Why do starfish regrow arms? Starfish—more accurately called sea stars—can regenerate lost arms because their tissues retain powerful wound-healing and developmental programmes that rebuild muscle, nerves, connective tissue, skin, digestive structures and the water-vascular system after injury. In many species, an arm can regrow from the body if enough of the central disc survives. A few species can even regenerate much of a new individual from a detached arm that retains part of the disc.
People searching for why starfish regrow arms, how sea stars regenerate, can a starfish grow from one arm, how long regeneration takes, why the central disc matters, whether starfish feel injury and how regeneration differs from healing are asking about stem-like cells, tissue patterning, nerves, wound closure, energy budgets and evolution at the same time. Regeneration is not instant repair; it is a prolonged rebuilding process.
For students, sea-star regeneration is a powerful systems example. The animal first seals the wound and prevents infection, then reorganises cells, establishes positional information, grows new tissues in the correct order and reconnects them to the surviving body. Successful regeneration depends on species, injury location, nutrition, temperature, health and how much of the central disc remains.
The Short Answer: Sea Stars Retain Strong Regenerative Programs
Sea stars can replace complex body parts because adult tissues remain unusually capable of dedifferentiation, proliferation and re-patterning after injury. Cells near the wound change behaviour, and new tissue is organised according to the animal’s radial body plan.
The process resembles development restarted locally in an adult body, with wound healing and pattern formation tightly linked.
Why Regeneration Is More Than Wound Healing
Ordinary wound healing closes damage and restores a barrier. Regeneration goes further by rebuilding missing structures such as nerves, ossicles, muscles, tube feet and digestive extensions inside the arm.
A healed stump and a fully restored arm are therefore very different biological outcomes.
Why the Central Disc Matters
The central disc contains major parts of the nervous, digestive and body-control systems. In many species, a detached arm without enough disc tissue cannot reconstruct a complete new animal.
The disc provides both essential organs and positional information needed to organise an entire body rather than just repair an arm.
Why Some Species Can Regenerate From an Arm Fragment
A few sea stars reproduce naturally by fragmentation and can regenerate a whole animal from an arm that contains a sufficient portion of the disc. Their tissues are especially capable of reorganising the missing body plan.
This ability is species-specific and should not be generalised to every starfish found on a beach.
Why Losing an Arm Can Be Survivable
Sea stars have a decentralised body plan with repeated structures arranged around the disc. Losing one arm does not necessarily destroy all essential functions at once.
The surviving arms can continue locomotion and feeding while the missing region is rebuilt gradually.
Why Radial Symmetry Helps
Most adult sea stars have five-part radial symmetry, although some species have more arms. Repeated anatomical units make the body partly modular.
This modularity means one damaged sector can sometimes be reconstructed from neighbouring tissues without rebuilding the entire animal at once.
Why the Nervous System Can Reconnect
Sea stars have a nerve ring around the mouth and radial nerve cords extending into each arm. After amputation, regenerating tissues must restore the radial nerve and reconnect it to the central ring.
Neural regrowth is therefore a central part of rebuilding a functional arm rather than an optional finishing step.
Why Tube Feet Must Regrow
Tube feet are used for locomotion, attachment, sensory exploration and feeding. A regenerated arm would be incomplete without them.
New tube feet develop in organised rows and reconnect to the water-vascular canals that power their hydraulic movement.
Why the Water-Vascular System Matters
Sea stars use a fluid-filled canal system connected to tube feet. This system distributes hydraulic pressure and helps operate hundreds of small appendages.
Regeneration must reconstruct both the visible tube feet and the internal canals that supply them.
Why Internal Skeleton Must Regrow
Sea stars have small calcified plates called ossicles embedded in connective tissue. These provide support without forming one rigid shell.
A new arm requires new ossicles arranged in the right pattern so it can bend while remaining structurally supported.
Why Muscles Must Rebuild
Locomotion and prey handling require muscles associated with the body wall, ossicles and internal organs.
Regeneration produces not only shape but contractile function, integrating new muscle fibres with surviving tissues.
Why Digestive Tissues Extend Into the Arms
Many sea stars have digestive glands called pyloric caeca extending into the arms. These organs help digest and store nutrients.
A mature regenerated arm therefore includes internal digestive structures rather than being a simple external appendage.
Why the Gonads Can Extend Into Arms
In many species, reproductive tissues occupy parts of the arms.
Regrowing an arm can eventually involve restoring reproductive capacity as well as movement and feeding structures.
Why Wound Closure Happens First
An open wound risks fluid loss, infection and further tissue damage. Sea stars rapidly constrict and cover the damaged surface before large-scale regrowth begins.
The first priority is stabilisation; pattern rebuilding starts after the animal has secured the injury.
Why Immune Responses Matter
Marine wounds are immediately exposed to bacteria and other microorganisms. Immune cells and antimicrobial processes help control contamination while damaged tissue is cleared.
Regeneration succeeds only if defence and tissue rebuilding remain balanced.
Why Inflammation Can Be Useful
Controlled inflammatory signalling helps remove damaged cells and recruit repair processes.
Too little response leaves debris and microbes; too much can damage healthy tissue, so regulation matters.
Why Cells Change Identity During Regeneration
Some cells near the wound can reduce specialised features and enter more flexible states, while resident progenitor or stem-like populations can proliferate.
This cellular plasticity provides building material for structures that did not exist at the wound margin before injury.
Why Cell Proliferation Is Necessary
A missing arm contains far more tissue than can be supplied by simply stretching neighbouring cells. New cells must be produced.
DNA replication and cell division therefore increase in regenerating regions as the new arm lengthens.
Why Regeneration Needs Pattern Information
Growing tissue must know where the tip, sides, oral surface and aboral surface belong. Random cell proliferation would create a lump rather than an arm.
Chemical gradients, gene-expression patterns and surviving tissue boundaries provide positional cues that organise the new structure.
Why Developmental Genes Reappear
Genes used during embryonic development can be reactivated during adult regeneration.
The adult animal reuses parts of the developmental toolkit to specify axes, tissues and organ relationships in the new arm.
Why Regeneration Starts With a Small Bud
After wound closure and early cellular reorganisation, a small outgrowth can form at the injury site.
This regenerative bud contains proliferating cells and organising signals that gradually produce the arm tip and internal structures.
Why the New Arm Is Tiny at First
Patterning must be established before full size is reached. The animal first creates a correctly organised miniature structure and then lengthens and matures it.
Shape precedes scale, much as an embryo forms basic body organisation before reaching adult dimensions.
Why Growth Takes Months
Sea stars are ectothermic marine animals with relatively slow tissue turnover compared with many warm-blooded vertebrates. Rebuilding bone-like ossicles, nerves and organs is metabolically expensive.
Complete arm regeneration can take months or longer depending on species and environment.
Why Temperature Changes Regeneration Rate
Higher temperature within a species’ suitable range can accelerate metabolism and cell division, while cold conditions slow them.
Excessive heat can cause stress or oxygen limitation, so warmer is not always better.
Why Food Supply Matters
New tissue requires proteins, lipids, minerals and energy. A well-fed sea star can devote more resources to regrowth than a starving one.
Regeneration therefore competes with maintenance, reproduction and movement for the same energy budget.
Why Starvation Slows Regrowth
When intake is low, the animal must prioritise essential organs and survival. Building a large new arm becomes less urgent than keeping remaining tissues alive.
The regenerative programme can continue slowly or pause until energy conditions improve.
Why Body Size Matters
A large missing arm requires more absolute material and energy than a small missing arm.
Young or small individuals can sometimes restore proportional length faster even though their overall metabolism and ecology differ.
Why Injury Position Matters
An amputation close to the disc removes more structures and creates a larger reconstruction problem than a cut near the arm tip.
The amount and location of surviving tissue influence how much pattern information and anatomy must be recreated.
Why Multiple Lost Arms Are Harder to Replace
Regrowing several arms at once divides energy among several wounds and reduces the animal’s remaining feeding and locomotor capacity.
The challenge is therefore multiplicative: more tissue must be built while the damaged animal is less capable of acquiring resources.
Why Sea Stars Sometimes Lose Arms Naturally
Predator attacks, traps, storms and conflicts with other animals can damage arms. Some sea stars can deliberately shed an arm through autotomy under extreme threat.
Regeneration turns what would otherwise be a permanent disability into a survivable temporary loss.
What Autotomy Means
Autotomy is controlled self-amputation at a specialised region of tissue.
The animal sacrifices a body part to escape capture, then later rebuilds it if conditions permit.
Why Autotomy Can Be Better Than Being Eaten
Losing one arm is costly, but losing the whole body is fatal.
Natural selection can therefore favour structures and behaviours that make controlled loss possible when the survival benefit is large enough.
Why Predators May Trigger Autotomy
A predator gripping an arm creates strong mechanical stress. Neural and connective-tissue responses can weaken the attachment region.
The sea star escapes while the predator retains only the discarded appendage.
Why Mutable Collagenous Tissue Is Important
Echinoderms possess connective tissues whose stiffness can change rapidly under nervous control.
These mutable tissues help sea stars alter body rigidity and can contribute to controlled arm loss as well as posture and feeding.
Why Echinoderm Connective Tissue Is Unusual
In most animals, connective-tissue stiffness changes relatively slowly. Echinoderms can alter mechanical properties rapidly without relying only on muscles.
This gives them a distinctive toolkit for both movement and injury responses.
Why Sea Stars Can Survive Major Injury Without Blood Loss Like Mammals
Their internal fluid systems and body organisation differ greatly from vertebrate blood circulation. Wound contraction and tissue closure can limit leakage quickly.
This does not mean injury is harmless, but the physiological consequences follow a different pattern from mammalian amputation.
Why Regeneration Can Change Feeding Ability
Some sea stars use their arms to open bivalves, position prey or move over surfaces. Losing arms reduces leverage and contact area.
Regrowth therefore restores ecological performance, not merely visual symmetry.
Why Tube Feet Make Regenerated Arms Functional
Hundreds of tube feet distribute traction across the seafloor.
As they develop and connect hydraulically, the new arm becomes increasingly useful for movement before reaching full adult size.
Why Regeneration Can Restore Sensory Capacity
Tube feet and arm tips contain sensory receptors for chemicals, touch and light.
A new arm rebuilds part of the animal’s environmental sensing network as well as its locomotor structures.
Why the Arm Tip Matters
At the end of each arm is a terminal tube foot and light-sensitive eyespot in many species.
Regeneration eventually restores this distal sensory region so the arm can contribute fully to orientation.
Why Sea-Star Eyespots Are Simple
The eyespot detects light direction and intensity rather than forming detailed images like a vertebrate eye.
Even simple vision can help a sea star orient toward reef structures or shaded shelter.
Why Regenerated Eyespots Need Correct Orientation
A light-sensitive structure pointing the wrong way would provide poor information.
Patterning mechanisms therefore must reconstruct both the organ and its spatial relationship to the arm.
Why Regeneration and Asexual Reproduction Can Overlap
If a species naturally splits and each fragment regenerates the missing body, the same cellular mechanisms serve both repair and reproduction.
Regeneration becomes part of the life cycle rather than only an emergency response.
What Fission Is
Some sea stars reproduce asexually by dividing through the central disc. Each portion regenerates missing arms and disc tissue.
This is a controlled reproductive process distinct from accidental injury, even though regrowth mechanisms overlap.
Why Fragmentation Can Produce Clones
Asexual fragments come from the same original individual and therefore share nearly identical nuclear genetic material.
This allows rapid local increase without mating but produces less genetic variation than sexual reproduction.
Why Sexual Reproduction Still Matters
Most sea stars also release eggs and sperm into the water, producing genetically varied offspring.
Genetic diversity helps populations respond to changing pathogens and environments over evolutionary time.
Why Regeneration Does Not Make Sea Stars Immortal
Animals still age, become diseased, starve, overheat and suffer injuries beyond their regenerative capacity.
Replacing arms cannot restore an animal after catastrophic loss of essential disc tissue or whole-body physiological failure.
Why Regeneration Can Fail
Infection, poor nutrition, unsuitable temperature, repeated injury or extensive tissue loss can interrupt the process.
Regeneration is a biological capacity, not a guarantee of perfect replacement after every event.
Why Aquarium Handling Can Damage Regenerating Tissue
New tissue is delicate and metabolically active. Rough handling can reopen wounds or damage developing structures.
Stable water quality and minimal disturbance support recovery better than frequent manipulation.
Why Water Quality Matters
Sea stars exchange gases and ions across body surfaces and tube feet. Pollutants, salinity changes and low oxygen can stress the entire animal.
A regenerating individual has fewer energy reserves to absorb additional environmental stress.
Why Salinity Changes Are Dangerous
Echinoderms generally regulate internal salts poorly compared with many fish and crustaceans. Rapid changes in seawater salinity can disrupt cell function.
Regeneration therefore depends on a stable marine environment as well as tissue biology.
Why Ocean Warming Can Affect Regeneration
Temperature influences metabolism, oxygen demand and disease risk.
Climate change can therefore alter regenerative success indirectly even if the genetic machinery for regrowth remains intact.
Why Sea-Star Wasting Disease Matters
Sea stars can suffer wasting syndromes involving lesions, tissue breakdown and arm loss.
Such disease is fundamentally different from clean controlled amputation and can overwhelm normal regenerative capacity.
Why Regeneration Research Interests Medicine
Scientists study sea stars and other regenerative animals to understand how adult tissues reactivate growth without producing uncontrolled tumours.
The goal is to discover general principles of wound healing, cell plasticity and pattern control rather than to copy sea-star regeneration directly into humans.
Why Humans Cannot Simply Regrow Arms the Same Way
Human limb tissues scar rapidly and lack the same adult patterning and cellular plasticity found in highly regenerative animals.
Our immune, developmental and connective-tissue systems respond to major injury differently.
Why Scar Formation Can Conflict With Regeneration
Fast fibrotic repair closes wounds effectively but can create a barrier to recreating the original developmental programme.
Regenerative animals often balance wound closure with tissue re-patterning more flexibly.
Why Cancer Control Is a Regeneration Problem Too
Regrowing an arm requires extensive cell division, but that proliferation must stop when the correct structure and size are reached.
Understanding how regenerative animals control growth could illuminate how tissues distinguish healthy rebuilding from uncontrolled proliferation.
Why Positional Memory Is Fascinating
Cells in different body regions behave as though they know whether they are near the disc, middle arm or tip.
This positional information allows missing structures to be rebuilt in the correct order.
Why Sea Stars Are Good Development Lessons
Adult regeneration reuses genes and signalling pathways associated with embryonic development.
The process demonstrates that developmental programmes can remain accessible long after an animal reaches maturity.
Why Sea Stars Are Good Stem-Cell Lessons
Regeneration involves cells capable of producing multiple tissue types, whether through resident progenitors, dedifferentiation or both.
Adult bodies can therefore retain more cellular flexibility than ordinary daily maintenance reveals.
Why Sea Stars Are Good Evolution Lessons
Predator-rich marine environments make arm loss common enough that regenerative capacity can strongly improve survival.
Selection favours rebuilding when the energetic cost is lower than the lifetime cost of permanent disability.
Why Sea Stars Are Good Systems-Thinking Lessons
Wound closure, immunity, cell proliferation, patterning, nerves, skeleton, water-vascular canals and nutrition all have to work together.
A regenerated arm is the product of coordinated whole-body biology, not one magical “regeneration gene.”
Common Myths About Starfish Regeneration
Not every detached arm becomes a new starfish, regeneration is not instant, and a central-disc injury can be fatal.
The strongest model is species-specific rebuilding that depends on surviving tissue, pattern information and energy.
Frequently Asked Questions
Can a starfish regrow a lost arm? Many species can. Can one arm grow a whole starfish? Only in certain species and usually only if part of the central disc remains. How long does it take? Often months or longer.
Does the new arm work normally? If regeneration completes successfully, nerves, tube feet, skeleton and organs can become functional again.
Where to Go Next
Sea-star regeneration connects naturally to the wider marine biology, development, wound healing and evolution ecosystem on eduKateSingapore.
Starfish regrow arms because adult sea-star tissues retain powerful programmes for wound closure, cell growth and body-pattern reconstruction. The animal does not merely heal a cut; it rebuilds a missing anatomical module and reconnects it to the whole body.
Why Regeneration Must Restore Left-Right and Oral-Aboral Patterning
A sea-star arm is not a simple cylinder. One surface faces the mouth, another faces away, tube feet occupy organised grooves, and internal organs follow specific positions. Regeneration must recreate all of these spatial relationships correctly.
That means the new arm needs a coordinate system, not just a supply of growing cells.
Why the Oral Surface Matters
The oral side contains the ambulacral groove and rows of tube feet used for attachment and locomotion. Rebuilding the wrong tissue on this surface would make the arm mechanically useless.
Patterning signals therefore distinguish the mouth-facing side from the upper aboral side throughout regrowth.
Why the Aboral Surface Matters
The upper surface carries protective epidermis, ossicles, spines and other structures depending on species. It experiences different mechanical and sensory conditions from the tube-foot side.
Regeneration restores that surface identity alongside internal anatomy.
Why Arm Number Is Usually Restored Correctly
Most species maintain a characteristic radial plan, commonly five arms. Positional signals in the surviving disc and wound edge help determine where one missing arm belongs.
The system therefore tends toward restoration of the species-typical body plan rather than producing random extra appendages.
Why Abnormal Regeneration Can Still Occur
Severe injury, unusual wound geometry or developmental disruption can produce malformed arms, branching or other irregularities. These cases reveal what happens when pattern information is incomplete or misread.
Regeneration is robust, but it is not error-proof.
Why Regrowth Requires Mineral Resources
New ossicles contain calcium carbonate, so rebuilding an arm requires access to the ions and metabolic energy needed for skeletal formation.
Mineral chemistry therefore contributes to regenerative success alongside proteins, lipids and cellular energy.
Why Ocean Acidification Can Matter
Lower seawater pH changes carbonate chemistry and can make calcification more energetically challenging for marine organisms. Responses vary among sea-star species and life stages.
Regrowing calcified tissues can therefore become more costly under changing ocean chemistry.
Why Oxygen Availability Matters
Cell division, protein synthesis and tissue remodelling require ATP. Low dissolved oxygen can limit aerobic metabolism and slow rebuilding.
A regenerating sea star therefore depends on respiratory conditions throughout the habitat, not only on food supply.
Why Regeneration Can Affect Reproduction
Energy invested in rebuilding an arm cannot be used simultaneously for producing eggs or sperm. An injured animal may therefore reduce reproductive investment during recovery.
This is a classic energy-allocation trade-off between repair and reproduction.
Why Regeneration Can Affect Growth
Young animals must divide resources among ordinary body growth, maintenance and repair. A major injury can slow overall size increase while the missing arm is rebuilt.
The animal survives, but recovery has a measurable biological cost.
Why Predation Pressure Can Favour Regenerative Ability
In habitats where arms are frequently bitten or damaged, individuals able to restore lost function gain a strong survival advantage. Natural selection can therefore maintain powerful regenerative systems.
The capacity is especially valuable when partial injury is common but whole-body destruction is not inevitable.
Why Regeneration Is Not Free
Building new tissue consumes food energy, amino acids, minerals and time. During recovery, feeding efficiency and locomotion may also be reduced.
Regeneration is advantageous because the alternative—permanent disability—would often cost even more across the animal’s remaining life.
Why Some Species Regenerate Faster Than Others
Species differ in metabolism, habitat temperature, body size, tissue organisation and evolutionary history. These differences change both the speed and completeness of arm regrowth.
There is therefore no universal timetable for “a starfish arm.”
Why Aquarium Estimates Can Be Misleading
Captive temperature, feeding and water quality may differ strongly from wild conditions. A regeneration time measured in one aquarium cannot be assumed for every species or habitat.
Good comparisons require matching species and environment carefully.
Why Regeneration Research Uses Histology
Microscopic tissue sections reveal which cells are dividing, where nerves are growing and when skeletal elements appear. Researchers can therefore map the sequence of events beneath the visible surface.
Histology turns an apparently simple growing arm into a timeline of organised tissue reconstruction.
Why Gene-Expression Studies Matter
Sequencing RNA from regenerating tissue reveals which developmental and repair genes become active at different stages. Changes in gene expression provide clues about signalling pathways controlling pattern and growth.
Molecular data therefore complement anatomical observation.
Why Imaging Helps Track Regrowth
Repeated photographs, microscopy and three-dimensional imaging can measure arm length, skeletal development and tissue organisation over time without relying only on final appearance.
Longitudinal tracking makes regeneration a quantitative process rather than a before-and-after curiosity.
Why the Same Animal Can Regenerate More Than Once
Many sea stars retain regenerative capacity throughout life, so a later injury can trigger another repair cycle.
Repeated regeneration still consumes resources and can be limited by age, health and cumulative damage.
Why Regeneration Does Not Restore Exact History
The new arm can restore normal function and pattern without recreating every microscopic feature exactly as it existed before injury. Scar-like differences or growth asymmetry can remain during maturation.
Biological regeneration aims for functional structure, not a perfect molecular copy of the lost tissue.
Why the New Arm Can Look Paler at First
New tissues may contain less pigment or thinner skeletal structures during early growth. Colour and texture can become more similar to older arms as maturation continues.
Visible difference therefore does not necessarily mean regeneration has failed.
Why Proportion Matters
A tiny regenerated arm must eventually lengthen enough to match the mechanical role of the other arms. The animal adjusts growth over time until body symmetry is improved.
Regeneration therefore includes both pattern formation and later scaling.
Why Sea Stars Are Good Regeneration Models
They can rebuild complex adult structures while remaining experimentally accessible in marine laboratories. Their evolutionary distance from vertebrates also reveals which regenerative principles are ancient and broadly shared.
Comparing sea stars with salamanders, planarians and mammals helps separate universal repair mechanisms from lineage-specific solutions.
Why Regeneration Is a Good Energy-Budget Lesson
An injured animal must choose biologically among maintenance, feeding, reproduction and rebuilding. These are not conscious decisions but physiological allocations controlled by hormones and nutrient availability.
Regeneration makes those hidden trade-offs visible in the rate at which new tissue appears.
Why Regeneration Is a Good Patterning Lesson
Cells must know not only what tissue to become but where that tissue belongs. Positional information is therefore just as important as cell division.
A functional arm requires correct geometry from disc to tip, oral side to aboral side and nerve cord to tube feet.
Why Regeneration Is a Good Evolutionary Trade-Off Lesson
Maintaining regenerative machinery may be metabolically costly, but in animals frequently exposed to partial predation it preserves lifetime reproductive potential.
The benefit depends on ecology: regeneration is most valuable when lost parts are common and survivors can continue functioning.
What Students Should Be Able to Explain
A strong answer should connect wound closure, cell proliferation, positional patterning, nerve regrowth, tube-foot reconstruction and the importance of the central disc. It should also explain why only some species can regenerate a whole individual from an arm fragment.
If the learner can separate regeneration from simple healing and from asexual fission, the topic has become transferable developmental biology.
The Final Mechanism to Remember
The durable chain is simple: injury closes, cells near the wound become developmentally active, signalling recreates body coordinates, new tissues grow in the correct arrangement and the arm gradually reconnects to the animal’s nervous, skeletal and hydraulic systems.
Regeneration succeeds because the adult body retains enough developmental memory to rebuild a missing part.
The Big Picture
Sea-star arm regeneration is not a magical replacement trick. It is a long coordinated reconstruction project carried out by living tissues that still know how to build complex anatomy.
That capacity makes sea stars valuable models for understanding how adult organisms repair, pattern and control new growth after major injury.
Why Regeneration Restores Function Before Full Size
A new arm does not need to reach adult length before it becomes useful. As nerves, tube feet and water-vascular canals reconnect, even a short regenerating arm can begin contributing to locomotion and sensing.
Functional recovery therefore occurs in stages, with basic performance returning before the final proportions are complete.
Why Regeneration Is Coordinated With the Whole Body
The new arm must connect to central nerves, digestive tissues and hydraulic canals. Growth cannot be planned locally without reference to the surviving animal.
Signals from the disc and systemic physiology therefore influence how quickly and accurately the missing region is rebuilt.
Why the Central Disc Is the Organising Hub
The disc contains the nerve ring, mouth, portions of the digestive system and major connections among the arms. It is the structural and physiological centre of the sea star.
That is why survival of sufficient disc tissue is so important when discussing whether an isolated arm can regenerate an entire individual.
Why Regeneration Can Be Fast Enough to Matter Evolutionarily
Although regrowth takes months, sea stars can live for years. Restoring an arm over part of a lifetime can therefore recover feeding and reproductive capacity long before the animal reaches old age.
The return on that regenerative investment can be substantial across many future breeding seasons.
The Practical Takeaway
The safest general rule is that many sea stars can regrow lost arms, but whole-animal regeneration from a detached arm is limited to particular species and usually depends on retaining part of the central disc.
Regeneration is powerful, gradual and biologically expensive—not an unlimited ability to survive any injury.
In other words, sea-star regeneration works because repair, developmental patterning and whole-body integration remain connected in adulthood. The animal closes the wound, rebuilds the missing anatomy in the right spatial order and restores enough function for the regenerated arm to rejoin the radial body plan as a working part rather than a simple scar.
Function returns completely.