Crabs blow bubbles because the crab inhales too much air, and the contact area between the gills and the air is large. The water contained in the gills is spit out together with the air, forming countless bubbles. In addition, crabs are on land. You still use your gills to breathe, and you can judge the quality of the crab based on how much bubbles it blows. Generally, crabs that blow a lot of bubbles are more energetic and grow stronger.