Physical Description

  • Flat bottom, domed top of body. Color ranges from red to dark burgundy.
  • Exoskeleton made of ten fused plates covered in red spines.
  • Spines connected to body by ball joints so can move in any direction.
  • Exoskeleton has holes for long, flexible tube feet poking out among the spines.
  • Mouth is at center bottom of animal. Called Aristotle’s lantern (he was the first to describe it). It is made up of five tooth-like plates that are moved by 60 muscles.
  • Length can reach 5 inches in diameter and spines can reach 2-3 inches in length in California. In British Columbia, can reach 7 inch diameter with 3 inch long spines.


  • Alaska to Baja California, Mexico.


  • Live just below the low tide line to about 300 feet deep.
  • Found in rocky, subtidal habitats.
  • They avoid sand or muddy sediment.


  • Female urchins release orange eggs and males release white sperm into the water column.
  • When the eggs hatch into larvae, they are bilaterally symmetric (meaning a single line divides the larvae into two mirror images).
  • As the larvae grow and settle to the bottom, they become pentaradially symmetric, meaning their body parts are symmetric around a center axis in sets of five.


  • Seaweed, kelp, especially giant kelp.
  • Use Aristotle’s lantern to scrape algae off rocks.


  • Sea otters, octopus, sunflower stars, wolf eels, crabs, birds.

Interesting Facts

  • Red sea urchins are the largest of all sea urchins.
  • Sea urchins are echinoderms, related to sea stars, sea cucumbers, and sand dollars. Like all echinoderms, this means they are pentaradially symmetric, meaning their body parts are symmetric around a center axis in sets of five.
  • Like all echinoderms, sea urchins do not have a brain or heart!  
  • California red sea urchins can live to be about 50 years old, while those in British Columbia can be over 100 years old. There are some in Canada over 7.5 inches long that are estimated to be 200 years old! 

Sources: Aquarium of the Pacific

Photo: Kevin Lee

The Hamdoun Lab at Scripps Institution of Oceanography uses sea urchins as their model organism to study cell and developmental biology and environmental toxicology in developing organisms. 

Researchers at the Jacobs School of Engineering and Scripps Institution of Oceanography banded together to develop a bio-inspired device based on the Aristotle’s lantern that was incorporated into a rover able to collect sediment in space!