I don't know monocots very well and this ID is a wild guess.
Chytrid on diatom
Diatom with a chytrid
Scanned old photo. It had been caught by an old man, who was kind enough to let us take a picture. Locally very appreciated in a dish called "caranguejo toc-toc" (literally "toc-toc crab"), named after the sound you make when you crack the crab's exoskeleton. ID more of an educated guess, but still I think I looks similar to a male Ucides cordatus.