Types of Starfish.

4. Starfish vary in size.

The Sunflower Sea Star (Pycnopodia helianthoides), known as the largest starfish, can reach an astounding diameter of up to 3.3 feet. These giant adult starfish mainly inhabit the Pacific Ocean, from Alaska to Southern California.

On the smaller side of the spectrum, the Patiriella parvivipara species measures a modest 0.4 inch (1 cm) in width - about the size of a human fingernail! These tiny stars live on the shallow, rocky shores around Australia, where they busily feed on organic material and help recycle nutrients within the ecosystem.

5. Not all sea stars have five arms, and not all look like stars.

While the familiar image of a sea star often features just five arms (prevalent in many species), these captivating marine creatures display a striking diversity in arm count. Their arms can range from a mere four up to over 40, depending on the species.

The Sun Star (Pycnopodia helianthoides) stands out due to its astounding arms that range from 16 to 40. This impressive array of limbs offers a competitive edge in hunting and locomotion—conversely, the Slender-armed Star (Luidia clathrata) sports just four arms.

Furthermore, not all species of sea stars look like stars. The Cushion Starfish is the exception to the rule. They have a cushion-like body, and most don't even have a defined form.

6. They can regenerate.

The most famous ability of sea stars is their ability to regenerate. Few animals, like the salamander, axolotl, and sea cucumber, have this ability. 

At the heart of this regrowth process are specialized stem cells. These cells can morph into various cell types required to reconstruct the missing body part². As these cells multiply and differentiate, they eventually give rise to a fully functional arm with muscles, nerves, and skin. In some cases, a lost limb could lead to the birth of an entirely new sea star.

This ability is also a key component of their survival strategy. When faced with danger, these creatures can deliberately shed one of their arms to distract predators while escaping. 

7. Sea stars have built-in armor.

Sea stars have diverse textures and looks, which they use to defend themselves. Their outer skin can range from smooth and leathery to slightly prickly and rough, depending on the species. This variety stems from calcium carbonate plates embedded within their skin.

These features form a durable covering and unique natural armor on a sea star's dorsal side, which shields them from predators such as birds, sea otters, sharks, manta rays, and certain fish species, including other sea stars.

This armor is even armed with specialized venomous spines in some species, making them particularly unpalatable. The crown-of-thorns starfish stands out in this regard, armed with long, venomous spines that deliver a painful sting to any would-be attacker.

Moreover, some species have developed specialized skin cells called pedicellariae, which grasp, bite, and inject venom into predators or other threats. A particularly intriguing example is the sunflower starfish. When threatened, it produces a foul-smelling, toxic substance that effectively deters predators from attempting to eat it.

8. Sea stars eat by expelling their stomachs.

This particular starfish fact is rather nasty. These marvelous marine animals have adapted a unique way of eating their food (particularly those larger than them). Due to their tiny mouth, they eat food outside their body by expelling their stomach.

When they encounter a bivalve, such as a clam or an oyster, this marine creature catches the prey with its flexible arms and tiny suction cups. The sea star then pushes its stomach out through its mouth, reaching into the narrow space of the bivalve's shell.

Once inside, the starfish's stomach bathes the soft, helpless bivalve tissue with enzymes to break it down and absorb the essential nutrients. After devouring its prey, it retracts its stomach back into its body. Although this unconventional dining strategy suits bivalves, starfish possess a versatile palate.

As omnivores, they are not limited to eating clams and oysters. They also consume sea grass, algae, coral, and tiny organisms. However, when food becomes scarce, some species resort to cannibalism. This behavior is also present in baby sea stars.

9. They feel and perceive things through their eyespots.

Have you ever wondered how these eyeless animals see and feel their surroundings? Starfish perceive their surroundings through their peripheral nervous system (sense organs). Each arm holds eyespots. These light-sensitive cells allow them to sense light and help them navigate their underwater habitat.

Moreover, the number of sea stars' eyespots matches the number of their arms, giving them a wide range of visual capabilities. For instance, a five-armed starfish has five eyes, whereas the 40-armed sun star has 40 eyes, thus ensuring an efficient exploration of their habitats, like kelp beds and reefs.

Aside from these remarkable eyespots, starfish rely on their tube feet to sense and interact with their environment. These tube feet play a crucial role in detecting food sources. Equipped with specialized sensory cells, they can identify chemical changes in the water, such as the presence of potential prey nearby. This ability allows starfish to zero in on their next meal with incredible speed and accuracy³.

10. They reproduce in two ways.

One fascinating fact about starfish is their reproduction method. While most organisms have only one, sea stars have two reproduction methods! Adult sea stars reproduce sexually and asexually, improving their diversity and abundance.

Interestingly, male and female sea stars look strikingly similar on the outside. However, they house their respective reproductive organs within their arms. These organisms reproduce sexually by releasing sperm and egg into the water, where external fertilization happens.

Once fertilized, the eggs quickly develop into swimming larvae called bipinnaria. These larvae drift through the ocean currents until they eventually settle on the ocean floor, transforming into mature sea stars.

On the other hand, sea stars reproduce asexually through fragmentation and regeneration. When a sea star loses an arm due to predation or injury, it can regenerate the lost limb; sometimes, it can generate an entirely new organism from the missing limb. This unique form of asexual reproduction enables sea stars to bolster their populations swiftly, particularly when environmental conditions favor them.

11. Sea stars have a dark side.

Certain starfish species, such as the infamous crown-of-thorns, live as invasive species in various regions around the globe¹. The damage they inflict on fragile coral reef ecosystems, particularly in tropical areas, is nothing short of alarming.

The crown-of-thorns starfish are notorious for their insatiable hunger for coral polyps. They can obliterate entire reefs, leaving eerie, bleached limestone skeletons in their wake. Consequently, the once vibrant and thriving underwater habitats and the myriad species that depend on these coral ecosystems suffer.

Numerous control methods and strategies have attempted to reverse the havoc wreaked by invasive starfish populations. One of the more conventional techniques entails the laborious manual removal of these creatures by dedicated divers. Although practical, this strategy can be time-consuming and expensive.

On the other hand, cutting-edge solutions, such as implementing targeted biological controls, seek to reestablish equilibrium within the ecosystem by encouraging the proliferation of natural predators like the giant triton snail, which feeds on crown-of-thorns starfish.

12. Though resilient, these species face numerous threats as well.

A primary factor heightening their vulnerability is the loss of habitat triggered by coastal development, human activities, and the deterioration of reefs. Moreover, pollution from plastic waste, chemical contaminants, and oil spills has far-reaching effects on starfish health and their chances of survival.

In addition, the impacts of climate change are intensifying. As sea temperatures rise, coral bleaching occurs, and vital food sources vanish. Unsurprisingly, the International Union for Conservation of Nature's Red List of Threatened Species has classified some starfish species, such as the African Red-knobbed Sea Star, as critically endangered.

Conservation efforts are targeting several vital areas to safeguard and preserve starfish populations. One such initiative is habitat restoration, which entails developing artificial reefs or repairing damaged coral reefs.

Anti-pollution measures like imposing stricter regulations on industrial and agricultural runoff are vital to maintaining water quality and protecting starfish habitats. Moreover, ongoing research into climate change's effects on starfish populations offers invaluable insights that can shape conservation strategies and anticipate future challenges.

We hope you enjoyed this list of interesting facts about starfish!

Related: To further explore the animal kingdom, check out some of the other animals that start with S.