|Source: Shark Facts|
Scientific Name Selachimorpha
Chondrichthyes have skeletons made up of cartilage rather than bone, and lack a swim bladder. Cartilage is lighter, more durable, and flexible than bone, contributing to their overall agility whilst also saving energy. This is vital when sharks must constantly move to prevent sinking.
Sharks have powerful jaws. The jaws of sharks are not attached to their skull. It moves separately with independent upper and lower jaws, allowing them to lift their head and thrust their mouth forward to bite its prey. The surface of shark’s jaws have extra support in the form of tiny hexagonal plates called ‘tesserae’ – calcium salt deposits which give shark cartilage more strength.
TeethTheir teeth are sharp and pointy. Sharks may have up to 3,000 teeth at one time and are fully embedded into the gums. A shark bites with its lower jaw first and then its upper. Each type of shark has a different shaped tooth depending on their diet. Interestingly, sharks never run out of teeth – they continuously grow multiple rows of replacement teeth in a groove inside of the jaw, and are usually replaced one at a time as opposed to entire rows. It is estimated that some sharks may lose 30,000 or more teeth in their lifetime, with replacement rates varying from several days to several months.
The shape and size of the teeth vary depending on their purpose. There are four main types of shark teeth:
· Needle-like teeth are found in sharks whose diet consist of small to medium sized fish and are effective at gripping onto agile and slippery fish.
· Serrated, wedge like teeth are found in larger species that feed on larger prey. It is effective at cutting off chunks of flesh for easy swallowing.
· Teeth which serve no purpose are found in plankton feeders (for example the basking and whale shark who uses their gills to filter feed).
· Dense, plate like teeth are used to crush the shells of prey (for example bivalves and crustaceans). Includes smaller sharks like nurse or angel sharks.
Fins and Tails
The majority of sharks have 8 rigid fins:
◾A pair of pectoral fins that lift the shark as it swims
◾One or two dorsal fins offering stability
◾A pair of pelvic fins offering stability
◾An anal fin offers stability
◾A caudal fin (tail) that propels the shark forward
The shark’s tail provides its forward motion, with speed and acceleration dependent on shape and size. Some tails have large upper lobes for slow cruising with short and sudden bursts of speed, whereas others have larger lower lobes for continued pace.
Their skin is made of denticles and not from ordinary fish scales and act as an outer skeleton for easy movement and for saving energy in the water. It is constructed like hard, sharp teeth and help to protect the shark from injury. Sharks wounds heal quickly.
The upper side of a shark is normally dark to blend in with the water from above and their undersides are white or lighter coloured to blend in with the lighter surface of the sea from below which helps to camouflage them from predators and prey.
Their size vary from 17cm (Spined Pygmy Shark) to 12 metres (Whale Shark).
Most shark species are carnivorous. The range of prey is exceptionally broad, including small bivalves, crustaceans, plankton, krill, marine mammals, and even other sharks (for example, a tiger shark might eat a bull shark, a bull shark might eat a blacktip shark and a blacktip shark might eat a dogfish shark). Generally sharks eat live prey, but have been known to feed on large whale carcasses. Sharks also have a very acute sense of smell that allows them to detect blood in the water from miles away. The gentle giants (for example the whale and basking shark) feed on plankton, filtering the water and trapping small organisms with sieve-like filaments. Sharks eat normally alone, but sometimes one feeding shark attracts others and all begin to try to get a piece of the prey. Some shark species attack and surprise their prey (such as seals and sea lions) from below.
|Source: Animal Fact Guide|
Only a few species are solitary hunters (including the great white) but, they too, often coexist at active hunting or breeding grounds. Most execute a range of social behaviour, hunting in packs or congregating in large numbers. Sharks characteristically cruise at an average speed of 8km per hour because they need to move in order to breathe. This forces water over their gills, delivering oxygen to the blood stream. Conversely, some shark species have adapted to benthic living, resting on the sea bed, and pumping water over their gills. Interestingly, they never enter a true state of sleep. Their eyes remain open and even track the movements of their surroundings during periods of inactivity. Some species are even able to ‘sleep swim’ by being unconscious while meandering around the ocean as a result of swimming being coordinated by their spinal cord and not their brain. Most sharks are particularly active at night when they hunt. Some shark species are solitary, while others display social behaviour. For example, hammerhead sharks school during mating season around seamounts and islands.
|Source: Animal Fact Guide|
Sharks have excellent senses. Two-thirds of a shark's brain is dedicated to its smell sense. They are able to detect a drop of blood from very far away and it gives them the ability to determine the direction of a particular scent based on the time it takes to reach one nostril compared to the other. Their nostrils are primarily used for smelling (as opposed to breathing) and are located on the underside of their snout.
Furthermore, they have great eyesight. New research shows that sharks may be colour-blind. A mirror like layer in the back of the eye called the tapetum lucidum doubles the intensity of incoming light and allowing them to see remarkably well in dim conditions. They don’t blink, even though they have eyelids. They rely on surrounding water to clean their eyes. When hunting or being attacked, some sharks have tough membranes that slides over and protects the eyes. Species that don’t have this membrane, roll their eyes backwards when striking prey.
Sharks do have ears which are located within a small opening on each side of their head, but it isn’t visibly present. Sound travels faster in water and sharks rely on it to alert them to prey or danger which can detected from over 800 feet away.
They are able to feel vibrations in the water using a line of canals that go from its head to its tail and are filled with water and contain sensory cells with hairs growing out of them. These hairs move when the water vibrates and alerts it to potential prey. Sharks also have a sensory organ called the "ampullae of Lorenzini" which they use to "feel" the electrical field coming from its prey.
Sharks mostly live 20-30 years. They mature slowly and reach a reproductive age from 12 to 15 years. Sharks are a k-selected species (they produce a small number of larger, more developed young), resulting in a relatively high survival rate. Nevertheless, mating between sharks is still rarely observed, especially between the larger species such as the great white. Many species only give birth to one or two pups at a time and have difficulty recovering after their populations have declined.
There are also different ways that sharks come into this world, including: eggs are laid (like birds); eggs hatch inside the mother and then are born; and pups sharks grow inside the mother (like humans).
It is problematic to estimate population numbers since there are many dissimilar species spanning a large geographic area. But it is undeniable that overall shark numbers are on the decline due to several threats they face in the wild. Their lifespan is about 20-30 Years in the wild.