# Are Starfish Really Fish?

> Starfish are not fish. They are marine invertebrates called sea stars, members of the echinoderm group alongside brittle stars, sea urchins, sand dollars and sea cucumbers. They have no backbone, fins, gills or scales and their internal anatomy follows a plan unlike...

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Published: 2026-08-30T14:04:35+00:00
Categories: Explainer, Nature

![High angle of bright blue Linckia laevigata sea star on massive textured Goniastrea retiformis coral on ocean beach](https://www.argo.net/wp-content/uploads/2026/08/sea_star_moving_across_rocky_seafloor.jpg)

Starfish are not fish. They are marine invertebrates called sea stars, members of the echinoderm group alongside brittle stars, sea urchins, sand dollars and sea cucumbers. They have no backbone, fins, gills or scales and their internal anatomy follows a plan unlike that of any true fish.

NOAA's answer to whether [starfish are really fish](https://oceanservice.noaa.gov/facts/starfish.html) recommends the more accurate common name sea star. The old name persists in everyday speech, but it describes appearance rather than biological relationship.

About 2,000 sea star species inhabit the world's oceans. They occur in tide pools, kelp forests, coral reefs, polar seas and deep trenches. Five arms are common, although some species have many more.

## Sea stars belong to the echinoderms

Echinoderm means spiny skin. The group is exclusively marine and includes animals with a hard internal skeleton made of calcium carbonate plates. Small projections, movable spines or grasping structures may cover the outer surface.

An adult sea star usually has **radial symmetry**, with body parts arranged around a central disc. Fish have bilateral symmetry, meaning the left and right sides mirror each other. Sea star larvae are bilateral before changing form during development.

Echinoderms are deuterostomes, placing them on a major evolutionary branch that also includes chordates. That relationship does not make a sea star close to a fish in ordinary classification. It instead reflects a very ancient split among animal lineages.

The [Smithsonian Ocean sea star guide](https://ocean.si.edu/ocean-life/invertebrates/sea-stars) shows the group's diversity, from small species hidden beneath rocks to sunflower sea stars with many arms. Color and shape often match a particular habitat or feeding strategy.

## A water vascular system powers tube feet

Sea stars move using rows of small tube feet beneath their arms. Seawater enters a porous plate called the madreporite and travels through internal canals. Muscles and hydraulic pressure extend or retract each foot.

Thousands of coordinated contacts can pull an animal over rock, hold it against waves or manipulate prey. The feet use adhesive chemicals rather than behaving as simple suction cups. Another secretion helps release each attachment.

**Tube feet serve several functions.** They contribute to movement, feeding, sensory exploration and gas exchange. A light-sensitive eyespot near the end of each arm helps many species detect broad shapes and changes in illumination.

Speed varies greatly. NOAA Fisheries reports that a sunflower sea star can use as many as 15,000 tube feet and move roughly a meter per minute. Other species progress so slowly that their movement is difficult to notice without time-lapse video.

## Feeding can happen outside the body

Many sea stars prey on mussels, clams or oysters. A predator grips both shell valves and applies steady force until a narrow gap opens. It can then push part of its stomach through its mouth and into the shell.

Digestive enzymes break down prey tissue outside the sea star's central body. Liquefied nutrients pass inward and the stomach retracts. The method lets the animal consume prey that would not fit through its mouth.

Not every species hunts shellfish. Some graze on films, capture small animals, scavenge carrion or swallow sediment rich in organic matter. Crown-of-thorns sea stars feed on coral polyps and can alter reefs when outbreaks become dense.

The [NOAA sunflower sea star account](https://www.fisheries.noaa.gov/species/sunflower-sea-star) explains how the loss of a major predator can affect kelp forests. Without enough predators, sea urchin populations can rise and consume extensive kelp.

## Regeneration has real limits

Sea stars can often regrow a damaged arm. Regeneration may take months or years and requires energy that would otherwise support growth or reproduction. Species differ in how much of the central disc must remain.

A detached arm does not automatically become a new animal. In most species it needs a portion of the central disc to survive and rebuild the rest of the body. A few species can reproduce by deliberately dividing.

*Regrowth is a survival mechanism*, not proof of immortality. Disease, prolonged heat, low oxygen or severe tissue loss can overwhelm it. Sea star wasting disease has killed enormous numbers along parts of the Pacific coast.

## Sea stars sense a world without a brain

A sea star has a nerve ring around the mouth and radial nerves extending through the arms. It lacks a centralized brain, yet the network coordinates movement and responses to food, light and touch.

Chemical receptors help locate prey. Tube feet sample surfaces and water, while eyespots influence orientation. Individual arms can respond locally, then integrate their activity through the ring.

The animal exchanges gases across thin body surfaces, including tube feet and small projections called papulae. It has no fish-like gills. Internal fluid circulates materials over short distances within the compact body.

The [IUCN sunflower sea star assessment](https://www.iucnredlist.org/species/178290276/197818455) illustrates why correct classification matters in conservation. A sea star's physiology determines its response to warming and disease. Calling it a fish may be harmless conversation, but understanding it as an echinoderm explains how it moves, feeds and survives.

## The name starfish hides extraordinary diversity

*Collection can damage local populations.* A sea star lifted from water may experience stress and handling can injure tube feet. Tide-pool visitors protect the community by observing animals where they are and replacing any accidentally moved rock carefully.

Scientific names avoid ambiguity because common names vary among regions. The word starfish remains culturally familiar, while sea star communicates the key correction. Either label should lead to the same biological fact: the animal is an echinoderm.

Sea stars have a mouth on the underside and an anus on the upper surface in many species. The central disc contains most major organs, while digestive glands and gonads extend into the arms. This distributed arrangement differs from the head-to-tail organization of fish.

They also lack blood circulation like that of vertebrates. Coelomic fluid moves nutrients and gases internally, assisted by body movements and cilia. Gas exchange occurs across thin surfaces, so low oxygen can become a serious stress.

## Health depends on habitat conditions

**Temperature affects disease outcomes.** The causes of wasting can involve several interacting conditions rather than one universal pathogen. Monitoring programs record species, symptoms and water conditions to distinguish local events from coastwide change.

Aquarium visitors may see a sea star gripping glass and mistake the feet for suction cups. Microscopic studies show chemical adhesion and release. This mechanism works on uneven natural surfaces where a simple vacuum seal would be unreliable.

## Arm number and ecology vary among species

**Five arms are only the familiar pattern.** Sunflower sea stars can have more than twenty, while cushion stars have short arms merged into a thick body. Deep-sea species may be delicate or adapted to soft sediment.

Many species release eggs and sperm into water. Embryos develop into swimming larvae that feed in plankton before settling. Metamorphosis reorganizes a bilateral larva into a radial juvenile.

*Habitat sets physical demands.* Intertidal animals endure waves, drying and abrupt temperature changes. Deep-water species face darkness, cold and limited food. Tube feet, body walls and feeding behavior vary accordingly.

## Why classification improves understanding

Predators include fish, crabs, sea otters and other sea stars. Defenses range from tough spines to chemical compounds or deliberate arm loss. The energetic cost of replacement prevents regeneration from being a free escape.

**Ecological roles differ.** Some control mussels or urchins, some consume detritus and others graze coral. Removing a strong predator can reorganize a community, while an outbreak of another species can reduce coral cover.

Using sea star encourages a better question: which species? Classification separates the animal from fish, but species biology determines habitat, prey, vulnerability and ecological influence. Recognizing an echinoderm explains the tube feet, water vascular system and external feeding method that the misleading older name cannot.

**Related reading:** [animals that live on coral reefs](https://www.argo.net/what-animals-live-in-coral-reefs/) and [marine biogeography](https://www.argo.net/what-is-marine-biogeography/).

 **Explore this topic:** [What Is the Biggest Fish in the Ocean?](https://www.argo.net/what-is-the-biggest-fish-in-the-ocean/) and [What Is the Biggest Fish in the Ocean?](https://www.argo.net/what-is-the-biggest-fish-in-the-ocean/).