On this page
A sea star can regrow a lost arm. The harder question runs the other way: can the lost arm regrow the sea star? In most species tested, it cannot, and the central disc is usually what decides.
Can starfish regrow arms?
NOAA's Ocean Service says that a sea star that loses an arm can regenerate it (NOAA Ocean Service). The same page counts approximately 2,000 species of sea star, all living in the sea.
Regrowth can be slow. Zoologist Charles Howard Edmondson spent about three years testing Hawaiian sea stars (Edmondson, Bishop Museum Occasional Papers, 1935). In one species, *Pentaceros hawaiiensis*, arm stumps cut close to the disc showed no appreciable regrowth after four months.
Some sea stars also drop arms on their own, a process called autotomy. In *Linckia multifora*, Edmondson saw the tube feet pull arm and body apart slowly, over about 60 minutes. In one group of 137 animals he counted 298 breaks where arms had been lost earlier.
Can a single starfish arm grow into a whole new starfish?
Only in a few species. Edmondson wrote that researchers had generally believed part of the disc had to come away with an arm for a new sea star to form. He added that growing one from the arm alone seemed to be limited to a small number of species, with members of the family Linckiidae showing it “to a very high degree” (Edmondson, 1935).
His timeline for detached *Linckia* arms, measured in the lab and roughly matched in the sea:
about 10 days: the torn end is sealed over by new tissue
six to seven weeks: a yellowish “germinal crescent” appears at the cut end
about three months: the new arms start to spread beyond the old one
six months: the new arms are about 8 mm long
ten months: the new arms are about 10 mm long
An old arm with a cluster of tiny new arms at one end looks like a comet, which is why it is called one.
Other species failed the same test. Severed arms of *Coscinasterias acutispina*, *Dactylosaster cylindricus pacificus* and *Pentaceros hawaiiensis* all died without growing new arms. *Dactylosaster* is in the same family as *Linckia*, so Edmondson suggested the ability may be an adaptation of particular genera, not of sea stars as a whole.
The disc seemed to make the difference. When he cut a tiny sea star, probably a *Nepanthia* species, into three sectors, each holding part of the mouth, all three had regenerating arms after 60 days. Single arms of the same species lived more than 60 days without forming any.
The comet has been seen in India too. In the CMFRI research aquarium at Mandapam, Tamil Nadu, a *Linckia multifora* collected from the local coast shed an arm that became a comet and developed into a separate sea star with its own central disc (CMFRI Cadalmin newsletter, 2012).
Do other echinoderms regenerate too?
Sea stars belong to the phylum Echinodermata. A 2021 review by Joshua Medina-Feliciano and José García-Arrarás lists its five major classes: feather stars (Crinoidea), sea stars (Asteroidea), sea urchins (Echinoidea), brittle stars (Ophiuroidea) and sea cucumbers (Holothuroidea) (Frontiers in Cell and Developmental Biology). The review notes that regrowth varies by class:
brittle stars are well known for regrowing arms
sea cucumbers can regrow internal organs, including the digestive tract
feather stars regrow arms and even whole crowns
sea urchins have the lowest capacity, but still regrow spines and parts of their shell
The review also describes echinoderms as invertebrate deuterostomes, closely related to the chordates. NOAA's page adds two defining features: five-point radial symmetry and a water vascular system (NOAA Ocean Service). For the full feature list, read why a starfish is not a fish.
How do you answer a starfish regeneration question?
Most questions test three cases:
Arm lost, disc intact? The animal regrows the arm. Call it regeneration.
A piece that includes part of the central disc? It can grow into a whole sea star. Call it regeneration working as asexual reproduction.
An arm alone, with no disc? In most species tested, it dies. Only a few, such as *Linckia*, regrow a body from it, forming a comet.
A model two-mark answer: “A sea star can regenerate a lost arm. If a cut piece contains part of the central disc, it can grow into a complete new animal, so regeneration also acts as asexual reproduction. A few species, such as Linckia, can do this from an arm alone.”
Worked example: “Any piece cut from a starfish grows into a new starfish.” Mark it false. In Edmondson's tests, arms cut from several species died without regrowing (Edmondson, 1935).
To compare other regrowers, see regeneration vs fragmentation in Planaria and Spirogyra and how axolotls regrow limbs.
If these terms keep blurring together, the Fermi Concept Library gives each biology concept its own lesson and its place in the map. For practice, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
