Can Starfish Regrow Their Arms? Regeneration Explained

How do starfish regrow arms?

Regrowing an arm happens in stages. Studies of species such as the Mediterranean red sea star (Echinaster sepositus) and the common starfish of European coasts describe a repair phase, an early regrowth phase and a long phase of growth.

First, the wound closes. Within hours to days, muscles pull the edges together and wandering cells from the body fluid gather to seal the opening, followed by a thin layer of new skin. This stops the sea star from losing body fluid and limits infection.

Next, a small bud of new tissue forms at the end of the stump. Cells near the wound change their roles and multiply, and new tissue moves in to rebuild the nerve, the water canals, the skeleton plates and the tube feet. In some studied species the tip of the new arm, with its eyespot, forms early, and the rest of the arm is added behind it.

Finally, the little arm slowly grows. For months it looks like a short stub among full-length arms, as in the photo above. Bit by bit it catches up, although a regrown arm often stays slightly shorter or differently shaped for a long time.

Why do starfish lose arms?

Sea stars lose arms to predators, storms, falling rocks and accidents. Crabs, fish, gulls and other sea stars may grab or bite off an arm, and the victim often survives.

Many species can also drop an arm on purpose. This is called autotomy, from Greek words meaning “self-cutting”. The sea star’s body plates are joined by catch connective tissue, a special tissue that can stiffen or soften within seconds under nerve control. When an arm is seized, the tissue near its base can suddenly soften, the arm breaks off at that weak point and the rest of the animal crawls away.

A few species use arm loss as a way to reproduce, which is explained below.

Can a starfish regrow from one arm?

It depends on the species. In most sea stars, including the ochre sea star of North America’s Pacific coast, a detached arm on its own cannot grow a new body. A new animal can only form if a piece of the central disc, where key nerves and organs meet, is still attached.

A few species are different. Sea stars of the genus Linckia, found on tropical reefs, can regrow a complete star from a single arm with no disc at all. While it grows, the animal looks like one long arm with four tiny new arms sprouting at one end. Scientists call this shape a “comet”, because it resembles a comet with a tail.

How long does it take?

There is no single answer. Small arms regrow faster than whole arms, and warm water and plenty of food speed things up. In many species a lost arm takes several months to regrow, and a year or more to reach the size of the others. Growing a whole star from a single arm, as Linckia does, takes much longer.

Regrowth also tends to slow down when food is short, because the animal has to divide its limited energy between staying alive and rebuilding.

What does regeneration cost a starfish?

Regrowing an arm is expensive. While a sea star is rebuilding, it has fewer tube feet to grip rock and pull open prey, so it may eat less. Energy that would normally go into growth and into producing eggs or sperm is diverted to the new arm, and studies on several species have found smaller reproductive organs in regenerating animals.

The open wound is also a risk. In the first days after losing an arm, sea stars are vulnerable to infection. This is one reason why a sea star that has lost several arms at once may not survive, even though it can regrow each of them in theory.

Starfish that clone themselves

Some sea stars use regeneration as a way to reproduce without a mate. In a few species the central disc slowly tears in two, a process called fission, and each half regrows the missing arms. Other species, such as some Linckia sea stars, shed arms that grow into new “comet” stars.

These clones are genetically identical to their parent. Most of the same species also reproduce sexually by releasing eggs and sperm into the water, so they can switch between both methods. The larvae of several sea stars can even bud off pieces that grow into extra larvae when food is plentiful.

Myth: cut a starfish and you get two

A popular story says that chopping a sea star into pieces just creates more sea stars. For most species this is not true. A piece without enough of the central disc usually dies, and even pieces that do survive face months of slow, risky regrowth. Only the few species that can regrow from an arm alone come close to the myth.

How does this compare with other animals?

Sea stars are among the best regenerators in the animal world, along with flatworms, which can regrow from tiny fragments, and many of their echinoderm relatives. Brittle stars regrow arms readily, and sea cucumbers can rebuild large parts of their gut.

Other animals regrow body parts in different ways. Crabs and lobsters replace lost legs over several molts, as explained in Can crabs regrow their legs?. Among animals with backbones, the axolotl is the champion, able to rebuild whole limbs, as described in How do axolotls regenerate?. Humans can regrow only small things such as skin, the lining of the gut and, to a degree, the liver.

Because sea stars are relatives of vertebrates, scientists study their regeneration to learn which genes and cells allow a body part to be rebuilt. To learn more about how these animals are built, see Is a starfish a fish?.