Few invertebrates have returned to the sea after a land ancestry, but many have invaded freshwater.
Invertebrates have complex life histories with both juvenile and adult stages.
The juveniles either breathing water or air but the adults are usually air breathers even though many live in water. Water beetle larvae have gills but the adults acquire an air bubble at the water surface, which they hold under their wing covers when they dive. Through their spiracles they absorb oxygen from the bubble, and to maintain gaseous equilibrium, more oxygen diffuses into the bubble from the water, which is itself in equilibrium with the atmosphere. The bubble may give oxygen for some time but eventually collapses. This is because as oxygen is absorbed by the animal, the percentage of nitrogen in the bubble temporarily increases.