What’s actually slowing this PC down?
Pick the symptom - the matching free tool is one click away.
Sea stars—commonly called starfish—are marine invertebrates, not fish. They move with seawater-powered tube feet, and some species can digest prey outside their bodies or regrow damaged arms. Here are nine facts about their biology, with species-specific examples clearly distinguished from traits shared by all sea stars.
1. Sea stars are not fish
Despite the familiar name, sea stars are echinoderms, a group of marine invertebrates. As NOAA explains, they do not have fish-like gills, scales, or fins. “Sea star” is a useful common name because it avoids suggesting that they are fish.
Their echinoderm relatives include sea urchins and sand dollars. Brittle stars are related, but they are a distinct group.
2. There are about 2,000 kinds of sea stars
NOAA National Ocean Service estimates that there are approximately 2,000 sea-star species, all living in marine waters. This is an approximate estimate, not an exact or newly counted total.
#1 Best Overall
Sea stars occur in varied marine habitats, from shallow coastal areas to the deep sea.
3. Five arms are common, but arm number varies
The familiar five-armed shape is common, but it is not universal. NOAA notes that some species have 10, 20, or 40 arms; Smithsonian Ocean also describes species with up to 50. These are examples, not a universal maximum.
Arm number and shape can help identify a sea star, but they are not enough on their own. Other features and the animal’s habitat may also matter.
4. Tube feet help sea stars move and cling
Rows of tiny tube feet cover the underside of a sea star. They are operated by the water-vascular system, a network of fluid-filled canals that uses water pressure to help power movement and attachment.
Coordinated tube feet let sea stars creep over surfaces and can help them handle prey. NOAA’s Woods Hole Science Aquarium notes that some sea stars can climb smooth rocks and aquarium walls; this does not apply to every species.
5. A sunflower sea star can move quickly
Sea stars are often slow-moving, but speed varies by species. NOAA National Ocean Service reports that an adult sunflower sea star can move at up to one meter per minute using approximately 15,000 tube feet. These figures describe sunflower sea stars, not sea stars generally.
6. Some can digest prey outside their bodies
When some sea stars feed on bivalves such as clams, tube feet help them pry open the shell. The sea star can then extend its cardiac stomach through its mouth and begin digesting the prey outside its body.
After digestion begins, the stomach returns inside. This feeding method lets some sea stars consume prey they could not swallow whole.
Free tools Windows power users keep installed
One-click scans. No signup required.
7. Many sea stars can regrow arms, but regeneration takes time
Many sea stars can regenerate damaged or lost arms, but the ability varies by species and can take months. A regrowing arm may remain smaller while it develops.
Rank #4
A detached arm does not automatically become a new sea star. Whether a new individual can form depends on the species and which structures, including parts of the central disk, remain.
8. Sea stars have no heart
Sea stars have no heart. NOAA explains that their water-vascular system circulates nutrient-filled seawater through the body. This specialized echinoderm system is not the same as a human heart and blood circulation.
9. Their water-vascular system has several jobs
The water-vascular system is a network of fluid-filled canals that operates tube feet and supports movement. Tube feet also help sea stars attach to surfaces and handle prey.
Its hydraulic action makes the system distinctive, but it is not simply a replacement for every function of blood. Sea-star anatomy is specialized for their echinoderm body plan.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Frequently Asked Questions
Are starfish really fish?
No. Sea stars, commonly called starfish, are marine invertebrates in the echinoderm group, not fish. NOAA notes that they lack fish-like gills, scales, and fins.
How do sea stars move?
They move with tube feet on their undersides. Water pressure in the water-vascular system helps operate the tube feet, which work together to move the animal over surfaces.
How does a sea star eat a clam?
Some sea stars use tube feet to pry open a clam’s shell, then extend the cardiac stomach through the mouth. Digestion begins outside the body before the stomach returns inside.
PC Slower Than It Used to Be?
A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11Crashes, No Sound, or Screen Glitches?
Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteCan a sea star regrow an arm?
Many can regrow damaged or lost arms, but regeneration varies by species and can take months. A detached arm does not always grow into a new sea star; that depends on the species and which structures remain.
Quick Recap
Sources
- NOAA National Ocean Service: “Are starfish really fish?”
- Smithsonian Ocean: “Echinoderms: Sea Stars, Urchins, Sand Dollars, and Relatives”
- NOAA Fisheries: Woods Hole Science Aquarium FAQs
- Smithsonian Science Education Center: “All About Starfish”
- NOAA National Ocean Service: “Heartless or hopelessly romantic?”
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
