To become a Gatherer, a Landwalker must pass two tests, depicted on the walls of the Landwalker's Bubble. The first test is that of being able to breather underwater, accomplished by the placing of a breathing polyp in the throat. The second test is the ability to understand the strange tongue of the Maerum, accomplished by the mixing of a black pearl, Tanyenn, and some blood, which creates a golden pearl that has to be swallowed by the Landwalker.
This may also serve as a simple but rather brutal induction test: if the Landwalker is not intelligent enough to understand the murals, nor hardy enough to cut themselves and create a golden pearl, they will drown when trying to leave the Bubble, or - without the Maerum's instruction - not learn of the dangers of the undersea creatures and be eaten if they leave the safety of the city. This ensures that anyone who passes both tests is likely to be intelligent and canny enough to make a good Gatherer, and the Maerum do not waste their time with unworthy Landwalkers.
The training of a Gatherer takes "six cold oceans", or six years.
April was condemned to become a Gatherer, until she revealed herself as the Waterstiller.