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The thresher sharks have an extreme example of this tail in which the upper lobe has evolved into a weapon for stunning prey. Bottom-dwelling sharks such as catsharks and carpet sharks have tails with long upper lobes and virtually no lower lobe. The upper lobe is held at a very low angle, which sacrifices speed for maneuverability.
The bluntnose sixgill shark (Hexanchus griseus), often simply called the cow shark, is the largest hexanchoid shark, growing to 18 ft (5.5 m) in length. [2] It is found in tropical and temperate waters worldwide and its diet is widely varied by region.
Blacknose sharks are preyed upon by larger sharks, [7] and captives have been observed to perform an apparent threat display towards encroaching divers or newly introduced members of their species. The display consists of the shark hunching its back, lowering its pectoral fins, gaping its jaws, and swimming with an exaggerated side-to-side motion.
With his hand close to razor sharp teeth, shark expert Neil Harvey attempts tonic immobility in a large reef shark. Tonic can be induced by turning a shark upside down, but that's not the only way.
It is recognizable because of its seven gill slits, while most shark species have five gill slits, with the exception of the members of the order Hexanchiformes and the sixgill sawshark. This shark has a large, thick body, with a broad head and blunt snout. The top jaw has jagged, cusped teeth and the bottom jaw has comb-shaped teeth.
The Atlantic sharpnose shark's maximum species length is known to be about 110–120 cm (3.6–3.9 ft). Although its average adult size tends to be about 91.4–99 cm (3.00–3.25 ft). Reports exist of these sharks living up to 19 years in the wild. [2] A distinctive feature is that juveniles have black edges on the dorsal and caudal fins. [3]
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Many sharks can contract and dilate their pupils, like humans, something no teleost fish can do. Sharks have eyelids, but they do not blink because the surrounding water cleans their eyes. To protect their eyes some species have nictitating membranes. This membrane covers the eyes while hunting and when the shark is being attacked.