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Marine lifespecies group

Sea stars

Asteroidea

It eats a mussel by pushing its own stomach into the gap between the shells.

An animal that eats by pushing its stomach out through its mouth and digesting prey outside its body. One species on one coast produced ecology’s idea of a keystone — and then a disease removed it across thousands of kilometres.

A sea star has no jaws and no teeth. To eat a mussel it grips both shell halves with hundreds of tube feet, pulls steadily until a gap of a fraction of a millimetre opens, everts its stomach through its own mouth and into that gap, and digests the animal inside its own shell. The prey is absorbed before it is swallowed. Everything else on this page follows from that being an effective way to eat a mussel. The ochre sea star on the Pacific coast of North America is where ecology’s keystone concept came from: prise it off a stretch of shore, keep it off for years, and mussels take over the rock while species number roughly halves. It became the most-cited manipulation in the field, and the risk of that is a habit this site tries to avoid — the result belongs to one species, on one shore, in a system where rock space is the limiting resource. Most of the two thousand or so sea star species are doing something else entirely. Then, from 2013, something removed the experiment’s subject at a scale no experiment could. A wasting disease killed sea stars from Alaska to Mexico, in some places nearly completely, across more than twenty species. What followed in some areas resembled the removal plots — mussel expansion, urchin increases where sunflower stars had been the predator — and it was not an experiment: there was no control coast, and the same years brought marine heatwaves. It is the strongest kind of evidence you can get by accident and the weakest kind you would ever design, and the honest reading is that it is consistent with the experimental result rather than a confirmation of it.

Early coverage · 38% complete · reviewed 2026-09-04

What this page covers

About two thousand species with very different diets and ecological roles. Nearly everything on this page concerns one of them, Pisaster ochraceus, on one coast — which is exactly the generalisation the page warns against.

Often confused with: Fish, which they are not — the alternative name is a nuisance rather than an error; All sea stars being keystone predators, which almost none are; Regeneration meaning a cut-up sea star always makes several new ones

Quick facts

Digestion outside the body
The stomach is everted through the mouth into the prey’s shell
The keystone case
Removal halved species number on a shore; the control plot did not change
One species, one coast
Around two thousand sea star species; almost none are keystone predators
Then disease did it at scale
Mass mortality from Alaska to Mexico from 2013 — with no control coast

Digesting a mussel from the inside

No jaws, no teeth, and a stomach that leaves the body.

Hundreds of tube feet, each working by water pressure, grip both halves of a mussel and pull. The mussel’s adductor muscle is strong but it tires; the sea star does not have to open the shell, only to hold a gap of a fraction of a millimetre. Through that gap it pushes its own stomach, wraps it around the animal inside, and digests it in place. What returns to the sea star is already liquid.

This is worth understanding before the ecology, because it explains the rate. A predator that must overpower and dismantle its prey is limited by handling time; one that clamps on and waits is limited mainly by how many mussels it can reach. That is why a modest number of sea stars can hold back a competitive dominant across a whole shore.

The shore that produced the keystone idea

And the caution that belongs with it.

Take one sea star off a patch of rocky shore and keep it off, and mussels take over the rock. Species number fell from about fifteen to about eight — while the untouched plot beside it did not change.

Established

Specialists would state this without hedging. Multiple independent lines of evidence agree.

Experimental exclusion of Pisaster ochraceus from an intertidal plot over several years resulted in competitive monopolisation of primary space by Mytilus californianus and a decline in species richness from roughly fifteen to eight, relative to an adjacent unmanipulated control.

Who this applies to
One rocky shore system on the Pacific coast of North America.Do not extend this beyond the taxa listed — the popular version over-reaches.
Studied in
Pisaster ochraceus, Mytilus californianus
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

A sustained manipulation with a matched control and a clear, large effect, repeatedly reproduced in related systems since.

How far it can be extended

A single removal plot and control on one shore, where the limiting resource is rock space — an unusually simple case.

Caveats

  • One shore, one plot, one control — the result is strong and its geographic scope is narrow.
  • Rock space as the limiting resource makes competitive monopolisation unusually direct; systems with more complex limitation may behave differently.

Still unanswered

  • How the same community responds to sea star loss from disease rather than from experimental removal, which affects a far larger area at once.

Last reviewed 2026-09-04

The evidence (1 study)

How we know

Taking the sea stars off the rock, and keeping them off

What does a predator do to the community it hunts in — beyond reducing the numbers of what it eats?

A section of rocky shore was cleared of its predatory sea stars by hand, and they were removed again whenever they returned, for several years. An adjacent comparable stretch of shore was left untouched. On both, the species present and the area each occupied were recorded through time.

What happened

On the cleared plot, mussels spread and progressively took over the rock, and species number fell from around fifteen to about eight. The untouched plot kept its composition.

What it shows

That a predator can maintain the diversity of a community rather than diminish it. Mussels are the best competitor for rock space and will take all of it; the sea star eats them faster than they can monopolise, and everything else lives in the space that leaves. The predator is not controlling numbers so much as preventing a takeover — which is a different mechanism from the one most people picture.

What it does not show

One shore, one plot, one control. It does not show that predators generally increase diversity: where the prey is not a competitive dominant, removing its predator does nothing of the kind, and in other systems predation reduces diversity. Rock space is also an unusually simple limiting resource, and the clarity of the result partly reflects that.

The controls — what makes this evidence rather than a story
  • The adjacent untouched stretch, which is what turns years of change into a comparison rather than a story.
  • Sustained removal rather than a single clearance, so the effect is of the predator’s absence and not of the disturbance.
  • Area occupied recorded as well as presence, so competitive displacement is visible rather than inferred.

From Food web complexity and species diversity

Where the idea went

When disease ran the experiment

At a scale nobody could design, and with no control.

From 2013 a wasting disease killed sea stars along thousands of kilometres of the Pacific coast, affecting more than twenty species and in places removing them almost entirely. The sunflower star, a large and mobile predator of urchins, was among the worst affected. In some regions what followed looked like the removal plots at continental scale: mussels spreading, urchins increasing where their predator had gone, kelp declining.

It is tempting to call this a confirmation and it is not one. There was no control coast, the same years brought unusual marine heatwaves, and disease severity varied with conditions that also affect the other species involved. The event is consistent with the experimental result and cannot separate the predator’s absence from everything else that was happening. Recording it that way is more useful than either dismissing it or treating it as proof.

The research behind this page

5 studies, newest first. Each one has a page explaining what it found and what it could not show.

2010Ecology

Are wolves saving Yellowstone’s aspen? A landscape-level test of a behaviorally mediated trophic cascade

Aspen were not recovering preferentially in high-risk areas.

1996BioScience

Challenges in the quest for keystones

The term had been applied to species of every abundance and to effects of every size, including cases where the species was simply dominant.

1974Science

Sea otters: their role in structuring nearshore communities

Islands with otters had few and small urchins and extensive kelp.

1969The American Naturalist

A note on trophic complexity and community stability

Some species have effects on community structure disproportionate to their numbers or biomass, so that their removal changes the community more than their abundance would suggest.

1966The American Naturalist

Food web complexity and species diversity

On the removal plot, mussels expanded and progressively monopolised the rock.

This page is a stop on a longer route

A guided journey reads several subjects in a deliberate order, with an argument for why one follows another. You can join in the middle.

Where to go from here

Each of these follows from something on this page — a relationship in the evidence, a claim people ask about, or the next mechanism along.

How complete this page is, and what it is still missing

NatureHQ publishes its own gaps. This page is at 38% completeness against what we would call a finished subject, and was last reviewed on 2026-09-04. It carries 4 claims and answers 0 mapped search questions.

  • no research from the last few years is attached — check for newer work
  • more experiments could be explained in plain English
  • no popular claim about this subject has been checked yet
  • Sea star biology beyond feeding — water vascular system, regeneration, reproduction — is barely covered.
  • The cause of sea star wasting disease is not settled and is not examined here.
  • The crown-of-thorns starfish, ecologically important on coral reefs, is not covered.