How do starfish regenerate their limbs? The short answer
A starfish (better called a sea star) can regrow a lost arm because its body is built in repeating parts. Each arm carries its own copy of vital organs, like the digestive glands, so losing one arm doesn't remove anything the animal can't live without.611 After the loss, the wound seals itself off, then special cells move to the injury and slowly rebuild the arm.2 Some of those cells were skin or muscle cells that change back into flexible cells able to become something new, a trick our own cells can't do.9 Regrowing a whole arm takes from a few months to over a year.211
So how do starfish regenerate their limbs, step by step? It's a slow rebuild in stages: seal, cells change jobs, nerves and skeleton first, then muscle and growth. The rest of this page walks through each stage.
Is a starfish a fish?
No. Sea stars live underwater, but that's where the resemblance ends: they have no gills, no scales and no fins.1 They are echinoderms, a group of spiny-skinned sea animals that includes sea urchins, sand dollars and sea cucumbers. Echinoderms have bodies arranged in fives around a centre, called five-point radial symmetry.1 That's also why scientists prefer the name "sea star". There are roughly 1,600 to 2,000 species, living in every ocean.56
A sea star's body is strange in other ways too:
- No brain. Sea stars have nerves but no brain, yet they still react to light.2
- No blood. Seawater, pumped through a network of tubes called the water vascular system, carries nutrients around the body instead.1 Water enters through a small sieve-like plate on top, the madreporite.3
- Rows of tube feet. The same water pressure powers rows of tiny tube feet under each arm, used for walking and gripping prey. A big sunflower star has about 15,000 of them.14
- An eye on every arm. At the tip of each arm is a simple eye spot that senses light and dark.23
- Not always five arms. Five is most common, but some species have 10, 20 or even 40 arms.1
Why can starfish regenerate their limbs?
Two reasons work together.
- A spread-out body. The mouth and stomach sit in the central disc, but every arm has two long digestive glands, its own water canal and its own nerve cord.11 Think of five backpacks strapped around a small body: lose one backpack and the rest still work. The sea star can keep eating and moving while it regrows.11
- Cells that can switch jobs. In humans, once a cell has become skin or muscle, it can't turn back. Starfish cells can "de-differentiate": change back from skin or muscle into a more flexible state, move to where they're needed, and become new tissue, even nerve cells.910
What happens when a starfish loses an arm?
Sea stars lose arms to predators, but many can also drop an arm on purpose to escape, a defence called autotomy (from Greek words for "self" and "cutting"). Lizards dropping their tails do the same thing.7
In 2024, scientists at Queen Mary University of London worked out part of how the common starfish, Asterias rubens, does it. Stress makes its nerves release a chemical messenger (a neuropeptide similar to a human hormone called CCK). This makes a band of muscle at the base of the arm, the "tourniquet muscle", squeeze tight at a special break zone. When the scientists injected the chemical and gently clamped an arm, 85% of the starfish let go of that arm.7 The same muscle stays clamped after the arm comes away, sealing the body.7
Starfish arm regeneration, step by step
Scientists who study the red starfish Echinaster sepositus and other species describe three phases.811 The timings below are typical, not exact: speed varies with species, size and water temperature.
seal → cells change jobs → nerves + skeleton → muscle → growth
Starfish don't grow a single ball of new cells at the tip the way salamanders do. Instead they mostly rearrange and reuse existing tissue, which is one reason their regrowth is slow.8 The calendar in the toy above uses this rough, illustrative timeline.
Can a starfish regenerate from one arm?
It depends on the species, and on how much of the central disc comes along.11
- Most sea stars can regrow missing arms as long as half or more of the body remains. This is the most common kind.
- Some species can rebuild a whole star from a smaller piece, as long as it includes part of the central disc, where the mouth is.
- A few species, like the tropical Linckia sea stars, can grow a whole new body from a single arm with no disc at all. With no mouth, the arm lives off food stored inside it. An arm growing a tiny star at one end is called a comet.11 In Linckia multifora, the break heals in about 10 days, and a comet takes about 10 months to grow a new disc with arms around 1 cm long.12
This is why chopping up sea stars backfires. In the 1960s, some efforts to control coral-eating crown-of-thorns starfish cut them into pieces and threw them back, which may have made outbreaks worse.11
Starfish vs salamanders, lizards and humans
Sea stars aren't the only animals that regrow body parts, but they're among the most dramatic.
Our liver is the champion human organ here: when part of it is removed, it regrows lost mass until it matches the body's size again.15 Doctors first noticed fingertips regrowing in young children, and later saw it in teenagers and adults too.16 But a whole arm or leg never grows back.
That gap is exactly why scientists study sea stars. Researchers at Carnegie Mellon University are trying to find out what tells starfish cells to change back and start over, in the hope that one day similar tricks could help human injuries heal.910
Sea stars need protecting, too. Since 2013, a disease called sea star wasting has killed billions of sea stars on the west coast of North America, including an estimated 5.75 billion sunflower stars.417
Common misconceptions
"Starfish are fish." They're echinoderms, cousins of sea urchins, with no gills, scales, fins, brain or blood.
1
"Any piece of a starfish becomes a new starfish." Most species need the central disc. Only a few can regrow a body from one arm alone.
11
"The arm grows back in a few days." The wound closes in days, but the arm takes months to a year or more.
2
"Losing an arm is always an accident." Many sea stars drop an arm on purpose to escape, using a special muscle at a break zone.
7
"Humans can't regrow anything." We can regrow liver tissue, and children can sometimes regrow a fingertip, just not whole limbs.
1516
Keep exploring
At the shore: look at sea stars in the water, but don't lift them out, and put any rock you turn over back the way you found it.
FAQ
How long does it take a starfish to regrow an arm?
Usually from a few months to over a year, depending on the species, its size and the conditions. The wound closes within days, nerves and skeleton start forming in the first weeks, and a visible arm bud appears after about six weeks, then keeps growing slowly.
Can a starfish regenerate if cut in half?
Often, yes. In most species, a piece with half or more of the body (including the central disc) can regrow the missing arms. But cutting starfish is not a way to control them: in the 1960s, chopping up crown-of-thorns starfish on coral reefs may have made outbreaks worse.
Can a starfish grow a whole new body from one arm?
Only some species. Most need at least part of the central disc. A few, such as Linckia sea stars, can grow a new body from a single arm with no disc at all; the arm with a tiny new star at one end is called a comet.
Why can starfish regenerate their limbs?
Their body is spread out: each arm has its own copy of the digestive glands, water tubes and nerve cord, so losing one arm doesn't remove anything vital. And unlike most of our cells, starfish cells near the injury can change back into flexible cells that take on new jobs.
Does it hurt a starfish to lose an arm?
Scientists don't know exactly what sea stars feel, since they have no brain, only a network of nerves. What is clear is that regrowing an arm takes months of work, so please never pull an arm off a sea star.
Which animal regenerates the fastest?
It depends on what is regrown. Among animals with backbones, axolotls regrow a whole leg in weeks, and lizards regrow a working tail in about two months. Starfish are slower, taking months to a year to rebuild an arm.
Can humans regenerate limbs like starfish?
No. Humans can regrow part of the liver, and young children can sometimes regrow a fingertip, but not a whole arm or leg. That's why scientists study starfish and salamanders: to learn how their cells switch jobs.
Sources
- NOAA Ocean Service, Is a starfish a fish?
- Seattle Aquarium, Sea star
- Monterey Bay Aquarium, Bat star
- Monterey Bay Aquarium, Sunflower star
- Britannica, Sea star
- National Geographic, Starfish
- Tinoco AB et al. (2024), Discovery of a neuropeptide that acts as an autotomy-promoting factor, Current Biology (see also the Queen Mary University of London news release)
- Ben Khadra Y et al. (2015), Wound repair during arm regeneration in the red starfish Echinaster sepositus, and Re-growth, morphogenesis, and differentiation during starfish arm regeneration, Wound Repair and Regeneration
- Carnegie Mellon University (2020), The future of human healing lies in the brain of a starfish
- Carnegie Mellon University (2022), Research reveals ways neurons are regenerated in starfish
- Wikipedia, Starfish regeneration (review article citing the primary research)
- Wikipedia, Linckia multifora
- Smithsonian Magazine (2020), Some salamanders can regrow lost body parts. Could humans one day do the same?
- Arizona State University (2014), How lizards regenerate their tails; Lozito & Tuan (2016), Development
- Michalopoulos GK (2007), Liver regeneration, Journal of Cellular Physiology
- Muneoka K et al. (2008), Regrowing human limbs, Scientific American
- NOAA Fisheries, Sunflower sea star