Tredgeons are much larger, prettier versions of sand crabs, first encountered in the book Neverseen. They have an opalescent shell, iridescent claws, and numerous legs and antennae. They also find the smell of congealed selkie skin irresistible. Mr. Forkle once mentioned that King Enki's crown was made of a piece of tredgeon shell.

In Neverseen, on the way to the prison Gethen was held in at the time, Sophie, Keefe, and Mr. Forkle encountered a tredgeon used to open the pathway to the prison by tunneling out of the sand.

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