This is a zoosporangium of Halophytophthora epistomium, an oomycote that can be found regularly, though rarely, in saltmarsh ecosystems, on leaves of plants at the marsh edge (e.g., live oak) that fall into marsh pools. When zoospores have matured, the sporangium pops the cap of the dehiscence tube off, a plug is extruded, and the zoospores swim out of the opened tube. H. epistomium can enwrap hyphae of true fungi with its own, and feed on the true-fungal hyphae. See Newell & Fell, 1996, Cues for zoospore release by marine oomycotes in naturally decaying submerged leaves, Mycologia 88:934-938.