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SKELETON: Cartilaginous skeleton means they have no real bones.
SPECIES: Over 500 known species
SIZE RANGE: 17 Centimetres -12 Metres
KNOWN SINCE: 450-420 million years ago (That’s right: sharks existed before dinosaurs.)
Sharks and other cartilaginous fish (skates and rays) have skeletons made of cartilage and connective tissue. Cartilage is flexible and durable, yet is about half the normal density of bone.
Even though sharks don’t have bones, they still can fossilize. As most sharks age, they deposit calcium salts in their skeletal cartilage to strengthen it. The dried jaws of a shark appear and feel heavy and solid; much like bone. These same minerals allow most shark skeletal systems to fossilize quite nicely. The teeth have enamel so they show up in the fossil record too.
NO SWIM BLADDERS
Unlike bony fish, sharks do not have gas-filled swim bladders for buoyancy. Instead, sharks rely on a large liver filled with oil that contains squalene, and their cartilage. Their liver constitutes up to 30% of their total body mass and is about half the normal density of bone.
Sharks have very good senses which help them to hunt for prey. Two thirds of a shark’s brain is dedicated to its sense of smell.
They can hear much deeper sounds than humans and some species can detect sounds more than 700 feet away.
Sharks can see in the dark which is great for hunting at night. The back of sharks’ eyeballs has a reflective layer of tissue called a tapetum. This helps sharks see extremely well with little light.
Sharks also have an extra one called the Ampullae of Lorenzini. This allows them to feel the electrical charge of prey in the water and is very useful for helping them to find food. They number in the hundreds to thousands.
These sensing organs are also called electroreceptors, forming a network of jelly-filled pores. They are mostly found in cartilaginous fish (sharks, rays, and chimaeras). These sensory organs help fish to sense electric fields in the water.
Ocean currents moving in the magnetic field of the Earth also generate electric fields. So, these electroreceptors help sharks in navigation.
HOW DO SHARKS BREATHE UNDER WATER?
Sharks are a type of fish which means they can breathe easily under water. They breathe through gills that are located on either side of their bodies.
Every shark has multiple rows of teeth on the top and bottom of their mouth. These razor-sharp teeth help the shark to rip its food into smaller pieces. The teeth often fall out, but they are soon replaced by new teeth that move quickly into place. A shark can go through an average of 20,000 teeth in its lifetime.
Sharks live in oceans all over the world. They generally do not live in freshwater although there are a few known exceptions, such as the bull shark and the river shark, which can be found in both seawater and freshwater.
Most shark species prefer warmer climates and are often found in the sea around Australia, Florida and Mexico. However, other types of sharks like cold water and live in the icy waters of Alaska and Iceland.
ARE SHARKS REALLY DANGEROUS?
Sharks have always been considered the bad guys of the ocean. They have been known to hurt humans before, which make us very scared of them. Although they are sometimes seen as ‘man eaters’, they only usually attack when they mistake a person for a seal, or they feel threatened and try to defend themselves. You’re more likely to be hit by an asteroid than to die from a shark attack.
On average there are only about 100 shark attacks each year.
The most dangerous sharks are the Great White shark, the Hammerhead shark and the Tiger shark.
However, not all sharks are fierce carnivores. Some species such as the Basking shark and the Whale shark are gentle giants that actually feed on plankton.
About 75 species of shark are in danger of becoming extinct. This is mainly because of humans who hunt sharks for food and sport or because they are accidentally caught in fishing nets. Fishermen kill around 30 to 100 million sharks every year.
Sharks are an important part of the ocean eco-system so we humans must try to protect them from becoming extinct. There are many organisations who are dedicated to saving the shark. One such organisation is The Shark Trust.
Sharks are not mammals due to a number of reasons:
· Sharks don’t produce milk to feed their young.
· Sharks are cold-blooded and cannot regulate their own body temperatures.
· Sharks don’t have lungs, and breathe using gills.
· Sharks don’t have hair. Their bodies are covered in hard scales.
·Sharks don’t feed their young with milk, neither do they have hair or lungs. Sharks are cold-blooded, whereas mammals are warm blooded.
We’ve also found that sharks are fish.
MORE FACTS ABOUT SHARKS
· The Great White shark is the only type of shark that puts its head above water.
· Scientists age sharks by counting the rings on their vertebrae. Vertebrae contain concentric pairs of opaque and translucent bands. Band pairs are counted like rings on a tree and then scientists assign an age to the shark based on the count. Thus, if the vertebrae has 10 band pairs, it is assumed to be 10 years old.
·Sharks exhibit a great diversity in their reproductive modes. There are oviparous (egg-laying) species and viviparous (live-bearing) species. Oviparous species lay eggs that develop and hatch outside the mother’s body with no parental care after the eggs are laid.
· Speed: 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 speeds upwards of 19 kilometres per hour (12 mph).
· Sharks can travel 70 to 80 km in a day.
· The exterior of shark teeth contains fluoride – kind of like built-in toothpaste…Great!!!
·A few sharks appear fluorescent under blue light, such as the swell shark and the chain catshark, where the fluorophore derives from a metabolite of kynurenic acid.
· They are common to depths of 2,000 metres (6,600 ft).
· Two-chambered heart.
·Well-known species are Tiger shark, blue shark, great white shark, mako shark, thresher shark, and hammerhead shark.
We have our e-books published on Amazon for Grade 3 and Grade 4. The books serve as an important guide for Science Olympiads organized by SOF, Silverzone, Unified Council and others. Books are designed to help students understand key science concepts.
The key highlights of the book are:
· Well explained topics
· Use of diagrams and images for students to visualize
· Test exercise after each chapter for self-assessment and evaluation
· Interesting facts sections spread across the book
Here are the links: