Starfish look simple, but they run on a hydraulic system of seawater, see with eyespots on their arm tips, digest food outside their bodies and can regrow lost parts. This guide covers how they work, how they live and why so many have died since 2013.

Is a starfish a fish?

No. That is why many scientists prefer “sea star.” Starfish are echinoderms, the spiny-skinned group that also includes brittle stars, sea urchins, sea cucumbers and crinoids. Echinoderms have five-part radial symmetry as adults, a skeleton of small calcite plates (ossicles) and a water vascular system that powers their tube feet. They are not crustaceans either. See echinoderms for the whole phylum.

How many species of starfish are there?

About 1,900 living species, found in every ocean from tropical reefs to polar seas and from the shore to abyssal depths of about 6,000 m. They have no tolerance for fresh water, so there are no freshwater starfish.

Starfish	Scientific name	Size	Notable for
Chocolate chip sea star	Protoreaster nodosus	to ~30 cm	Dark “horns” on a tan body
Blue sea star	Linckia laevigata	~30–40 cm	Brilliant blue; regrows from a single arm
Sand sifting sea star	Astropecten polyacanthus	~12–20 cm	Buries in sand; contains tetrodotoxin
Crown-of-thorns sea star	Acanthaster spp.	25–35 cm (to ~80 cm); up to 21 arms	Coral predator, venomous spines
Sunflower sea star	Pycnopodia helianthoides	arm span to ~1 m	Critically Endangered since 2020

How do starfish move without legs?

Under each arm runs a groove lined with tube feet, usually in two rows. Each foot is connected to a bulb (ampulla) in the water vascular system. Muscles squeeze the bulbs to push water into the feet, extending them, and the feet grip with adhesive chemicals rather than true suction. Working in waves, hundreds of feet pull the animal along. Most starfish are slow: the leather star manages about 15 cm a minute. Burrowing sea stars such as Astropecten have pointed feet without suckers and move faster over sand. A starfish turned upside down can flip itself by bending two arms back and pushing off with a third.

How do starfish eat?

Many starfish are opportunistic predators of clams, mussels, snails, barnacles and sponges, while others scavenge or sift detritus. Their stomach has two parts. In advanced species, the larger cardiac stomach can be pushed out through the mouth. When attacking a clam, the starfish pulls on the shell with its tube feet until a tiny gap opens, then slips part of its stomach inside and digests the clam in its own shell. Primitive groups such as Astropecten swallow prey whole instead. The crown-of-thorns uses its everted stomach to digest living coral tissue.

Can starfish see?

Yes, in a basic way. Each arm ends in an eyespot made of about 80–200 simple ocelli, which can detect light, dark and coarse shapes. Many starfish also have light-sensitive cells scattered over the body, and their tube feet, spines and pedicellariae sense touch and chemicals. When one arm smells food, it becomes the leading arm and steers the whole animal.

Do starfish have a brain or blood?

Neither. A nerve ring around the mouth connects to a radial nerve in each arm, so control is spread through the body. Instead of blood, starfish circulate seawater through the water vascular system, which moves oxygen, nutrients and waste. Gas exchange happens through the tube feet and through small skin gills called papulae.

How do starfish regenerate arms?

Most starfish can regrow lost arms, and many drop an arm on purpose to escape a predator. A few species, including the blue sea star and its relatives, can regrow a whole animal from a single arm attached to a piece of the disc, and some can regenerate from fragments only about 1 cm long. Regrowth takes months to years, and a damaged starfish is vulnerable to infection meanwhile. Read more in starfish regeneration.

How do starfish reproduce?

Most species have separate sexes and spawn eggs and sperm into the water, often in synchronized events; the larvae drift as plankton before settling. Some species are hermaphrodites, and many polar and deep-sea starfish brood their eggs under their bodies or in special pouches. Some can also reproduce asexually by splitting the disc or shedding arms that grow into new animals, as the small Asterina starfish of reef aquariums do.

Why are starfish keystone species?

In 1960s experiments on the Washington coast, ecologist Robert Paine removed ochre sea stars (Pisaster ochraceus) from rocky shores. Without them, mussels took over and the number of other species fell sharply. The result gave ecology the term “keystone species,” and starfish remain its textbook example.

What is sea star wasting disease?

Starting in 2013, a wasting disease swept the Pacific coast of North America. Affected sea stars develop lesions, lose arms and dissolve within days. More than 20 species were hit. In 2025, Prentice and colleagues identified a strain of the bacterium Vibrio pectenicida as a cause of the disease in sunflower stars; its role in other species is still being studied. The sunflower sea star, once the largest and most important predator of sea urchins on the coast, declined by an estimated 90.6 percent, with as many as 5.75 billion animals dying, and the IUCN listed it as Critically Endangered in December 2020. Without sunflower stars, urchin numbers exploded in parts of the coast and grazed down kelp forests.

What is a starfish body made of?

A starfish skeleton is not one shell but hundreds or thousands of small calcite plates called ossicles, set in the body wall and linked by muscle and connective tissue. Many ossicles carry spines, granules or tubercles, which give each species its texture. Between the plates, thin-walled papulae poke out to exchange gases with the water, and in many species tiny pincer-like organs called pedicellariae keep the surface clear of settling larvae and debris.

The connective tissue is unusual. Like that of other echinoderms, it is a catch connective tissue that the animal can switch from soft to stiff within seconds, under nervous control. This lets a starfish lock its arms in place for hours while it pulls on a clam, wedge itself into a crevice so firmly that it is almost impossible to remove, or go limp and squeeze through a gap.

On the upper surface, usually off-centre on the disc, sits the madreporite, a small sieve plate through which seawater enters the water vascular system. The mouth is on the underside, and in most species a small anus opens on the top of the disc, although the burrowing order Paxillosida lacks one. Inside, the stomach is divided into a large cardiac part that can be pushed out of the mouth and a smaller pyloric part that always stays inside, with digestive glands (pyloric caeca) running into every arm. Because digestion can start outside the body, a starfish can feed on prey far larger than its mouth.

How does a starfish grow from egg to adult?

In broadcast-spawning species, fertilized eggs develop into larvae that look nothing like the adult. The first stage, the bipinnaria, is bilaterally symmetrical, with bands of beating cilia that it uses for swimming and feeding on phytoplankton. In many species it then becomes a brachiolaria larva, growing three extra short arms and a sucker. When it is ready to settle, the brachiolaria attaches to the seabed with a short stalk and undergoes a radical metamorphosis: the five-part radial body of the juvenile forms inside the larval tissues, the mouth and anus move to new positions, and the larva’s left and right sides are reorganised into a star. Members of the Paxillosida skip the brachiolaria stage and settle directly from the bipinnaria.

Larvae can also multiply on their own. Some starfish larvae bud off small pieces that develop into new larvae, especially when food is plentiful, so one fertilized egg can give rise to several adults. Planktonic larvae can drift for weeks, which helps species spread along coastlines and recolonise areas after die-offs.

Not all starfish have a drifting stage. Many polar, deep-sea and small coastal species brood their young on the underside of the body, in pockets on the upper surface or, in a few cases, inside the stomach or gonads. In Parvulastra parvivipara of southern Australia the young develop inside the parent and feed on other eggs and embryos in the brood pouch. Brooding gives each offspring a better start but produces far fewer of them.

How long do starfish live?

Lifespan varies widely and tends to be longer in larger species and in those with planktonic larvae. The small brooding Leptasterias hexactis of the North Pacific weighs about 20 g as an adult, matures in two years and lives about ten. The ochre sea star, Pisaster ochraceus, can weigh up to about 800 g, takes about five years to mature and has a maximum recorded lifespan of 34 years. Ageing a wild starfish is difficult, because the skeleton does not lay down clear annual rings, so many estimates come from long-term studies of marked individuals or from growth rates.

What eats starfish, and how do they defend themselves?

Starfish have more enemies than their armour suggests. They are eaten by fish, crabs, gulls, sea otters, sea anemones, other starfish and large predatory snails such as tritons. The harlequin shrimp of the Indo-Pacific feeds almost entirely on starfish, flipping them over and eating the tube feet and soft tissues. Juvenile sea stars are also cannibalistic, and have been seen eating each other within days of metamorphosis.

Their main chemical defence is a family of soapy compounds called saponins, or asterosaponins, which make the body wall taste unpleasant. Some species add stronger toxins. The sand-sifting star Astropecten polyacanthus contains tetrodotoxin, the nerve poison of pufferfish, and the slime star can release large quantities of repellent mucus. The crown-of-thorns starfish is protected by long, sharp spines carrying toxins that cause painful wounds in people who handle it. Arm-dropping (autotomy) is the last line of defence: a starfish caught by one arm can shed it and leave the predator with a mouthful while the rest of the animal escapes.

When do starfish become pests?

Some starfish cause serious damage when their numbers explode or when they turn up somewhere new. The crown-of-thorns starfish eats live coral, and outbreaks of thousands of animals can strip whole reefs. A study in French Polynesia found that coral cover fell from about 50 percent to under 5 percent within three years after an outbreak that began in 2006. Outbreaks on the Great Barrier Reef have been linked to nutrient run-off that boosts the food supply of the larvae, though the causes are still debated. See crown-of-thorns starfish for more.

The northern Pacific seastar, Asterias amurensis, is native to the coasts of northern China, Korea, Russia and Japan. Its larvae are thought to have reached Tasmania in ships’ ballast water; it was first collected there in 1982 and reported in the Derwent River estuary in 1985, and it reached Victoria by 1998. A female can carry up to 20 million eggs. In Australia it preys on native shellfish and on the egg masses of the rare spotted handfish, and settled juveniles foul salmon cages, oyster lines and scallop longlines. It is on the list of the world’s 100 worst invasive alien species. Shellfish farmers elsewhere also treat native starfish as pests, because a large starfish can eat many mussels or oysters.

How old are starfish as a group?

Echinoderms appear in the fossil record in the Cambrian, and the first starfish-like animals in the Ordovician, around 450 million years ago. Starfish fossils are rare, because the ossicles usually fall apart soon after death; complete skeletons survive mainly in a few exceptional deposits known as starfish beds, where animals were buried suddenly by sediment. The living starfish, the Neoasteroidea, look quite different from their Palaeozoic ancestors. Many lineages died out in the great extinctions of the late Devonian and the end of the Permian, and the few survivors diversified rapidly in the Early and Middle Jurassic.

Although adult starfish are radially symmetrical, their larvae are bilateral, a clue to their ancestry. Echinoderms are deuterostomes, the group that also includes chordates, which makes starfish more closely related to fish and humans than to snails, crabs or jellyfish. The scientific name Asteroidea, from the Greek for star, was given to the class by the French zoologist Henri de Blainville in 1830.

What is known about the cause of sea star wasting disease?

For a decade the cause of the 2013 epidemic was unclear. A 2014 study pointed to a densovirus, but later research failed to confirm it as the driver. In August 2025 a team from the University of British Columbia, the Hakai Institute and the University of Washington reported in Nature Ecology & Evolution that a strain of the bacterium Vibrio pectenicida, named FHCF-3, causes the disease in sunflower sea stars. They compared the body (coelomic) fluid of sick and healthy animals, grew the bacterium from diseased stars, and showed that injecting it into healthy sunflower stars reproduced the disease. In their experiments, more than 90 percent of the stars died within a week of showing symptoms. Whether the same strain drives wasting in the other affected species is still being tested, and warmer water appears to make outbreaks more severe.

Identifying the pathogen opens the way to testing for infection, screening animals bred in captivity and choosing safer places and times to release them. Legal protection has moved more slowly. The US National Marine Fisheries Service proposed listing the sunflower sea star as threatened under the Endangered Species Act in March 2023, but as of mid-2026 no final decision had been issued, and the Center for Biological Diversity filed a lawsuit to force one.