I have a lot of different sources that I use to come up with animals for this blog. Sometimes, though, all I need to do is look around my room for inspiration. Today is one of those days, for when I was searching for an animal to blog about, I caught sight of a wonderful poster I have of adorable animals, one for each letter of the alphabet. My favourite animal on the poster is what I’m going to blog about today, the sea urchin. Here’s the picture from the poster (art by Jenn Ski):


Isn’t it adorable? Now, it’s not a very accurate picture of a sea urchin, because they don’t have eyes, but it’s the eyes that make the picture, so I can’t really complain. Sea urchins and sand dollars make up the class Echinoidea, and the variety of species in this class is quite impressive. There are around 950 species, which inhabit all the world’s oceans, and can live anywhere from the intertidal zone to depths of 5000 meters. So there’s not much I can say about sea urchin habitat, except that on the whole they prefer rocky areas while sand dollars like softer ground.

Sea urchins have fivefold symmetry, meaning that they have a centre, and five equal parts branch out from this area. Aside from this cool body pattern, sea urchins have some strange anatomy. The bottom of a sea urchin contains the animal’s mouth, which is collectively called Aristotle’s Lantern (apparently because he thought it was shaped like a lantern). The organ consists of a ring of soft tissue in which strong, bony plates are embedded. These act like teeth, and are strong enough to chew through rock. In many species of urchin, the Lantern is also surrounded by five pairs of gills. The anus of the sea urchin is on the top of the animal, opposite the urchin’s mouth. A fairly simple digestive system connects the two (though not in a straight line, it circles around the urchin’s interior for a while).

A picture of Aristotle’s lantern, some tube feet, and some spines. It kind of reminds me of a sand worm.
Image By Bundesstefan – Own work, CC BY-SA 3.0

The hard shell that covers sea urchins is called the test, and it is composed of a number of overlapping bony plates made of calcium carbonate. The test is covered in a number of different things, of which the spines are the most obvious. Spine lengths can be as large as 30cm, though most urchins have more sensible spine lengths. Sea urchin spines are not usually dangerous, if they are removed properly. In between the spines are little claw-like structures called pedicellariae, which contain venom. I wouldn’t worry about getting stung by a sea urchin though, because you’d have to work your way through all those spines to get to the pedicellariae. Finally, the test also has numerous rows of tube feet on it, which allow the sea urchin to move. Through hydraulic pressure, the urchin moves the feet and thus can amble around the ocean.

Sea urchins mostly eat algae, but will also consume other invertebrates if they get the chance. Sea urchins tend to destroy ecosystems if there is no one to stop them. They eat everything, leaving an area known as urchin barren. Sea otters, which are a major predator of sea urchins, are key in keeping urchin populations under control. So otters are not only super cute – they have a key purpose, too!

Urchins working on creating an urchin barren. Image By Ed Bierman from CA, usa – Purple Sea Urchins, CC BY 2.0,

Sea urchins are pretty cool animals – they have pretty much five of everything and are basically a ball of spines. Still, I wish they had adorable eyes like in my poster, because then they’d also be some of the cutest animals on the planet. Oh well.

Cover image By Frédéric Ducarme – Own work, CC BY-SA 4.0