Purple sea urchin

purple sea urchin


Yes, this thing shown above is an animal not a plant. This is the purple sea urchin, a medium-sized invertebrate living along the Pacific North American coast. This species is well named, after all the picture above shows that they definitely are purple. The adults have spines that are completely purple, but the younger urchins have spines that are mostly pale green with a little bit of purple. The individual shown above is probably part way in between juvenile and adult as the spines are mostly purple but the bottom parts are the pale green color. All of the spines are connected to an inner body called the “test”. On the bottom side of the test, not visible in the picture above, the sea urchin has tentacle-like appendages covered in suckers. These are known as tube feet. They are used in locomotion, feeding, and breathing. Instead of having gills or lungs to absorb oxygen with, the sea urchin has these tube feet which accomplish the same purpose. Also on the underside of this animal is its mouth. This mouth contains five sharp triangular teeth which move separately side to side to open and close the mouth. This structure is commonly called the Aristotle’s lantern.

I said earlier that these animals move around using their tube feet. Sometimes, though, they get flipped over and cannot use these tentacle to move. When this happens, they use highly coordinated movements of their spines to turn themselves right side up. When getting around normally on their tube feet, purple sea urchins are extremely slow. They move on average just about one to two inches (2.5-5 cm) in one minute!

Radial symmetry

All sea urchins exhibit something called radial symmetry. This is a special way of being symmetrical. Humans, along with most animals, are symmetrical right to left, but no other way. Sea urchins are different. When seen from above they are a circle shape, and any straight line passing through the center of that circle is a line of symmetry. No matter what way the line faces, the sea urchin will be symmetrical along that line. Now they won’t be perfectly symmetrical (some of the spines may not line up perfectly), but they are basically symmetrical.


Purple sea urchins can be up to about four and a quarter inches (11 cm) wide with a height of up to two inches (5 cm). The test of these sea urchins can be three and a quarter inches (8.5 cm) wide, and the spines are usually about half an inch (a little over 1 cm) long.


Because purple sea urchins move extremely slowly, they cannot eat quickly moving food. In fact, the food they eat does not move on its own at all. They are almost completely herbivores, and their main food is algae. They use their tube feet to grab pieces of floating algae out of the water. The five sharp teeth are used to scrape algae off rocks so it can be ingested. The favorite type of algae of the purple sea urchin is by far giant kelp. The majority of the time, sea urchins will just feed on kelp strands that drift by.

When kelp forests are destroyed by storms such as El Niño, drifting kelp is harder to find. These animals will then feast on kelp that is rooted on the rocks. This causes a sudden decrease of kelp in that area, as the sea urchins eat any kelp the storm didn’t destroy. This causes what is called a sea urchin barren. The name barren makes it seem like the area would not have much life, but that is far from true. Although it is barren of kelp, there are hundreds or thousands of invertebrates there including sea urchins and various types of sea stars. Click here to see a video about how sea urchin barrens are made and what life is like for the invertebrates there.

Sometimes these creatures will eat other foods besides algae. This normally takes the form of decaying plant or animal matter.

Habitat and range

The map below shows this animal’s range marked, appropriately, in purple. As you can see, they live only on the Pacific coasts of Canada, the United States, and maybe a little bit along Mexico. They even inhabit a little bit of the coast near Alaska. The reason these creatures only live very close to the shore is that they cannot live in deep waters. They cannot swim and therefore they sink to the bottom of the ocean. If they were too far out from shore, the pressure would crush them since they couldn’t get off the ocean floor. Usually purple sea urchins do not go deeper than about 525 feet (160 m) below the surface.

Purple sea urchin range

As you can probably tell by this animal’s diet, it likes to live in areas near kelp forests. They also like places where the rock is soft enough for them to be able to dig out an area to hide from predators.

Status and threats

There is not a special status for how threatened these invertebrates are. They are thought to be least concern right now, but in the future they may become endangered if they are not helped. They have both natural threats and threats from humans. Some of their natural predators include sea gulls, triggerfish, otters, sea stars, and eels. These predators help keep the kelp forests in good shape by eliminating their main threats in the sea urchins. The purple sea urchin has an interesting defense technique using its spines. They have been observed, when a shadow passes over them, to point their spines in the direction of the shadow. This way, if a predator passes over them trying to attack, suddenly many spines are pointing right at them to deter the predator.

There are some threats by humans as well. The main reason for this is that they are being exported to Japan in large numbers do to their reputation as being a delicacy. The California Department of Fish and Game is currently trying to control the harvesting of these animals so their population is not destroyed. There are also other conservation efforts that are currently being discussed but have not been implemented yet.


Purple sea urchins breed annually between January and March. These animals practice spawning as reproduction. The females release their eggs into the ocean, and the males all fertilize them. For the first six to twelve weeks, the tiny young sea urchins are platonic, floating around near the surface. After this time, they have become large enough to sink to the ocean floor. Here they grow quickly, but this growth rate depends on how much food there is. After about one year, the juveniles are about 1.2 inches (3 cm) wide. At two years of age, the young are ready to participate in spawning. In the wild, these creatures can live up to 75 years.

Don’t forget to scroll down and comment your guess about what the next animal is!

Mystery animal


Photo credits:

  • Purple sea urchin – Taollan82
  • Purple sea urchin range – Craig Pemberton
  • Mystery animal – Public domain
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