Some of the most beautiful creatures in the sea are so small they are rarely seen and easily escape our attention.  “Sea butterflies,” called pteropods by scientists, are lentil-sized marine snails that flit through oceans around the world. While a land snail uses its muscular foot to slide along a surface, pteropods beat their foot like tiny wings to hoist their translucent body upward in the water. Delicate shells house their soft bodies, and shell spirals and spikes have inspired artists and scientists alike.

In the last decade, pteropod shells have also revealed signs of a struggle: scientists have discovered pitted, rough shells in some pteropod populations. Ocean acidification, caused by burning fossil fuels, is reducing the mineral that pteropods use to build their shells, rendering them weak and fragile and causing slower shell growth.