I found this larva on the sea lettuce.
Looks a bit worried, don't you think?
Something wasn't right; it looks like a maggot, not a marine organism. It didn't belong in the aquarium. It seemed a little lost, too; not swimming, but not holding on to the seaweed very carefully, either. I fished it out and looked it over.
As far as I can tell, it's some kind of fly larva, possibly house fly. Those "eyes" are at the tail end; they're spiracles, breathing holes. The front end is pointed, with dark mouth parts (throat structure and mouth hooks) visible through the flesh. It's about 1 cm. long, stretched out.
On the move.
Compare it to the larva of the Indian Meal moth I captured a couple of weeks ago:
Same general shape and size, but the fly larva has those tweezer-like mouth parts. I checked the moth larva again; it's gone into the pupal stage in my little bottle.
Moth pupa, under webbing.
Even in this stage, it's bigger than the maggot.
I gave my rescued critter a choice; salt water, fresh water (in a paper towel), damp paper, or dry land. It chose the damp paper towel; it's wandering about on it now, nibbling here and there.
I don't know how it got in the aquarium, through the lid. It eventually would have drowned and been eaten by the hermit crabs. As it is, if it pupates and hatches out as a house fly, it will feed a spider, instead. I'm not very good with house flies.