This is a zoosporangium of Halophytophthora vesicula, a eukaryotic mycelial decomposer that is an oomycote, not a fungus. H. vesicula is found in coastal marine waters throughout the world's oceans; its main source of food is leaves that fall into seawater. This sporangium is full of mature zoospores, and just about to go into release mode. The zoospores are released through a balloon-like vesicle that is formed by eversion and expansion of the inverted cone just inside the smaller end of the obovate sporangium. The biflagellate zoospores escape to the outside through a pore in the tip of the membranous vesicle, and then swim away trying to find new substrates. Successful zoospores (i.e., those that hit, e.g., newly fallen leaves of red mangroves or live oaks) will attach and germinate into hyphae that penetrate into the solid substrate. This sporangium is of the most common, "delicate", type; the "robust" type (see also photo in Newell, 1996, J Exp Mar Biol Ecol 200:187-206) is rare, and has a more sturdy inverted-cone, pre-vesicle structure. These two types may be inappropriately placed under the same binomial.