Sea Urchin
Echinoidea
Quick answer
Sea urchins are round, spiny marine invertebrates related to starfish, with a hard shell called a test covered in movable spines. They graze on algae across reefs and rocky seabeds worldwide, using a five-toothed mouth on their underside known as Aristotle's lantern. Most live around 10 to 30 years, and some species are very long-lived.
Sea Urchin facts at a glance
| Scientific name | Echinoidea (class) |
|---|---|
| Diet | Mostly herbivore (algae and kelp) |
| Habitat | Reefs, rocky seabeds, and kelp forests worldwide |
| Lifespan | 10–30 years (some far longer) |
| Size | Shell usually 3–10 cm (1–4 in) across |
| Top speed | Very slow; creeps on tube feet and spines |
| Conservation status | Least Concern (IUCN, most species) |
| Kingdom | Animalia |
|---|---|
| Phylum | Echinodermata |
| Class | Echinoidea |
Where it lives
Reefs, rocky seabeds, and kelp forests of oceans worldwide, from shallows to the deep sea.
Spines and shell
A sea urchin's body is a rounded shell called a test, built from fused plates and covered in movable spines. The spines provide defense against predators, help with movement, and in some species carry mild venom. Between the spines, tiny tube feet extend to grip surfaces and gather food.
Aristotle's lantern
On its underside, a sea urchin has a complex five-toothed mouth structure called Aristotle's lantern. It uses this to scrape algae and kelp from rocks and to chew tough plant material. The self-sharpening teeth can even slowly grind into rock to carve out a sheltering hollow.
Diet and ecology
Most sea urchins graze on algae and kelp, and in balanced ecosystems they help control plant growth. When their natural predators, such as sea otters, decline, urchin numbers can explode and strip kelp forests into bare 'urchin barrens.' This makes them an important species for the health of coastal habitats.
Slow life and long age
Sea urchins move slowly using tube feet and their spines, creeping across the seabed in search of food. Many live for decades, and the red sea urchin is among the longest-lived animals, with some individuals estimated to be over 100 years old. They reproduce by releasing eggs and sperm into the water.
Frequently asked questions about the Sea Urchin
Are sea urchins dangerous?
Sea urchin spines can puncture skin and break off, causing pain and possible infection, and some species have mild venom. They are not aggressive, but stepping on one is a common painful injury for swimmers.
What do sea urchins eat?
Most sea urchins are grazers that eat algae and kelp, scraping it from rocks with a five-toothed mouth called Aristotle's lantern. Some also eat small bits of animal matter.
How long do sea urchins live?
Many sea urchins live 10 to 30 years, but some are far older. The red sea urchin is among the longest-lived animals, with some individuals estimated at over 100 years.
Are sea urchins related to starfish?
Yes. Sea urchins and starfish are both echinoderms, a group of spiny-skinned marine invertebrates that also includes sand dollars and sea cucumbers.
Why are urchin barrens a problem?
When predators like sea otters decline, sea urchin numbers can surge and overgraze kelp, turning lush kelp forests into bare 'urchin barrens.' This damages habitat for many other marine species.
