Castalians appeared in All Great Neptunes Ocean and were an enthustiastic nation that wanted to join the New Commonwealth. The planet is a watery world with 20 billion inhabitents, just under three times the population of the Earth in 2007. There are two types of Castalians, air breathers and water breathers. The water breathers had a silvery skin round their face with gills to allow them to breathe. The air-breathers have no gills or any differing colours on their skin.