Dune Universe by Frank Herbert
Seaworms are a genetically modified form of the sandworm. Tylwyth Waff created them, and released them into the ecosystem of Buzzell.
Waff modified some sandtrout to thrive in a watery environment when matured into worm form. The experiment succeeded beyond all expectation, and soon the soostone harvesting came to an almost-complete halt, as the seaworms were viciously territorial and killed many Phibians engaged in the harvesting.
When captured and dissected, an internal organ found in the seaworm contained a particularly potent form of spice, later referred to as ultraspice.