Seaweed grows in the oceans of habitable planets in Spore. In the later stages of the Cell Stage, seaweed is the food for herbivores and omnivores, preceded by algae; in the Tribal Stage, herbivorous tribe members will gather seaweed, rather than fish, when ordered to fish.
Seaweed can be a good, plentiful source of food for herbivores and omnivores in Cell Stage, but other herbivores may hang around it; these can both deprive the player of access to seaweed by consuming it first, and also attract predators.
It does not appear in the Creature Stage, but can be seen in the Early Creature editor, while in the Civilization and Space Stages, it can appear when herbivore tribes are seen fishing. Due to a bug, it can also appear in the player's city on a platter. It does not feature at all in Spore Galactic Adventures, even as an object that can be placed in adventures, although the expansion pack does introduce new plants that resemble aquatic species (albeit mainly coral and sponges).