air bladder Definition

a sac containing gas and especially air : such as

a saclike organ present in most bony fishes which contains gas and especially oxygen and nitrogen and that serves chiefly to control buoyancy and in some primitive fish (such as the lungfish functions as an accessory respiratory organ —called also swim bladder

a gas- or air-filled vesicle in certain algae and aquatic plants (such as rockweed) that helps keep photosynthetic blades and fronds buoyant —called also pneumatocyst