FaunaHub

Sea Urchin (class Echinoidea)

Marine invertebrateEchinodermGrazer

A sea urchin (Sphaerechinus granularis) on the seabed off Madeira.

Sea urchin (Sphaerechinus granularis), Madeira.

Image: Diego Delso, CC BY-SA 4.0, via Wikimedia Commons.

Overview

Sea urchins (class Echinoidea) are spiny, rounded marine animals belonging to the echinoderms — the same group as sea stars (starfish) and sea cucumbers. Their body is built on a five-part radial symmetry and encased in a rigid internal shell (the test) covered in movable spines and tiny tube feet.

Despite their simple appearance, sea urchins are important grazers that can shape entire underwater habitats, especially kelp forests and reefs.

Note: there are many sea urchin species with differing spines and habits, and some have spines that can deliver a painful jab or mild venom, so wild urchins should not be handled. Treat figures as general and verify against authoritative marine sources.

Habitat & Range

Sea urchins live on the seabed in oceans worldwide, from rocky shorelines and coral reefs to kelp forests, seagrass beds, and the deep sea. They are found from shallow tide pools to great depths, often tucked into crevices or moving slowly across rock and sand.

Diet

Most sea urchins are grazers, scraping algae and other growth from rocks with a remarkable five-toothed jaw apparatus known as “Aristotle's lantern.” Many also eat detritus and small encrusting organisms. In places where urchin numbers explode, their grazing can strip kelp and create barren areas, showing how influential they are in their habitats.

Behavior

Sea urchins move using hundreds of tiny tube feet, part of the water-driven system shared by all echinoderms, along with their spines. The spines and tube feet handle locomotion, defence, and feeding. Sea urchins are an important food for predators such as sea otters, certain fish, and sea stars, and these predators in turn help keep urchin numbers in balance.

Human Interaction & Conservation

Sea urchins are ecologically important and, in some regions, harvested for their roe as a delicacy. They are central to well-known ecological stories, such as how sea otters protect kelp forests by preying on urchins. The main concerns relate to overfishing of their predators and habitat change, which can let urchin populations boom and damage kelp. Consult authoritative marine sources for specifics.

More photos of the sea urchin

A spiny sea urchin on the deep seafloor.

A spiny sea urchin on the seafloor.

Image: NOAA Office of Ocean Exploration and Research, Public domain, via Wikimedia Commons.

Frequently Asked Questions — Sea Urchin

Is a sea urchin related to a starfish?
Yes. Sea urchins and sea stars (starfish) are both echinoderms, along with sea cucumbers, brittle stars, and others. They share features like a five-part radial body plan and a water-driven system of tube feet, even though they look quite different.
What is Aristotle's lantern?
It's the sea urchin's mouth apparatus — a complex structure of five hard teeth and supporting parts on the underside of the body, used to scrape algae and other food from rocks. The name comes from an old description likening its shape to a lantern.
Are sea urchins dangerous to touch?
They should not be handled. Many sea urchins have sharp spines that can puncture skin and break off, and some species have spines or small pincer-like organs that deliver mild venom. Stepping on or grabbing an urchin can be painful, so they are best observed without touching.
Why do sea urchins matter to kelp forests?
Sea urchins graze on kelp and other algae. When their predators (such as sea otters) decline, urchin numbers can surge and overgraze the kelp, creating bare "urchin barrens." This makes urchins a textbook example of how one animal can shape an entire underwater ecosystem.

Sources and further reading

Authoritative wildlife references used for general educational context. Conservation status should always be verified against current IUCN Red List data. External links open in a new tab.