Barnacles are invertebrate creatures mentioned in the How to Train Your Dragon Book series.
Barnacles are marine-dwelling invertebrates which spend their adult life adhered to a surface. The surface can be rock, wood, human structures, or even other living creatures. Despite what their shelled immobile appearance might suggest, barnacles are actually classified in the Phylum Arthropoda, Class Crustacea, making them related to Lobster, Crabs, and Shrimp.
The main function of barnacles in the Book series is as an epithet. However, another - perhaps inadvertent - use of barnacles in the Book series is as a sort of hard crusty armor for colossal-sized Sea Dragons such as Seadragonus Giganticus Maximus.
|“||He [ Green Death ] was so encrusted with barnacles he looked like he was wearing a kind of jeweled armor but, where the little crustaceans and the coral couldn't get a grip, in the joints and crannies of him, you could see his true color.||”|
|— Book 1|
In reality, one species of barnacle has enough meat to be edible. Other than that, barnacles can be a nuisance with they encrust on human structures such as ship hulls to cause damage and decreased function.
Gobber yells at his Viking Novice students to try and bring them in line.
|“||NEVER ... in MY FOURTEEN YEARS ... have I come across such a load of HOPELESS BARNACLES as you lot.||”|
Fishlegs exclaims, "Well, blister my barnacles".
Barnacles are also mentioned as adhering to other living creatures as in real life, but in this case, on a Dragon:
|“||He [Green Death] had been there so long that he almost seemed to be part of the ocean floor itself, a great underwater mountain, covered in shells and barnacles, some of his limbs half-buried in the sand.||”|
|— Book 1|
In the epilogue of Book 1, Hiccup imagines Stoick's shield with the Green Death's tooth embedded in it sitting at the botton of the sea, the tooth still singing the Supper's Song.
|“||... and I see in my mind's eye a shield, strangely changed by a rich encrusting of jewel-like barnacles and cold-water coral. with an eight foot tooth sticking right out of the middle of it.||”|