Reef Chemistry Question of the Day #114 Sea Urchins

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #114

Sea urchins have five continuously growing teeth with a complex mineral structure. Their teeth are amazingly rugged, and sea urchins have been known to eat through ferroconcrete piers, and lead-ensheathed deep-water telephone cables. What is the highest concentration component of sea urchin teeth?


A. Calcium sulfate
B. Aragonite
C. High magnesian calcite
D. Organic polyimide

Good luck!























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[evolved]

C

And they're self-sharpening!

 

[bpcass]

Calcium sulfate is gypsum, I use it to harden my water when making certain beers. Very soft mineral used in making plaster of paris so that's out.
I'm not entirely sure what organic polyimide is but from the name it is an organic polymer. Another organic polymer is chitin, the exoskeleton on a bug or crustacean, and I'd expect it to have similar properties. I wonder if it is what makes up the urchin spines. I'd expect it to be tough but not hard. It could, however, be used as the matrix of the teeth with a much harder material mixed in, like a diamond drill bit. Is high magnesian calcite a form of dolomite? If so it is basically calcium carbonate with a large amount of magnesium included. This would make it harder than aragonite and also less soluble. Of the choices I'd say that high magnesian calcite is the hardest material but likely brittle. If I were designing the echinoderm tooth I'd use a tough matrix with hard inclusions so I'll guess the highest concentration component is D the organic polyimide with perhaps the high magnesian calcite as an inclusion.

 

[beaslbob]

b aragonite sounds familiar. Could make the urchin a sand blaster.

 

[fuzzone99]

"C" ???

 

[Randy Holmes-Farley]

And the answer is...C. High magnesian calcite

I give lots of interesting chemical info about sea urchins here:

http://reefkeeping.com/issues/2004-08/rhf/index.htm

from it:

Sea Urchin Teeth


Like sea urchin spines, sea urchin teeth have also received significant study. Sea urchins have five continuously growing teeth (Figure 10) with a complex mineral structure. Their teeth are amazingly rugged, and sea urchins have been known to eat through ferroconcrete piers, as well as lead ensheathed deep-water telephone cables. It turns out that their teeth are formed, like the spines, of high magnesian calcite (4.5 - 13 mole percent magnesium). For some urchins, the magnesium content varies considerably by location within the tooth, with the most magnesium concentrated at the tip, just the opposite of the magnesium distribution in spines.37,38 The tip of an urchin tooth undergoes extensive erosion, both mechanical and chemical. The mechanical erosion from constant chewing on tough foods may explain why an urchin would want the tip of the tooth to be as strong as possible, driving the development of higher magnesium calcite in tooth tips.

The chemical aspects of tooth wear may also be important for some species. Strongylocentrotus droebachiensis, for example, eats certain kelp species in the genus Desmarestia that pack sulfuric acid into vacuoles (small reservoirs) in some of their cells. After grazing on such algae for a while the urchin teeth begin to dissolve and develop a gap between them. This gap allows the algal blades to more frequently slip out of their grip. I have not seen any analysis of the magnesium content of the teeth of these urchins, but high magnesium calcites are generally more soluble than lower magnesium calcites (or pure calcite), so one might predict that these urchins may have less magnesium in their teeth than other species.


In addition to the magnesian calcite, some parts of the tooth are amorphous plates and rods of calcium carbonate, although crystalline calcite predominates. In many cases, the calcite is in the form of long crystalline fibers that are thin at the base (1 micron) and thicker at the end (20 microns).


These calcite fibers have a thin organic coating (probably the proteins that control deposition and growth) that is, in turn, surrounded by polycrystalline calcite containing up to 35 mole percent magnesium. This structure has been referred to as a "gradient fiber-reinforced ceramic matrix composite, whose microhardness and toughness decrease gradually" from the tip to the base. Additionally, there are plates of calcite with an interior of amorphous calcium carbonate in the tooth that serve to prevent the propagation of tooth cracks.


Overall, the teeth of sea urchins have a complex array of techniques for making them as hard and durable as possible, and they have served to inspire scientists and engineers trying to make similarly durable structures.