Tidal Moon

A day and a half before the tide is due to ebb away from Aquia, Carol Kent discovers that her father's notes on his red moss distillation process are missing.

Amherst remembers a rumour he heard that red moss has been discovered on Io; Scaler, he realizes, must be working for Ionian Products, a company seeking to break Cree, Inc.'s crephine monopoly.

As they return to Aquia, Carol Kent tells Amherst about her father's process, which involves exposing red moss to ammonia and treating it with an extract from the eggs of Ganymedian gall-ants.

Due to Jupiter's tidal pull, every spot on Ganymede's surface is inundated with water every three months except a small area of the south pole where the human settlement of Hydropole is located.

The Ganymedian natives, the Nympus, grow a mosslike plant called cree which is ordinarily red, but which turns blue when exposed to the ammonia in Ganymede's atmosphere.

The blue moss is collected by human traders in the employ of Cree, Inc. who travel among the native villages on an aquatic riding animal called a hipp (short for Hippocampus catamiti); it is then shipped to Earth to produce crephine, a combined anaesthetic and medicine.