The sandtrout ... was introduced here from some other place. This was a wet planet then. They proliferated beyond the capability of existing ecosystems to deal with them. Sandtrout encysted the available free water, made this a desert planet ... and they did it to survive. In a planet sufficiently dry, they could move to their sandworm phase. — Leto Atreides II, Children of Dune
Sandtrout are the larval form of sandworms, and develop from much smaller 'sand plankton'. They come from the Dune series.
Their leathery remains previously having "been ascribed to a fictional "sandtrout" in Fremen folk stories," Imperial Planetologist Pardot Kynes had discovered the 'Little Makers' (as they are also known) during his ecological investigations of the planet, deducing their existence before he actually found one.Kynes determines that these "sandtrout" block off water "into fertile pockets" and Alia Atreides notes in Children of Dune that the "sandtrout, when linked edge to edge against the planet's bedrock, formed living cisterns." The Fremen themselves protect their water supplies with "predator fish" who attack invading sandtrout. Sandtrout can be lured by small traces of water, and Fremen children catch and play with them; smoothing one over the human hand forms a "living glove" until the creature is repelled by something in the "blood's water" and falls off. This is despite their need to process water.
The sandtrout are described as "flat and leathery" in Children of Dune, with Leto Atreides II noting that they are "roughly diamond-shaped" with "no head, no extremities, no eyes" and "coarse interlacings of extruded cilia." Squeezing the sandtrout yields a "sweet green syrup", probably the result of the sandtrout processing water and food. When water is flooded into the sandtrout's excretions, a pre-spice mass is formed; at this "stage of fungusoid wild growth," gasses are produced which result in "a characteristic 'blow,' exchanging the material from deep underground for the matter on the surface above it." After exposure to sun and air, this mass becomes melange.
The sandtrout die "by the millions in each spice blow," and may be killed by even a "five-degree change in temperature."He notes that "the few survivors entered a semidormant cyst-hibernation to emerge in six years as small (about three meters long) sandworms." A small number of these then emerge into maturity as giant sandworms, to whom water is poisonous.