Gillyweed is a magical plant native to the Mediterranean and resembling a bundle of slimy, grey-green rat tails. When it is eaten by a witch or wizard, one grows gills and webbing between the fingers and toes. There is some debate among Herbologists as to the duration of the effects of Gillyweed on fresh water versus salt water, [1] but the effects of Gillyweed in fresh water seems to last about an hour.
In the film, Neville Longbottom tells Harry about Gillyweed (he learned of it in a book Moody had given him), and is given the actual Gillyweed by Longbottom and told to put it in his mouth by Crouch/Moody.
