• The origin of sharks is not clear, but their geologic record suggests sharks appeared 450 million years ago. That makes them slightly older than trees, which appeared on Earth about 390 million years ago.
• More than 500 species have been described.
• They vary greatly in size. The smallest is the dwarf lanternshark (Etmopterus perryi), which measures 16 to 20 cm. The largest is the whale shark, the biggest fish in the ocean, which averages 5,5 to 9,8 m long.
• The Greenland shark (Somniosus microcephalus) was estimated to be at least 272 to a maximum of 500 years old, making it the longest-lived known shark species and longest-living vertebrate known to science.
• On average, whale sharks (Rhincodon typus) grow to 12 m, they are the largest sharks and fish in the world. The largest measure up to 18 m.
• The fastest sharks, the shortfin mako (Isurus oxyrinchus), can reach a speed of about 80 km/h.
• Hammerhead sharks (family Sphyrnidae) can see in 360 degrees. Due to the shape of their heads and the wide distance between their eyes, they have excellent binocular vision, enabling them to gauge both depth and distance when scanning the sea for their meal. By waggling their heads back and forth, they can spot prey (or danger) all around them — above, below and also behind.
• A great white shark (Carcharodon carcharias) can lose up to 35.000 teeth in its lifetime. But sharks never stop growing new teeth. They have multiple rows of teeth in their jaws, so when a tooth is shed it is replaced by a new one — in as little as 24 hours — which moves forward to replace it.
• In locating food, sharks primarily use their chemical senses, particularly the olfactory. Visual perception is adapted to distinguishing moving objects more by reflection than by colour. Pit organs over the body serve as distance touch receptors produced by sound waves.
• Sharks have the ability to sense the electrical fields that every living creature emits – electroreception. Electrically sensitive organs called ampullae of Lorenzini allow sharks (and skates and rays) to detect very faint electric charges. Great whites can sense a charge as weak as one millionth of volt — helping them close in on prey that they can’t see or smell. Electroreception only works effectively at short distances.
• Studies suggest sharks show restful states, which could be analogous to sleep. There are signs of decreased activity in some species, which may indicate a resting phase. Sharks don’t have eyelids, making it harder to determine their sleep patterns. During periods of rest, sharks remain conscious to maintain the pumping action of their gills and respond to environmental stimuli. A level of consciousness is essential for survival, to stay alert to potential threats while resting.
• Sharks kill fewer than 10 people per year.
• Humans kill up to 100 million sharks per year.
• Sharks can have some of the slowest reproductive rates in the animal kingdom. It varies by species; the dogfish shark (family Squalidae) has one of the longest gestation periods at up to 2 years, while the blue shark can birth 25 to 50 pups per year.
• Sharks are apex predators in their ecosystems, which means they aren’t adapted to handle high levels of predation—so they don’t reproduce quickly and in large numbers like smaller fish. Because of overfishing, environment degradation, pollution and other threats to sharks, their numbers are declining and they can’t reproduce fast enough to keep up. 

Sources:

• https://www.britannica.com/
• American oceans
• https://www.conservation.org/learning/shark-facts
• https://www.ifaw.org/international/journal/shark-facts-statistics
• https://www.americanoceans.org/facts/how-do-sharks-sleep/

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