The Uniqueness of Urchins: Spines

Sea urchins–purple urchins especially–have a bit of a bad reputation on the West Coast of the United States right now. They are currently forming and maintaining urchin barrens, which are kelp forests’ “evil twin” or alternative stable state. But that’s a complicated story for another time!

Even though urchins aren’t exactly universally loved, I am fond of them. It probably has something to do with my general love for marine invertebrates. Regardless, there’s lots to learn about urchins! Let me show you with the help of some specimens, starting with their spines.

This is one of my specimens–a dried purple sea urchin with its spines still intact. As you can see, several of its spines have broken off over time. They can move each spine individually to defend against attacks. If you “tickle” an urchin on its exoskeleton, its spines will respond in a matter of seconds.

For reference, here’s a smaller, living purple sea urchin:

The color has faded in the dead specimen–living urchins are a brighter purple.

Typically, dead urchins don’t retain their spines for long. The spines tend to fall off, along with the skin, revealing what’s known as the test. The test is the urchin’s main, circular exoskeleton.

A purple urchin test

The spines have sockets that connect to the ball-like bumps protruding from the test. This enables them to rotate each spine. Unlike arthropods, urchins grow their test and spines continuously–no need to molt!

Urchins exhibit pentaradial symmetry, which means that their bodies are radially symmetrical in multiples of five. Sea stars and other echinoderms are also pentaradial. This test shows how urchins have alternating stripes of spine-bump sizes.

The little holes in the test are for the tube feet to extend from. You’ll hear more about tube feet in a future blog post!

It’s important to mention that some urchins have venomous spines–particularly tropical species. Neither purple urchins nor the closely related red urchins have venom in their spines, so they are completely safe to touch. The only threat these temperate urchins pose to humans is the threat of getting spines stuck in your skin.

Urchins can drop spines when living for a variety of reasons, including disease, duress, or trying to dispose of a fouling organism. It’s possible for urchins to recover from dropping a few or many spines if given enough recovery time, especially if it has enough nutrients in the water and in its diet.

Those are the basics about urchin spines! Stay tuned for more urchin facts and pictures!

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