What Are Sea Urchins

By D.C. Demetre •  Updated: 09/19/22 •  8 min read

The sea urchin is a spine-covered sea animal that lives in oceans throughout the world. It is closely related to sea cucumbers and starfish.

The majority of sea urchins are spherical in shape, with five-way symmetry. At first glance, one may appear stationary and inert, but soon reveals itself as a moving, living animal, with its waving spines and slow wandering walk.

A sea urchin will move more quickly when it senses one of its predators — which include sea otters, crabs, and humans – nearby. Yes, sea urchins are harvested by fishermen because the gonads of sea urchins, male and female, are eaten in various parts of the world, particularly in Japan, as a kind of sushi. The eggs are also eaten.

Where Are Sea Urchins Found


where do sea urchins live

Can you spot the sea urchin?

Sea urchin habitats include kelp forests, on sandy and rocky sea floors, in pits and crevices in rocks, on coral reefs and in rock pools in intertidal zones. Sea urchins live in oceans throughout all climate regions, from warm tropical seas to the frigid waters around the North and South poles.

The purple sea urchin (scientific name Strongylocentrotus purpuratus) is familiar to many in the United States, as it lives in the waters along the Pacific coast from British Columbia down to Mexico. There is another purple species, the Atlantic purple sea urchin (Arbacia punctulata) which inhabits shallow waters of the Atlantic Ocean coastline from Massachusetts to Cuba.

A third purple species is the helmet urchin (Colobocentrotus atratus). It is found in intertidal zones of the indo-pacific region, such as around the islands of Hawaii. These purple urchins are dome-shaped and have large flat spines.

Sea urchin populations are denser in areas that are barren of vegetation. In kelp forests, urchins hide from predators in rocky crevices and feed on falling algae debris, but when predator numbers decline, the urchins emerge and feed directly on growing kelp leaves.

Are Sea Urchins Animals?

Sea urchins are not fish, nor are they a plant. They are a type of animal called invertebrates. Sea urchins, along with all other echinoderms are invertebrate deuterostomes, to be more specific.

Scientists have traced the ancestors of sea urchins back to the late Ordovician period, 450 million years ago. These creatures’ stiff, spine covered skeletons have left behind an astounding amount of fossils.

sea urchin phylogeny

Phylogenetic relationships among major clades of Echinoidea. Credit: Nicolás Mongiardino Koch, et al. eLife CC-BY

Sea Urchin Characteristics

The shell, or test, of a sea urchin, is made up of overlapping calcium plates that are fused together. There are about 20 plates in all, arranged in a five-sided symmetrical pattern.

A small plate with holes, called the madreporite, is located at the of the shell. The holes in this plate let seawater flow in and pass through a system of tubes known as a water vascular system, which pumps water down into the tube feet and moves them.

The shell is covered with a layer of skin and muscle. The spines are attached to this layer, and its muscles move the spines so that they can point in different directions. Depending on the species, the spines can be long and sharply pointed, or shorter and stubbier. The spines on heart urchins (sea urchins in the order Spatangoidea) are thin and hair-like, giving them a furry appearance.

Sea Urchin Spines

A sea urchin can regrow a spine in a few days if it has been broken off or damaged. A study done in 2004 examined how urchin spines regrow and found something unusual.

Each spine is made of a single crystal structure from its base to the tip. Many crystals, like sugar and salt, grow from dissolved molecular components. But sea urchins use a different method.

The material of the spines is first compiled in non-crystalline form, which is called “amorphous calcium carbonate” (ACC). Packets of ACC are launched out of cells around the base of the broken spine up to the growing tip. Within hours of landing at their destination, the amorphous material, which is made of tightly packed, but jumbled up molecules, changes to a calcite crystal in which the molecules line up evenly in crystal lattice formations.

The process is basic yet difficult to study since the amorphous calcium carbonate phase does not last long, so Weizmann researchers had to develop new methods to capture images of the process.

The results show microscopic needles that initially grow straight out from the old spine’s stump, then branch out to establish a lacy structure that is hard but light. The crystalline structure of the old spine provides the template for the alignment of the molecules in the crystal, and thus controls the intricate, yet precise, growth pattern.

How Big Do Sea Urchins Get

Most sea urchins range from 1 to 4 inches in diameter, when measuring the shell. When you add the spines, they can appear much larger. The black longspine urchin (Diadema setosum), for instance, has spines that grow up to ten inches long. You would need a tank of at least 50 gallons to keep one in an aquarium.

The red sea urchin (scientific name Strongylocentrotus franciscanus), also known as the giant red sea urchin, is one of the biggest species, and grows up to 7 inches in diameter with the spines as long as 3”.

Do Sea Urchins Have Eyes?

Sea urchins don’t have eyes, but they can see with their tube feet. Currently, they are the only animals that are known to see without having eyes. They see with the use of light-sensitive cells in their tube feet, which resemble tentacles and are spread all over the body, like the spines.

You could say that the entire sea urchin is one single compound eye,”

said John Kirwan, who conducted a study on sea urchin vision as a part of his doctoral thesis, along with colleagues at Lund University.

Kirwan’s calculations showed that of the 360 degrees surrounding the sea urchin, an object must take up between 30 and 70 degrees for the sea urchin to see it. Humans only need an object to take up 0.02 degrees in order to detect it, so sea urchin eyesight is poor in comparison with human eyesight.

What Do Sea Urchins Do To Survive And Reproduce

To avoid predators, sea urchins typically forage at night and move slowly They bite and scrape their food with their teeth, catching food in their tube feet. They also use their tube feet for breathing, movement, catching prey, and attaching to the surfaces they live on.

To defend themselves against predators, some sea urchins have venom in their spines. Most stings from a sea urchin are not poisonous or dangerous to humans, although they may cause an infection, itching, swelling and discomfort.

helmet urchin

A helmet urchin (Colobocentrotus atratus). Credit: Ken-ichi Ueda CC-BY

The sexes of sea urchins are distinct. Millions of eggs are released into the water by females, where they combine with male sperm to become larvae. Animals that change form before becoming adults are known as larvae in the early stages. Sea urchin larvae float freely before sinking to the bottom and developing into juvenile creatures with an adult-like body structure.

Certain varieties of sea urchins have eggs that hatch inside of the females. There seem to be no freely floating larvae of these species.

What Phylum Are Sea Urchins In

Sea urchins belong to the phylum Echinodermata. Echinoderms are bilaterians, which means that they descended from mirror-symmetrical organisms. Adult echinoderms have a calcareous endoskeleton made of ossicles joined by a web of collagen fibres, as well as a water circulatory system with external tube feet.

Echinoderma are all marine animals, although they can be found in a variety of setting, from shallow intertidal zones to the deepest depths of the ocean. Many echinoderms possess exceptional regeneration abilities. Sea urchins, for example, are continuously regenerating damaged spines.


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  2. Braccini, S. 2001. Arbacia punctulata. Animal Diversity Web. Accessed September 17, 2022
  3. CJ Kazilek. (2015, August 22). Urchin Anatomy. ASU – Ask A Biologist. Retrieved September 16, 2022
  4. John D. Kirwan, Michael J. Bok, Jochen Smolka, James J. Foster, José Carlos Hernández, Dan-Eric Nilsson. The sea urchin Diadema africanumuses low resolution vision to find shelter and deter enemies. The Journal of Experimental Biology, 2018; jeb.176271
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  8. urchin (n.). Online Etymology Dictionary. Retrieved 17 Sept 2022
  9. Weizmann Institute of Science, How the Sea Urchin Grows New Spines. Weizmann Wonder Wander. Retrieved September 16, 2022

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