Mystery seed sprouts underwater

So, apparently a seed was dropped into the aquarium some time ago and it took root in the gravel. What you're seeing here is a sunflower (or some other plant whose seed is included in small animal food) growing underwater. I had no idea this could happen.

What now? Transplant it into soil? Leave it be and see how much it grows? Plant additional seeds of different types next to it? Seems like this calls for experimentation.