There are other photobionts with orifices, but this is the first where we've been able to see inside the orifice. I expected to see complex tissues, but it looks more like a bladder. It's open to the atmosphere- it seems like any water in there would evaporate quickly. Maybe it stores groundwater. Most of the tissue isn't pigmented - I don't know if they photosynthesize. --Jane Van Susteren, Exobiology Lead
Hydropods are found on land surrounding where the rover is disassembled by unknown creatures. They are mentioned during a phone call that you listen in on, with the implication that the hydropods have something to do with the Caretakers or other beings. Finally, in the glimpse of the underwater ecosystem that you get, a stalk can be seen with dozens of hydropods attached like grapes on a vine, sealed shut instead of wide open as they are seen when on land. One possibility is that the hydropods are actually capsules wherein the aquatic motobionts grow (the name "hydropod" even roughly means "aquatic capsule") before emerging, and the empty husks wash up on the shores where you initially see them and classify them as photobionts.