Sanibel Sea School Blog

Sea Urchins

March 29, 2021

By Emily White

Have you ever scooped up a spiny little sea urchin while snorkeling on Sanibel Island? There’s a lot to know about these “porcupines of the sea”, so let’s dive right in!

Sea urchins are part of the phylum Echinodermata, so they are related to animals like sea stars, sand dollars, and sea cucumbers. They’re covered in sharp spines that they use for protection, movement, and trapping food. Their outer skeleton is called the test and is made up of ten plates that are fused together, and every other plated section contains holes that the sea urchin can extend its tentacle-like tube feet through.

Interestingly, sea urchins don’t have eyes, but the ends of their hundreds of tube feet contain photoreceptors that allow them to get a blurry picture of the world around them- one study showed that if an object appears over a sea urchin, it will become alert and point its many spines towards the perceived threat. This way, they can keep an eye out (so to speak) and fight back against predators such as crabs, large fish, birds, and sea otters.

Sea urchins can typically be found along the ocean floor in both shallow and deep water, but they’re also vitally important to coral reefs. With their five hard, sharp teeth located right in the center of their bellies, sea urchins have the important job of eating up the algae that accumulates on coral reefs to keep the reefs from being smothered. Sea urchins also love to munch on decomposing matter like dead fish, mussels, and sponges- they’re like the vacuum cleaners of the ocean!

Next time you’re out snorkeling, keep an eye out and you might just see a sea urchin crawling across the sea floor!

A West Indian Sea Egg on the reef in Andros Island, BahamasA West Indian Sea Egg on the reef in Andros Island, Bahamas

A West Indian Sea Egg on the reef in Andros Island, Bahamas

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