From Creatures Wikia
Opal Sponge (Porifera opalite)
The larger and more colorful of the two variants of sponge provided with in Creatures 3, the Opal Sponge is, like most of the relatively scarce plant life in the Marine Terrarium, a major food source for fish. It will only grow in salt water. It releases large, pink seeds when it has grown to full size.
At the beginning of the game the Opal Sponge is seeded in the right half of the Marine Terrarium.