Reproductive Strategies in Marine Hydrozoan Jellyfish: Sexual Medusae and Asexual Polyps

Hydrozoan jellyfish belong to the phylum Cnidaria and generally have radial symmetry with diploblastic layers. Many species of hydrozoan jellyfish have two characteristic forms: a sessile polyp and a free-swimming medusa. Sessile polyps multiply through asexual reproduction, whereas free-swimming medusae undergo sexual reproduction by releasing eggs and sperm. Although these reproductive systems are known to be adaptations to seasonal changes in the physical environment of the sea, little has been written about when and exactly how hydrozoan jellyfish carry out asexual and sexual reproduction in nature. Here, we describe the life cycle of hydrozoan jellyfish, including polyp reproduction, medusa formation, oocyte maturation, spawning, fertilization, and metamorphosis. We discuss current research on the asexual and sexual reproductive systems of three marine hydrozoan species: Cytaeis uchidae, Cladonema pacificum, and Clytia hemisphaerica.