Apparently there is a eukaryotic phytoplankton called
Braarudosphaera bigelowii which surrounds itself with twelve regular pentagonal CaCO3 scales (each of which in turn is constructed from five isosceles trapeziums - yes, they really are - look carefully). Each pentagon edge measures about 5μm. I am not a marine biologist so I have no idea why it does this but what a stunning piece of micro architecture it is!

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