Head to head
Sea Urchin vs Starfish
Sea urchins and starfish share a beach bucket, a body built on fives, and a system of water-powered tube feet found in no other animals. They are close relatives, and a sea urchin is essentially a starfish that pulled its arms up over its back.
| Sea Urchin | Starfish | |
|---|---|---|
| Phylum | Echinodermata (class Echinoidea) | Echinodermata (class Asteroidea) |
| Body | Rigid fused globe with movable spines | Flattened disc drawn out into arms |
| Arms | None | Usually five; some species many more |
| Feeding | Grazes algae with a five-toothed jaw | Preys on shellfish; everts its stomach |
| Diet | Mostly herbivore | Mostly carnivore or scavenger |
| Movement | Tube feet plus spines as stilts | Tube feet along the underside of each arm |
| Defence | Spines, some venomous; tiny pincer organs | Tough body wall; spines; some toxic |
| Ecological trouble | Urchin barrens: kelp forests grazed to rock | Crown-of-thorns: coral reefs stripped |
Sea Urchin vs Starfish: common questions
Are sea urchins and starfish related?
Closely. Both are echinoderms, the phylum that also includes brittle stars, sea cucumbers, and crinoids, and all echinoderms share the five-part radial body and the unique water-vascular system that drives their tube feet. Within the phylum, sea urchins and starfish are separate classes, but developmental biology and fossils show the urchin body is essentially a modified version of the star body, as if the arms of a starfish were bent upward and joined over the top, carrying the rows of tube feet with them, and the whole skeleton then stiffened into a shell.
Do both have the eversible stomach?
No, that is mainly a starfish trick. Many starfish can push their cardiac stomach out through their mouth to engulf or envelop prey, which lets a starfish eat a mussel or oyster it cannot fit inside its body and even feed on things larger than itself. Sea urchins feed differently, using the hard, self-sharpening five-toothed apparatus on the underside, Aristotle's lantern, to bite and scrape food, mostly algae, directly into the gut. Some sea cucumbers, another echinoderm group, can eject their internal organs in defence, but that is a separate phenomenon.
Which is more of a problem for reefs and kelp?
Both can be, in different systems. On temperate rocky coasts, sea urchins are the classic threat: when their predators, such as sea otters, lobsters, and certain fish, are removed, urchin numbers explode and they graze kelp forests down to bare pink rock, creating urchin barrens that can persist for decades. On tropical coral reefs, the crown-of-thorns starfish is the equivalent problem, since outbreaks of this large, coral-eating sea star can strip the living coral from huge areas of reef. In both cases the underlying issue is usually an imbalance, often caused by overfishing of predators or by nutrient runoff that boosts larval survival.