Alright, I just got to the pre-boss save chamber in the body, (I assume that's what it is,) and I'm taking a break to go back and explore every last thing before I try beating the game. I don't like ruining things for myself, but I have to ask about something:
I was down in the dark and murky depths somewhere, and I sang open a plant, and instead of an item popping, it instead had an image that looked a bit like the ghostly fish floating at the top of its stalk. There were a couple of them swimming around there, but I couldn't seem to get any reaction to occur with anything. I tried notes, I couldn't bubble the fish, I couldn't pick them up, I tried fish form, I tried ghost form, I tried fish from
then ghost form...
I couldn't get anything to happen, and ended up killing both of the ghost fish there, so I just quit the game to save that bit for later, and haven't gone back since. Any hints or outright spoilers? I'm at a loss.
edit: in case it helps, here's a screenshot:

Hmm. Perhaps the fish it resembles is spawned by opening it. I'm not positive I've seen them elsewhere.