The Gretiu are a species of aquatic methane-breathing animals native to Bahalo, the homeworld of the Unggoy. They are a rare sight, but are found on the methane oceans of Balaho and are mostly avoided by the Grunts as they are very dangerous. The Gretiu can take down animals much bigger then themselves such as the Huturu. They have 2 forms, the larvae, and the mature state.also, Even though they are nicknamed Covenant sharks, they have no physical or evolutionary connections to sharks and are no big interest to the Covenant. The Gretiu can grow to be over 50 meters long and are one of the largest aquatic predators on Bahalo. The Gretiu also eat colonies of Pweda.