19th July: Shark Awareness Day

Today is Shark awareness day - so here are 10 things you didn't know about sharks.

1. Sharks have been around a long time. Sharks as we know them today have been around for 100 million years, while fossils of shark scales have been found which are 420 million years old. There are fossils even older than that, although scientists cannot agree on whether these came from a type of shark, or not. Until the 16th century, sharks were known to mariners as "sea dogs".
2. Shark fossils are mostly scales or teeth. Shark skeletons are made from cartilage, not bone and so will decompose when the shark dies. The teeth don't, and there are a lot of them, as a shark constantly replaces its teeth throughout its life and can produce up to 30,000 of them. The scales, or dermal teeth, which help reduce turbulence around them when swimming and help them swim faster.
3. Sharks are fast. In general, sharks swim at an average speed of 8 kilometres per hour (5.0 mph), but when feeding or attacking, the average shark can reach 19 kilometres per hour (12 mph) or more. The shortfin mako shark, the fastest shark can burst at speeds up to 50 kilometres per hour (31 mph).
4. There are about 500 different species of shark. They vary considerably in size from the small dwarf lanternshark which is only 17 centimetres (6.7 in) in length, to the whale shark (Rhincodon typus), the largest fish in the world, which reaches approximately 12 metres (39 ft). They are related to rays and skates, and some species of shark are thought to be more closely related to rays than to other sharks.
5. Some sharks give birth to live and fully functional young. Some lay eggs like other fish, but in others the eggs hatch while still inside the mother where they are nourished by the egg yolks, fluids secreted by the mother, and often by eating their unborn siblings as well.
6. A shark can detect as little as one part per million of Blood in seawater, but it is chemicals found in the intestines of their prey that is more likely to attract them. They can detect the direction of a scent based on the timing of it reaching each nostril, similar to the way humans detect where a sound is coming from. They also have huge numbers of electroreceptor organs. These help them find prey by detecting the electromagnetic fields that all living things produce, so hiding in the sand at the bottom of the sea to avoid being eaten by a shark is ineffective. It's thought they may use these receptors for navigation as well.
7. Sharks breathe through five to seven gill slits. In order to allow the shark to breathe, Water has to flow continuously over the gills. Some, but not all, species of shark have to be constantly on the move or they will suffocate. Swimming in these species is controlled by the spinal chord and not the brain, so they can sleep and swim at the same time.
8. Sharks are not as dangerous to humans as films like Jaws may have led us to believe. Only four out of the 500 or so species have been known to attack and kill people, and only about 4-5 people die from shark attacks each year. If you have Galeophobia (excessive fear of sharks), be reassured that you are more likely to be struck by Lightning or be killed by a falling coconut (150 deaths per year). You are more likely to be bitten by another human than by a shark. The sharks have much more reason to be afraid of us as we kill 100 million of them a year. In fact, the shark probably doesn't want to eat you. Great whites need a lot of fat in their diet, and typically sample potential prey before killing it. They can tell from one bite whether something will provide the nutrients they need. Humans are not really to their taste, so they would spit you out and go looking for a Seal or something with much more blubber. Still worried? You can avoid being attacked by not wearing shiny jewellery or metal while swimming, and not splashing about too much. And remember that a shark will only attack you if you are wet.
9. Other myths about sharks are that they are solitary hunters, roaming the ocean looking for food. Although some species can and do travel huge distances, most stay in local areas and live in schools, and have a "pecking order" when it comes to feeding. Nor is it true that sharks don't attack people in the middle of the day. As we've seen, they don't attack people that much at all, but if they were going to, it's less likely to be in the middle of the day because people get out of the water to have lunch. The Aztecs believed that attaching a string of chili peppers to a canoe would keep sharks away but there is no evidence that this was effective. There is no scientific evidence to support the myth that sharks are immune to cancer or that shark body parts can treat the disease.
10. There are some ancient myths about sharks, especially in Hawaii where there were said to be beings who could change between shark and human form. Sometimes corpses could be offered to the sea to be transformed into a type of ancestor guardian in the form of a shark. That shark would have markings similar to the clothing the corpse had been dressed in, and would look after the family by warding off danger and driving fish into their nets. The shark god Kamohoali'i is said to have banned sharks from eating people. In Fijian mythology, the shark god Dakuwanga would eat lost souls. The remains of sharks have been found in Aztec temples, so it is believed they were sacrificed to the gods.

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As well as this blog, I also write fiction and have published two novels (Death and Faxes and Glastonbury Swan) and a collection of short stories (Jigsaw). If you like ghost stories, crime stories, a bit of romance and anything slightly bizarre you might enjoy them. 

Further details on my books page