If you look at the spines on the worm, and the spines on the creature you can see from the surface, they are two different creatures. There is grimoire to suggest that there are leviathans in the methane oceans of titan, and the hydrocarbon pits of europa (like, the books of sorrow leviathans. from the fundament. that existed to stop any race from making a pack with the darkness). I think the creature you see from the surface is one of these. I found an interesting theory about the other that I think is plausible. The one you see underwater is Oryx's worm. Think about it, Oryx is one of the original 3 of the hive as we know them. It would make sense that his worm is so massive, because he has done so much to feed it, including annihilating the ecumene and the taishibethi, as well as killing one of his [i]gods[/i], akka.