Starfish vs Sea Urchin: Key Differences Explained

The main difference

A starfish is an armed predator that often digests prey outside its body; a sea urchin is a spined grazing globe that scrapes algae with a specialised jaw apparatus.

They are the two echinoderms everyone recognises, and comparing them makes the shared five-fold body plan and hydraulic movement system visible in two very different packages.

At a glance

Comparison of Starfish and Sea Urchin across 6 dimensions, with an interpretation of each difference.

Body form

StarfishArms radiating from a central disc, typically five

Sea UrchinA globe or flattened disc covered in movable spines

Both express five-fold symmetry, but in visibly different packages.

Feeding

StarfishPredatory — many species evert the stomach to digest prey externally

Sea UrchinGrazing — scrapes algae and encrusting organisms with a five-toothed jaw apparatus

One is a predator of attached animals, the other a herbivore shaping algal communities.

Movement

StarfishHundreds of hydraulic tube feet along grooves beneath the arms

Sea UrchinTube feet extended between the spines, with spines assisting

Both use the water vascular system, a mechanism unique to echinoderms.

Spines

StarfishShort and fixed, embedded in the body wall

Sea UrchinLong and movable, used in defence, movement and wedging into crevices

Movable spines are the sea urchin's most conspicuous feature.

Regeneration

StarfishExceptional — many species regrow lost arms, and some regenerate from a fragment

Sea UrchinRegrows spines and tube feet, but not on the same scale

Starfish regeneration is among the most striking in the animal kingdom.

Ecological role

StarfishPredator of bivalves, barnacles and corals; some are keystone species

Sea UrchinGrazer whose abundance can determine whether a reef stays kelp forest or becomes barren

Both can transform habitats when their numbers shift substantially.

How they are related

Both are echinoderms, a phylum whose members share five-fold radial symmetry as adults, a calcareous internal skeleton and a hydraulic water vascular system. Starfish form the class Asteroidea and sea urchins the class Echinoidea. Notably, echinoderm larvae are bilaterally symmetrical, and the radial adult form develops later — which is why echinoderms are more closely related to vertebrates than to most other invertebrates.

Telling them apart

There is no realistic confusion: arms radiating from a disc against a spine-covered globe. Both move slowly using tube feet, which in a starfish run along grooves beneath the arms and in a sea urchin extend between the spines, often reaching well beyond them.

Arms radiating from a central disc, moving over rock or sand: starfish. Spine-covered globe wedged into a crevice or grazing rock: sea urchin. Both are echinoderms — neither is a fish, which is why sea star is increasingly preferred to starfish.

Habitat and distribution

Both occur in all oceans from intertidal to deep sea. Starfish are found on rocky shores, reefs, sand and mud. Sea urchins are strongly associated with rocky substrate and kelp systems, where many wedge into crevices by day and graze at night, and some excavate depressions in rock over time.

Diet and ecological role

Starfish are largely predatory, taking bivalves, barnacles, snails and, in some species, coral. Many evert the stomach through the mouth to digest prey externally, allowing them to feed on animals they could not swallow. Sea urchins graze algae and encrusting organisms using Aristotle's lantern, a complex five-toothed structure that scrapes and cuts.

Behaviour

Both move slowly and continuously using tube feet under hydraulic control. Starfish prise open bivalves with sustained pull from the tube feet until the shell gapes slightly, then insert the everted stomach. Sea urchins graze over rock surfaces and many shelter in crevices by day, with some species covering themselves in shell fragments and algae.

Senses and adaptations

Starfish have simple eyespots at the arm tips capable of detecting light and, in some species, resolving coarse images that help them navigate back to reef habitat. Both rely heavily on chemical sensing to locate food. The water vascular system that drives their tube feet is unique to echinoderms and operates hydraulically rather than through muscle alone.

More on this topic: Animal senses and adaptations.

People and these animals

Both are ecologically consequential. Crown-of-thorns starfish outbreaks have caused substantial coral loss on Indo-Pacific reefs, and sea star wasting disease has devastated populations along parts of the American Pacific coast. Sea urchin population increases, often following loss of their predators, have converted kelp forests to urchin barrens in several regions. Urchin roe is also commercially valuable.

What they have in common

• Both are echinoderms with five-fold radial symmetry as adults.
• Both move using hydraulic tube feet driven by a water vascular system unique to the phylum.
• Both have a calcareous internal skeleton beneath the skin.
• Both can transform marine habitats when their populations shift substantially.

Key differences

• Starfish have arms radiating from a disc; sea urchins are spine-covered globes.
• Many starfish digest prey externally by everting the stomach.
• Sea urchins graze algae using Aristotle's lantern, a five-toothed jaw apparatus.
• Sea urchin spines are long and movable, while starfish spines are short and fixed.
• Starfish regeneration is far more extensive, in some species regrowing from a fragment.

Where people go wrong

• Calling either a fish — both are echinoderms, hence the preference for sea star.
• Assuming echinoderms are simple animals, when their larvae are bilaterally symmetrical and their relationships lie close to vertebrates.
• Expecting all starfish to have exactly five arms, when many species have more.

Read the full profiles

• StarfishAnimal profile
• Sea UrchinAnimal profile

Related comparisons

• Crab vs Lobster
• Jellyfish vs Squid
• Clam vs Oyster

Related FaunaHub guides

• All insects & invertebrates comparisons
• Fauna by region
• Animal taxonomy
• Animal encyclopedia

Sources and methodology

How this comparison is built: FaunaHub comparisons are assembled from the sourced animal profiles behind them rather than written as standalone claims. Figures such as size, mass, and lifespan are typical ranges that vary by subspecies, population, region, sex, age, and — where relevant — whether an animal lives in the wild or under human care, so they are presented as ranges rather than fixed values. Where a common name covers a group rather than a single species, the page says so instead of implying a species-to-species match. Conservation status changes over time and should be confirmed against the current IUCN Red List. Dimensions that could not be supported from the underlying profiles are omitted rather than filled in.

What this page does not do: it does not rank the two animals, predict the outcome of a confrontation, or present either one as stronger, smarter, or better. It is not veterinary, medical, first-aid, handling, capture, hunting, or pest-control guidance. For an animal in your care, or any situation involving injury, contact a qualified veterinarian, your local wildlife authority, or emergency services.