Thursday, April 26, 2018

Toothy lantern

What can you eat with only five teeth?

Sea urchin mouth, with its five teeth.

If you're a sea urchin, you can even chew into rock with them.

Small green sea urchin, Strongylocentrotus droebachiensis, with its spiky shadow.

The teeth are the bottom (because the mouth faces down) part of the mouth structure called Aristotle's Lantern, supposedly because when Aristotle drew one, he thought it looked like a lamp. They are made, like the test and mollusc shells, of calcium carbonate, CaCO3. We see CaCO3 every morning as we make breakfast; egg shells are made of calcium carbonate. So is chalk. And the antacid we take after too big a meal.

So how does a sea urchin bore a hole in rock with chalk?

A 2016 study by the Pacific Northwest National Laboratory shows how it's done, although they don't mention sea urchins specifically.

Calcium carbonate is one of the most important materials on earth, crystallizing into chalk, shells, and rocks. Animals from mollusks to people use calcium carbonate to make biominerals such as pearls, seashells, exoskeletons, or the tiny organs in ears that maintain balance. These biominerals include proteins or other organic matter in the crystalline matrix to convert the weak calcium carbonate to hard, durable materials.

If a tooth does chip, it repairs itself, filling in the gap with a freshly built CaCO3 and protein structure. No need for dentists!

The sea urchin's main diet is a lot softer than rock, of course; it eats mostly algae. A swarm of urchins can demolish a kelp forest. Chewing rock is construction work, making safe holes to sleep in.


  1. I enjoyed this post... taking me back to my marine biology days many years ago! We have a Strongylocentrotus here in S CA... can't recall the species but different than the one you mention. They used to be super-common all over the tide pools during low tide. The past few times I've taken my granddaughter to the tide pools during good minus tides, there haven't been any! Very odd.

    1. They seem to come and go. They do eat themselves out of house and home occasionally; that may be part of the picture.


If your comment is on a post older than a week, it will be held for moderation. Sorry about that, but spammers seem to love old posts!

Also, I have word verification on, because I found out that not only do I get spam without it, but it gets passed on to anyone commenting in that thread. Not cool!