Pteropods swim by flapping a pair of wing-like parapodia, and the whole animal is only millimetres across, with a shell thin enough to see through. That shell is aragonite, which dissolves more readily than the calcite used by many other shell-builders.
Dissolved carbon dioxide lowers seawater pH and reduces carbonate ion concentration, and cold polar water holds more carbon dioxide, so high-latitude seas cross the aragonite saturation threshold first. Surveys along the North American Pacific coast and in polar waters have recovered live pteropods with pitted, etched and thinned shells consistent with dissolution, and laboratory exposures reproduce the damage.
Two qualifications are important. Shell dissolution is not the same as population collapse; pteropods can partly repair shells and the demographic consequences are still being measured. And other stressors, including temperature and food supply, act at the same time. What makes the animal significant is its position in the food web: Limacina is a staple prey of juvenile salmon, seabirds and baleen whales, so its condition is watched as an early indicator rather than as an isolated curiosity.