The Sea Animal So Tough It Eats Rocks For Breakfast

Hiding in a hole is a great idea if you're a reef creature, because someone is always out there looking for a snack. Because of this, one of the best places to find interesting animals is in a hole or crevice. If you peer into these safe hidey-holes, you'll have the chance to see many cool eels, crabs, banded shrimp, octopus, and snails.

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For a long time, I thought these cool little habitats were a product of the natural rock formation. After all, Hawaiian rock is mainly volcanic, and volcanic rock is often full of air pockets and lava tubes. As it turns out, these handy little rock shelters were not created by air bubbles in cooling lava, but by something equally amazing. The holes and tunnels that so many reef creatures call home were literally chewed out of solid stone by an animal smaller than my fist; a sea urchin.

There are several types of rock-boring urchins around the world. These species of urchins literally use their teeth to chew holes in the rock. You read me right. And you thought the only scary thing about an urchin was its spines.

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Sea urchin teeth are made up of tiny mineral crystals, which are cemented together with calcium. The teeth have weak breaking points built right into them, so that when the tooth starts to get dull, it breaks and a new, sharp edge is exposed. This means that the teeth are always sharp, which is very handy if you have rocks to chew. The urchin's teeth are constantly growing to replace these broken off bits.

Urchin chewing can have a significant effect on the structure of the reef. For example some studies estimate that as much as 79-85% of all erosion caused by reef animals is caused by urchins. Larger urchins cause more bioerosion, with some larger urchins chewing up as much as 7 grams of rock per day. That's about as much as three pennies, which didn't sound like much until I calculated that if I chewed up the same proportion of rock per pound as an urchin, I would be chomping on about five pounds of rock per day. Yum!

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I should mention that rock-boring urchins don't actually feed on rock - they get nutrients from algae. Rock-chewing is more like building a house than eating a meal. But it's still building a house out of solid rock with their teeth, which is pretty darn awesome.

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References

Bak, R. P. M. (1994). Sea urchin bioerosion on coral reefs: place in the carbonate budget and relevant variables. Coral Reefs, 13(2), 99-103.

Killian, C. E., Metzler, R. A., Gong, Y., Churchill, T. H., Olson, I. C., Trubetskoy, V., & Gilbert, P. U. (2011). Self‐Sharpening Mechanism of the Sea Urchin Tooth. Advanced Functional Materials, 21(4), 682-690.

Wang, R. Z., Addadi, L., & Weiner, S. (1997). Design strategies of sea urchin teeth: structure, composition and micromechanical relations to function. Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences, 352(1352), 469-480.