why do crabs bubble

Crabs bubble at their mouths primarily because they are expelling carbon dioxide from their gills. When crabs are out of water, they need to keep their gills moist to continue breathing air, and as air passes over the moisture in their gills, bubbles form. This bubbling is a normal part of how crabs oxygenate themselves on land by keeping water circulating around their gills to prevent dehydration. In some cases, when crabs are stressed or threatened, they may also produce more foamy bubbles. This behavior helps protect their gills and maintain proper respiration while they are on land or in environments where they need to keep their gills wet.