ALEXANDER SEMENOV / SCIENCE PHOTO LIBRARY ALEXANDER SEMENOV / SCIENCE PHOTO LIBRARY
Hydromedusa. Medusa stage of a Halitholus sp. hydrozoan. 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. Photographed in the White Sea, Russia.
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