Invertebrates
Starfish
Asteroidea · also called sea star, asteroid
A starfish has no brain, no blood, and no bones, and it walks on hundreds of little feet run by water pressure. When it finds a mussel it does not carry the meal away — it turns its own stomach inside out through its mouth, slides it into the shell, and digests the animal where it sits. Lose an arm and it grows another; in some species, the lost arm grows a new starfish.
Overview
Starfish, or sea stars, are the class Asteroidea, around 1,900 species, in every ocean from tide pools to the deep sea and pole to pole — but never in fresh water. They are echinoderms, related to urchins and sea cucumbers, with a spiny skin and typically five arms around a central disc. Most are predators of slow or fixed prey — clams, mussels, snails, barnacles, coral.
Run on seawater
A starfish has no muscular legs. It pumps seawater through an internal water vascular system and uses hydraulic pressure to work hundreds of tiny tube feet, which grip and pull. There is no central brain — a nerve ring coordinates the arms — and no blood; seawater carries nutrients and gases. Each arm tip has a simple eyespot that forms a coarse image.
Eating inside-out
To open a mussel, a starfish pulls the shell apart by a fraction of a millimetre with its tube feet, then everts its stomach through its mouth into the gap and digests the animal in place before drawing the meal back in.
Keystone, and plague
Removing the ochre star from a Pacific shore let mussels take over and diversity collapse — the experiment that created the term keystone species. Meanwhile booms of the crown-of-thorns starfish have stripped large areas of the Great Barrier Reef of living coral.
Status
Starfish as a class are Not Evaluated. Most species are common, but since 2013 a sea star wasting disease has killed billions along the Pacific coast of North America, driving the sunflower star to Critically Endangered — and its loss, as a major urchin predator, has helped collapse kelp forests.
Starfish: common questions
Why is it wrong to call it a starfish, and does it matter?
Many marine biologists prefer sea star because the animal is not a fish in any sense. It has no backbone, no gills, no fins, and no scales, and it belongs to the echinoderms, a group whose only close relatives are sea urchins, sea cucumbers, and brittle stars, not to any vertebrate. Fish, by contrast, are backboned animals. The name starfish is very old and still widely used, including by some scientists, so it is not wrong exactly, but sea star is more accurate and is becoming the standard term in field guides and aquariums. Both names refer to the same class, Asteroidea.
How does a starfish eat a mussel or a clam?
With patience and an unusual trick. The starfish climbs onto the closed shell, anchors itself with the tube feet on some arms, and grips the two shell halves with the tube feet on others, then applies a steady pull. The mussel's muscles can hold the shell shut hard, but not forever, and after minutes to hours the shell gapes open by less than a millimetre. That is enough. The starfish everts its cardiac stomach, pushing it out through the mouth in the centre of its underside and squeezing the thin membrane into the gap, where digestive enzymes break down the mussel's body. The starfish then absorbs the partly digested soup and pulls its stomach back in.
Can a starfish really grow back from a single arm?
In some species, yes, though not from just any fragment. Most starfish can regrow a single lost arm from the central disc over several months. A smaller number, including several Linckia species and some others, can regenerate a complete new starfish from a detached arm, provided the arm carries at least a small piece of the central disc, and these arms are sometimes called comets while they are growing a new set. A few starfish even reproduce routinely by deliberately splitting the disc in two. Oyster farmers once made the problem worse by chopping up starfish and throwing the pieces back, unaware that some would regrow.
What is sea star wasting disease?
It is an outbreak of a still not fully understood illness that has devastated starfish along the Pacific coast of North America since 2013. Affected animals develop white lesions, lose the firmness of their bodies, twist and lose arms, and disintegrate within days, and it has killed billions of individuals across more than 20 species. The sunflower star, formerly one of the most abundant large starfish on the coast, crashed by an estimated 90 per cent or more and is now listed as Critically Endangered. The cause is debated, with a virus, bacteria, and low oxygen at the skin surface all proposed, and warmer than usual ocean temperatures appear to make outbreaks worse. The loss of the sunflower star, a major urchin predator, has contributed to the collapse of kelp forests in some areas.