How do starfish regrow their arms?

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, something our bodies don't do to rebuild a lost limb.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 scales, no fins and no fish-type gills (they breathe through tiny skin gills and their tube feet).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.

1. 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
2. Cells that can switch jobs. In humans, skin and muscle cells don't normally switch back like this to rebuild a lost limb. 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. The researchers think 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

In the red starfish studied, scientists found no single ball of new cells at the tip like salamanders grow. 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 the central disc and half or more of the body remain. 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

Regrowth is one reason chopping up sea stars is a poor way to control them. 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

Keep exploring

• Why do cells divide? The cell copying behind every kind of regrowth.
• How do octopuses change color? Another soft-bodied sea animal with an amazing trick.
• How are coral reefs formed? Home to many sea stars, including the crown-of-thorns.
• How do fish breathe underwater? Real fish breathe with the gills that sea stars lack.

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