Neat - For neat stuff you found

I wonder how critters that live at the bottom of the ocean do it.

My guess is that for creatures that live on a certain level their bodies are adapted to that level. Food would be eaten at that pressure, converted to poop at that pressure, and expelled at that pressure. The status quo.

But whales are a bit different because they have to come up for air. At the extreme end Cuvier's beaked whales have been known to go almost 3km down when the pressure is almost 300 times that at the surface. With a pressure gradient like that any poop that carry up with them will want to expand, so I guess they would need to either have enough flexibility to allow for expansion, develop special muscles/structures to actively maintain constant pressure, or just let it out.