Shark Week: 25 fascinating questions about sharks answered

It’s Shark Week and we’re celebrating the incredible world of the shark. Not sea monsters as they’re often portrayed in film, but the remarkable and intelligent species with many unique and surprising skills. Scroll down to find the answers to 25 amazing questions about this amazing, misunderstood and endangered species.

1. How do you tell the age of a shark?
It’s not easy. Because they shed their teeth so often, you can’t measure the growth rings in them to determine age. The vertebrae have growth rings too, but they aren’t added at a uniform rate, so researchers inject marker chemicals to captured sharks to provide reference samples.

2. How do sharks hear?
A small opening on each side of the head leads to the inner ear. Sharks have very good hearing that is tuned to lower frequencies than our ears. They can hear fish struggling from several miles away.

Sharks do actually have “ears”. Notice the dark spot level with end of the mouth and in line with the eye

3. How many teeth does a shark get through in a lifetime? Why is this number so high?
Shark teeth aren’t embedded in the jaw, but are attached to the skin. They are designed to be disposable and are continually replaced like a conveyor belt from the back of the mouth. A great white shark can get through up to 50,000 teeth in its lifetime.

4. How many shark species are there?
True sharks are classified in the superorder selachimorpha and there are more than 440 species alive today. A new species of shark, skate or ray is identified approximately every two weeks.

5. Do sharks have bones?
No. Sharks belong to the elasmobranch group of fish that have lighter, more flexible cartilage in their skeleton, instead of bone. Their muscles are anchored directly to the inside of their tough skin.

6. How do sharks stay buoyant?
Sharks don’t have a swim bladder like other fish, so they use their liver instead. The amount of squalene oil stored there is adjusted to leave the shark slightly heavier than water. The extra buoyancy is supplied by dynamic lift from the shark’s fins as it swims.

7. What’s the biggest shark?
The whale shark. As well as being the largest shark, this is also the largest fish. Only true whales are larger. Adults can be over 20m (66ft).

The whale shark is found in tropical and warm waters throughout the world, except the Mediterranean.

8. Does a shark’s skin help it swim faster?
Shark skin is covered with scales like other fish, but shark scales are made from dentine and actually resemble tiny teeth more than scales. These ‘dermal denticles’ generate tiny vortices on their trailing edges as the shark swims and this vastly reduces the shark’s drag through the water.

9. How long do sharks live?
Most species live for between 20 and 30 years. But the whale shark is estimated to live for up to 100 years.

10. What’s the smallest species of shark?
A kind of deepwater dogfish shark called etmopterus perryi. It lives in the
Caribbean Sea and grows no larger than 20cm (7.8in).

11. Do any sharks glow in the dark?
Yes, a few species use bioluminescence to lure prey. The brightest is the cookie cutter shark, which glows over its entire stomach, except for a dark band round its neck. This makes it look like a much smaller fish silhouetted against the sky.

12. Is It true that sharks don’t get cancer?
Sharks definitely do get cancer, but there’s some evidence that a compound called angiogenin, an inhibitor in shark cartilage, reduces the ability of tumours to grow.

13. What’s the most endangered species of shark?
Both the large sawfish and the common sawfish are listed as critically endangered by the International Union for Conservation of Nature. Their saw snouts become very easily entangled in fishing nets.

14. Which is the fastest shark ever?
The shortfin mako shark. An open-water hunter that chases fast fish such as tuna, it can reach up to 32km/h (20mph).

The mako shark is the world’s fastest shark

15. Do sharks sleep?
Not properly. Some species rest on the seabed, but their eyes will still follow nearby swimmers. Others may rest one half of their brain at a time, like dolphins do. The spiny dogfish uses its spine to co-ordinate swimming, so that it can rest its entire brain without stopping.

16. Are sharks solitary or do they live in groups?
Many species are highly social. Scalloped hammerheads can form schools of up to 100 sharks, for example. Even the normally solitary hunting species can congregate in groups around a rich food source or to breed.

17. Do sharks ever attack/prey on other shark species?
Most shark species have fairly specific diets – blue sharks mainly eat squid, for example. But great whites, tiger sharks and mako sharks will eat tuna, seals, sea lions, dolphins and even smaller shark species. Hammerhead sharks dine almost exclusively on rays, which are closely related to sharks.

18. Does any other animal prey on sharks?
Orca (killer whales) have been known to attack and kill great white sharks, and sperm whales are also occasionally believed to kill sharks. These are exceptions, though. Most of the time, sharks sit firmly at the top of the food chain.

19 How powerful are shark jaws?
A six-metre shark, such as a great white, can exert more than 18,000 newtons of force with a bite. That’s a huge force – twice as much as the largest alligators, which have the strongest bite of any land animal, and more even than current estimates of the bite of the T-rex. But it doesn’t end there. An alligator only exerts maximum bite force when its jaws are almost shut. The wider it opens its mouth, the less leverage is available for the jaw muscles and the weaker the bite becomes. Great white sharks have a unique arrangement of muscles in their jaw that enables them to exert maximum bite force, regardless of how wide their mouth is opened.

20. What are the weirdest items found in a shark?

Ladies’ pyjamas
A rubber tyre
A roll of chicken wire
Tar paper
A bag of potatoes
Odd shoes
A dog
A can of Spam
A sack of coal
The head and forequarters of a crocodile
(All found in the stomachs of tiger sharks, which are the most indiscriminate feeders.)

21. How do sharks attack?
Only a few species of shark are solitary ambush predators – many are filter feeders or eat small fish and crustaceans on the seabed. The aggressive hunters – tiger shark, bull shark and great white – usually patrol close to the surface. They attack at dawn or dusk, when light is poor, and from above so their light-coloured bellies make them hard to spot against the sky. Some species of shark have an extra transparent eyelid (called a nictitating membrane) that can shield their eyes, but the great white does not, so it rolls its eyes back in their sockets just before its strike connects, to protect them. Very often sharks will pursue a hit-and-run technique, taking a single bite out of their prey and then retreating to allow it to bleed to death.

The eyes roll back in their sockets before the shark strikes

22. How many people are killed/attacked by sharks a year?
In the last ten years, the number of fatalities caused by sharks averages out at less than five per year, worldwide.

23. How many sharks are killed/attacked by people a year?
About 100 million sharks are killed every year by humans. Some of this is recreational fishing, but most is commercial. Sharks are killed for their fins for shark fin soup, and in the Australian state of Victoria, shark is the most commonly used fish in fish and chip shops.

24. Can sharks smell blood from miles away, and if so how?
The open-water species can detect blood at concentrations as low as one part per million, but they are even more attracted to the smell of fish guts. By comparing the timing of the scent’s arrival at each nostril, they can tell its direction and quickly home in on distant prey.

25 Statistically, what are the most deadly places on Earth with regard to shark attacks?
Between 1580 and 2010 there have been 2,320 recorded shark attacks worldwide. Here’s a statistical breakdown of the most dangerous areas.