Gillyweed is a magical plant, apparently native to the Mediterranean Sea. It is described as looking like "slimy grayish green rat tails." Consumption of the raw weed results in gills appearing in the subject's neck, and apparently also hands and feet lengthening and growing webs to become effective fins.

When Harry Potter is having difficulty figuring out a way to complete the second task of the Triwizard Tournament (which requires him to rescue Ron from the bottom of a lake), Dobby steals some Gillyweed from Professor Snape's store cupboard and gives it to Harry to use.

More than breathing underwater, this plant forces partial adaptation to an underwater existence, providing Harry with gills and extending his feet and hands into flippers. These effects last for about one hour in fresh water.

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