The aptly-named Elephant Shark

Sharks are an amazingly diverse group of creatures and defining them is not as easy as it looks. Although they are all fish, they differ enormously in their body shape, size, habitat, behaviour and diet. Many of them look nothing like the animals we associate with the classic image of a shark – some are almost flat, bottom-dwellers, while others are strange-looking creatures that live at enormous depths. But there are some features that are common to all sharks:

• Unlike other fish, sharks have a skeleton made of cartilage, rather than bone. The skeleton is reinforced in some places with special plates, called tesserae, which are made of hard calcium salts.
• Sharks all have teeth that are produced and shed at regular intervals. Some sharks can produce several thousand teeth every year, the old ones working themselves loose and being replaced by a new row of teeth behind them.
• Even a shark’s skin has teeth! A defining characteristic of sharks is the presence of tooth-like scales covering its skin, called dermal denticles. It’s these denticles that give a shark’s skin its resemblance to sandpaper.
• Sharks have at least five pairs of vertical gill slits, which are almost always mounted on the side of the head. Some species have as many as seven pairs of gill slits.

Most other fish possess swim-bladders to help keep them afloat, but sharks lack any trace of swim-bladders and use other means of maintaining their buoyancy.