Barnacle

Barnacles were creatures found underwater, known for clinging onto the submerged outer edges of boats.

Space barnacles could be found on the outsides of spacecraft travelling through space. (PROSE: The Hungry Night) One kind of space barnacle, the electromagnetic barnacle, was a much more complex and sentient creature. During her time as a tour guide on Gendar, Maxie Masters recalled giving guided tours of the Babbling Bibliothèque to various alien tourists, including electromagnetic barnacles. (PROSE: Out of the Box)

In the Land of Fiction, Rob Roy MacGregor returned the Doctor's TARDIS to him once when it had been lost underwater, and had been recovered covered in barnacles. (AUDIO: Legend of the Cybermen)

Donna Noble once compared an alien tourist to a "walking" barnacle, with tentacles. (AUDIO: Death's Deal)