Hydromedusa. Medusa stage of the hydrozoan Sarsia tubulosa. Hydromedusae are the sexually reproductive stage of hydrozoan cnidarians. Hydrozoa live in large single-sex (dioecious) colonies made up of thousands of individual specialised polyps. Reproductive polyps bud-off asexually produced medusae. When mature, the medusae release gametes (sex cells) freely into the ocean. Gametes of opposite sexes, from different colonies, then combine to form free-swimming larval zygotes, that either settle on a suitable substrate, or swim and develop into another medusae or polyp directly.