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.