Facts About Sea Urchins

16 Interesting Facts About Sea Urchins

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What is an Urchin?

Sea urchins are small, spiny marine animals that are part of the phylum Echinodermata. They have a globular body enclosed in a hard shell covered with moveable spines. These spines help protect the urchin from predators and also assist in movement. There are around 950 known species of sea urchins that inhabit oceans all over the world. Most live in shallow coastal regions, but some species are found in the deep sea floor.

The name “urchin” comes from an old word meaning hedgehog, inspired by their round prickly appearance. While they do not have eyes, sea urchins have excellent senses of touch and smell due to specialized tube feet and pedicellariae. These help them detect vibrations and chemical changes in the water. Sea urchins play important ecological roles as grazers and bioturbators, helping to maintain balance in marine environments. Their numbers need to be kept in check by predators to prevent overgrazing of kelp forests. Humans harvest sea urchins for their edible roe which is considered a delicacy, especially in Japan where it is called uni.

Here are 16 fascinating facts about these unusual creatures:

Interesting Facts About Sea Urchins

Flower Urchin
Flower Urchin by Johan J.Ingles-Le Nobel is licensed under CC BY-ND 2.0 .
  1. Sea urchins have a round body covered in moveable spines which help protect them from predators. Their body shape has inspired their name – “urchin” comes from an old word meaning hedgehog.
  2. The spines can be venomous. While not all sea urchin spines are venomous, some species have small venom glands located at the base of their spines. These can inject toxins as a defense against predators.
  3. They come in a rainbow of colors. Sea urchins can be purple, black, green, red, or brown. Their colors help camouflage them against seabed environments like kelp forests or coral reefs.
  4. Sea urchins don’t have eyes but they have excellent senses of touch and smell thanks to specialized tube feet and pedicellariae. These help the urchin feel vibrations and chemical changes in the water.
  5. They eat with their feet. Sea urchins have a complex jaw structure called “Aristotle’s lantern” which they use to scrape food off surfaces. Their tube feet then pass the food to their mouth.
  6. Their poop is helpful. Sea urchin feces contain ammonia which fertilizes giant kelp and encourages new kelp growth. Some urchins will hold their feces to fertilize the kelp they eat!
  7. Sea urchins can live a long time. Some species have been known to live for more than 100 years (such as the red sea urchin). Their lifespan depends on their habitat and food availability.
  8. They are a key part of marine ecosystems. As herbivores, grazers, and bioturbators, sea urchins help maintain balance and diversity in ocean environments. Too many urchins can devastate kelp forests.
  9. Fishermen harvest them for their edible roe (eggs). Sea urchin roe is considered a delicacy, especially in Japan. Their spines are also sold as curios.
  10. You can find “urchin barrens” which are areas where sea urchins have completely grazed away kelp forests leaving bare rock. These urchin-dominated habitats have low biodiversity.
  11. They exhibit unique behaviors like clustering in shelters, forming feeding fronts, and even trap-building! Some urchins blow water jets to uncover food buried in sediment.
  12. Sea otters are key urchin predators. Their decline led to an explosion in urchin populations along North America’s Pacific coast, devastating kelp forests. Reintroduced sea otters are helping to rebalance ecosystems.
  13. There are around 950 sea urchin species. Most live in shallow coastal waters but some inhabit the deep sea floor where they feed on dead organic matter.
  14. Sea urchins are echinoderms. This means “spiny skin”. They are related to starfish, brittle stars, sea cucumbers, and crinoids. All echinoderms have a five-fold symmetry.
  15. You can find fossil sea urchins dating back around 450 million years to the Ordovician period. Their distinctive round shape with the mouth on the bottom makes urchin fossils easy to identify.
  16. Humans have used them for millennia. Indigenous cultures made tools, decorations, and even poison from sea urchin parts. Ancient Greek and Roman women used sea urchin shells as nail files!


From their unusual appearance to their importance in marine habitats, sea urchins are fascinating creatures. Learning more about their ecology, evolution, and human interactions reveals what remarkable animals they are. Their long history on Earth points to the sea urchins’ ability to adapt and survive threats. But modern challenges like climate change and human activity now jeopardize the future of these captivating echinoderms.

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