Sharks are a group of fish that first appeared around 420 million years ago and have since diversified into over 470 different species that inhabit every ocean on our planet. Their size ranges from the tiny dogfish measuring only 17 centimetres to the incredible whale shark, the biggest fish in the ocean, which can reach lengths of up to 12 metres and weigh over 20 tons. Although these fish come in many shapes and sizes, they all share unique characteristics that make them a shark.
1. MULTIPLE GILL SLITS
Most species of sharks have five pairs of gill slits, but a few have six and even seven, typically located on the side of the head. Tiny membranes in the shark’s gills extract oxygen from the water.
2. AMPULLAE OF LORENZINI
Located near the nostrils, around the head and on the underside of the snout are small jelly-filled pores. These sensory organs are able to detect electrical fields underwater and are used by sharks to help locate prey and navigate.
3. JAWS AND TEETH
Most sharks have multiple rows of teeth that continue to grow and replace the ones that are lost throughout their lifetime. Much of what we know about sharks has been learnt from fossilised shark teeth, as unlike the rest of the body, they can remain intact on the seabed for hundreds or even thousands of years
4. DERMAL DENTICLES
A shark’s skin is composed of hundreds of tooth-like scales called dermal denticles. These ‘teeth’ are what makes a shark’s skin tough and also incredibly effective at reducing drag.
5. CARTILAGINOUS SKELETON
Unlike other fish, sharks’ bodies contain no bones and their skeleton is made of cartilage. This material is considerably less dense and more flexible than bone, making sharks lighter and more agile. Sharks use forward movement to control vertical position. In most species, the tail acts as the shark’s propeller while the pectoral fins are used for balance and direction
6. Prominent Dorsal Fin
Most sharks have a prominent dorsal fin that is located on top of the animal behind the head. This fin is often seen breaking the surface and it plays a vital role in propulsion, buoyancy and direction
7. Lateral Line
A visible line along the side of the shark’s body consisting of sensory organs that help to detect movement and vibration in the surrounding water, providing spatial awareness and the ability to navigate
8. Pectoral fin
Like most fish, sharks have two pelvic fins behind the head, but these are rigid, not flexible, often elongated and are used for buoyancy and steering
9. LARGE LIVER
Most fish have a swim bladder that they fill with air to help control their buoyancy but sharks rely instead on a large, oil-filled liver to reduce their overall density and provide some measure of hydrostatic lift