Sandtrout were creatures native to the planet Arrakis. They were described as resembling large freshwater leeches, amorphous blobs or slugs.
Relationship with Sandworms
They were the premature form of a sandworm and if squeezed, they excrete an extremely potent form of Spice which gave one an incredible energy boost. Also, children in Fremen sietches attached them to sticks and play games with them to entertain themselves.
Sandtrout had the ability to transform a whole terraformed planet into desert by encapsulating large bodies of water allowing the Sandworm to survive, since Sandworms cannot survive in a water-logged environment.
Leto Atreides II
Sandtrouts were also used by Leto Atreides II to turn himself into a giant Sandworm. He managed to live through the metamorphosis because his blood was spice-concentrated and this led the sandtrout into thinking it was water. The first formed a glove over his hand, and the next ones linked to it by their cilia until most of his body was covered.
Later, Sandworms could only sense the sandtrout covering his body and not him, so the Sandworms would draw back to ensure they did not eat their own kind. Gradually, the sandtrout transformed Leto's body until his metabolism and biochemistry were a hybrid of human and Sandworm. However, he also modified the trout in the process, making the resulting sandtrout tougher and more adaptable, allowing them to be settled on other worlds. Thus he ensured the survival of the sandworm species. Leto also imparted some of his own DNA and genetic memories into the sandtrout that emerged from him when he died, and thus into the sandworms that evolved from them.