The diploid plants produce male (antheridia) and female (oogonia) gametangia by meiosis. Initially the developing zygospore contains thousands of nuclei contributed by the gametangia. The male gametangia are most commonly called antheridia. Inside the zygosporangium, the nuclei from each of the original two gametangia pair up. During sexual reproduction, compatible strains form short, specialized hyphae called "gametangia". The gametangia then fuse into a zygosporangium. Female gametangia are most commonly called archegonia. Female gametangia and gametes release a pheromone called sirenin that attracts the male gametes. Eventually, those gametangia merge and zygosporangia form. Extensions called gametangia are formed from each of the compatible haploid mycelia. At the point where two complementary gametangia fuse, a thick-walled, spherical zygosporangium develops. "P. megakarya" can be distinguished from other "Phytophthora" species by its production of gametangia. During the haploid phase, the thallus forms male and female gametangia that release flagellated gametes. The gametangia develop in conceptacles embedded in receptacles in the apices of the final branches. These organs resemble gametangia except that they contain only nuclei.