Justin Ries holds two tropical pencil urchins (Eucidaris tribuloides) reared under different levels of atmospheric carbon dioxide (CO₂). The urchin on the right, grown in seawater under today's air conditions (400 ppm), is healthy and has normal spines. The urchin on the left, grown under very high CO₂ conditions (2,850 ppm), is substantially damaged by the more acidic conditions. Most of its spines have fallen off, and the remaining ones show a pitted texture under the scanning electron microscope. Weighing the urchins confirmed that they were actually dissolving in the more acidic high CO₂ conditions.

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