Sharks are a large group of fish belonging to the Superorder Selachimorpha. They are commonly known for their sleek shape, characteristic dorsal fin, and predatory nature. Physiologically, sharks are characterized by a skeleton made of cartilage rather than bone, five to seven gill slits, and pectoral fins not connected to their heads. Though there is a wide range of morphology and species, overall sharks have changed very little since their first appearance in the fossil record in the Ordovician period about 450 million years ago.
Similar to a cat, sharks have a tapetum lucidum in their eyes that reflects light in dark environments, increasing their sight in dark light conditions. Some species also have a nictitating membrane, or third eyelid, also similar to cats. Like many non-fish vertebrates, sharks can also dilate their pupils.
Dogfish are a Family of sharks (Squalidae) with a typical 'shark' appearance, but are unique in that they produce a venom coating their dorsal spines.
In the Book series, sharks are mentioned as a descriptor for behavior and appearance. There is mention if Toothless catching a dogfish, suggesting that sharks may be eaten by dragons.
In real life, sharks are eaten by humans, and used for their hides as well. They are also kept in public aquariums and zoos.
Toothless is compared to a shark for his ruthlessness as a dragon.
|“||However, you only had to look into his big, innocent, heavily lashed eyes to realize that he was totally without morals. The eyes were ancient, the eyes of a killer. You might as well ask a crocodile or a shark to be your friend.||”|
|— Book 1|
When being trained to catch fish, Toothless brings Hiccup mackerel and a dogfish.
|“||But he came back when Hiccup called, eventually, and by the end of a couple of hours he had caught six medium-sized mackerel and a dogfish||”|
|— Book 1|
Sharks are also mentioned when describing the overpowering smell of the Green Death's breath:
|“||It was the stench of DEATH itself - a deep, head-spinning stench of decaying matter; of rotting haddock heads and sweating whale; of long-dead shark and despairing souls.||”|
|— Book 1|
|“||Anybody who can have a twenty minute conversation with a winged shark the size of a planet and come out of it alive is a better thinker than I am.||”|
|— Thuggory to Snotlout about Hiccup|
Sharks are mentioned again when Alvin describes loosing his hand.
|“||I opened the coffin, reached forward quite innocently to grasp something inside ... and the lid snapped shut with the force of a shark's jaws and in one stroke cut off my hand.||”|
|— Alvin the poor-but-honest farmer|
The Outcast ship, the Hammerhead is described as having "a sail curved like a shark's fin". Many of the Outcasts themselves "had filed their teeth into sharp little points, like the teeth of a shark."
Shark eyeballs appear on the Fat Consul's menu.
Sharks are also extensively referenced when comparing a particular shark-like dragon species called the Sharkworm. Though both have a characteristic dorsal fin, a shark's is smooth-edged while a Sharkworm's is serrated.
|“||Sharkworms are a kind of dragon that look a lot like sharks.||”|
Sharks are also used for comparison - again - to describe a dragon's temperament.
|“||Appearances, of course, were deceptive, for dragons are among the most selfish animals on the planet, and Toothless was, in fact, a shark in a baby seal's clothing.||”|
|— Book 3|
As a kidnapped Hiccup and Fishlegs sail into Fort Sinister with their kidnappers, "they sailed through the open gates, right into the heart of the Fortress, and the doors shutting behind them were like the closing of a shark's mouth."
|“||At first he though it was a great whale or shark, but eventually he realized it was something far, far worse. It was a tremendous Sea Dragon called a DOOMFANG.||”|
|— Book 4|
Later on in the epilogue of Book 4, an older Hiccup recounts that something large like a whale or shark was following his own ship. It turned out to be the very same Doomfang, but instead followed Hiccup and his ship like a guardian angel.