Gills

• Type: Magical Biological Transformation, Temporary Transfiguration
• Cause: Consumption of Gillyweed
• Known Users: Harry Potter
• Duration: Approximately one hour

Gills are a temporary magical feature that a wizard or witch can grow to enable breathing underwater. The transformation is induced by the consumption of Gillyweed. When Harry Potter ate a clump of Gillyweed, he experienced a burning sensation in his throat, followed by the appearance of a set of fully functional gills on his neck, just below his ears. These gills allowed him to extract oxygen directly from water, rendering his lungs temporarily unnecessary for respiration while submerged. In addition to the gills, the magical effect also causes webbing to grow between the user's fingers and toes. This webbing significantly enhances swimming ability, making movement through water faster and more efficient. A notable side effect of the transformation is the inability to speak clearly while above water. When Harry attempted to speak after the gills had formed, he was only able to produce a croaking sound. The effects are temporary, with the gills and webbing receding after approximately one hour, at which point the user's normal respiratory functions are restored.

The most prominent use of magically-induced gills occurs during the Second Task of the Triwizard Tournament on 24 February 1995. The champion, Harry Potter, was required to breathe underwater for an hour to rescue his hostage from the Merpeople at the bottom of the Great Lake. After other research attempts by Hermione Granger proved fruitless, Dobby the house-elf provided Harry with Gillyweed moments before the task began. Dobby had overheard a conversation, instigated by Barty Crouch Jnr. (disguised as Alastor Moody), and retrieved the plant from the private stores of Professor Snape. This was part of Crouch Jnr.'s larger plan to ensure Harry won the tournament. Originally, Crouch Jnr. had given a book, Magical Water-Plants of the Mediterranean, to Neville Longbottom, intending for Neville to suggest Gillyweed to Harry. When Neville was too intimidated to do so, Crouch Jnr. used Dobby to ensure the information reached Harry. Harry consumed the Gillyweed and successfully grew gills and webbed appendages, which allowed him to navigate the Great Lake, contend with Grindylows, and communicate non-verbally with the Merpeople. The gills enabled him to stay underwater long enough to rescue not only his own hostage, Ron Weasley, but also Gabrielle Delacour, the sister of Fleur Delacour. The magical effects wore off as Harry swam back to the shore, with the gills and webbing disappearing just before he reached the surface.

In the film adaptation of Harry Potter and the Goblet of Fire, the transformation sequence is depicted as more visceral and painful for Harry. The gills tear through the skin on his neck, and the webbing between his fingers and toes grows in a visually dramatic fashion. (film)