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The Sea Urchin

Sea urchins are echinoderms in the class Echinoidea. The name Echinoderm came from Greek. “Echinos” means ‘’Spiny” and “Derma” means “skin”. Some marine animals that are classified as echinoderms are sea cucumbers, sea stars or starfish, sand dollars, and sea urchins. Sea Urchins are like the porcupines of the sea and get their name from an Old English word for the spiny hedgehog.

Sea urchins inhabit all oceans and can be found from the shallows to 5000 meters deep, there are around 950 species.

 Echinoderm sea cucumber  Echinoderm sea star or star fish  Echinoderm sand dollar  Echinoderm sea urchins with an octopus hiding between them
 Sea cucumber Sea stars or starfish  Sand dollar Sea urchins

About the Sea Urchin

Sea Urchins mainly feed on algae and kelp and are therefore primarily herbivores, but are also omnivorous scavengers that will feed on animal matter.

Much like starfish, urchins have a certain regenerative ability.  If a spine is damaged or lost, a sea urchin can re-build it.

A few interesting facts about these spiny animals:

sea urchin test

Sea urchin outer skeleton is called a test

  • They have no brains.
  • Instead of having blood in their bodies, they have a water vascular system used for locomotion, food and waste transportation, and respiration.
  • They are grazers that mainly feed on algae.
  • They move by walking, using their many flexible tube feet in a way similar to that of starfish.
  • Hidden beneath their spines they have a hard, outer skeleton called a test.
  • They help keep the ecosystem in balance.
  • They are nocturnal.


'A Sea Urchins’ teeth never stop growing.'

When you look at a sea urchin, the first thing you see is the pointy spines. Their spines are used to protect themselves against predators. Their spines can be all sorts of different thicknesses, lengths, and strengths. Each spine is attached to the urchin by a ball joint, allowing them to be pointed in any direction.

Sea Urchins can grow between 3 cm to 10 cm. They have a radial symmetry body and are divided into five equal parts.  They come in different colors, mostly black, purple, brown, red, light pink, and green.

sea urchin shells or test colors

Sea urchin shells or test colors

Underneath the spines, you get an outer skeleton, called a test. Another word for it is the hard shell. It is made up of ten fused plates that encircle the sea urchin like slices of an orange.

Their bodies are also covered in tiny tube feet which are small tentacles with a suction cup-like end that can be extended far from their spines to move themselves at a very slow pace on the seafloor, climbing obstacles or even roll themselves over when they get flipped over.

 sea urchin mouth with teeth  sea urchin tiny tube feet small tentacles
Sea urchin mouth with teeth  Tiny tube feet with suction cup ends

Surprisingly, they do have a mouth… the mouth would be at the bottom with teeth known as pyramids. A Sea Urchins’ teeth never stop growing. It is connected to muscles that allow the urchin to chew, grind, grasp, tear, and are very strong.

The importance of Sea Urchins

‘The juvenile abalone are protected from octopus and rock lobsters by hiding in the spines of the Sea Urchin.’

sea urchins vulnareble

  • They help to maintain the balance between coral and algae.
  • They are important for coral reefs to recover after disturbances like coral bleaching because it controls the macro-algae.
  • Sea urchins have been with us for a very long time, therefore they have formed part of many research projects.
  • They act as a water filter.
  • They form an important part of some animals’ diet which would include sea otters, birds, crabs as well as rock lobsters.
  • Sea Urchins provide nurseries for abalone.  The juvenile abalone is protected from octopus and rock lobsters by hiding in the spines of the Sea Urchin. 

What is threatening them?

'They may become endangered in the future.'

Did you know, hurricanes and the rise in temperature of the water wiped out a great number of Sea Urchins. Sea Urchins are very vulnerable to any change, and with global warming, which is changing the temperature of the oceans and also the increased amounts of tropical storms, they may become endangered in the future.

sea urchin meat

Sea urchin meat

As you may already know, they are food for humans as well. This can potentially lead to alarming drops in numbers of the species because around 50 000 tons of sea urchins are captured each year for food! Now that is a lot.

Sea urchins are a very important species in the ocean. If you, like me, never realized how big a role they play in the ecosystem, now we know.


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monet bruwer

by Monet Bruwer

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