The first features that most people notice are their streamlined, torpedo-like shaped body, fins and sub terminal mouths (under their heads). A shark’s body is adapted to living in the ocean, where every tail movement requires hard work to move through the dense medium. Many sharks need to keep swimming to breathe, it is very important that there is nothing to prevent the shark from moving efficiently. This is the reason why their fins, tail and scale alignment are adapted to allow water to flow easily over their body, reducing drag and energy consumption. Fast swimming hunters such as Mako and Silky have a streamlined body shape and are considered hydro- dynamically silent. The ability to pass through water with the least resistance is achieved by the shark’s rough skin.
The texture is rough since is has small scales similar to teeth, called dermal denticles. Each species has a unique shaped denticle. They have a covering of dentine, a central pulp canal containing blood vessels and a single nerve. If you rub your hand along the body from front to back, there is little resistance and it feels smooth. However, when you rub your hand back to front, the skin feels like sand paper. The denticles play an important part in swimming efficiency. The water is channeled by the ‘skin teeth and flows across the fins and around the body. The teeth also break up the interface between skin and water, reducing the friction between the two entities. The teeth skin also help protect the shark from injuries and several elements in the water. It's like a suit of armor for sharks.
Jaws and Teeth
The jaws of a shark are not attached to the skull; they move as separate parts. The upper and lower jaw work independently. This versatility provides the shark with a very powerful pull and latch.
Lined with multiple rows of shark teeth, the jaws of a shark are perfect for biting, chomping and slicing its way through its prey. Sharks teeth come in all different sizes and you can tell what type of prey its eats just by the shape of its teeth. For example, the triangular, serrated teeth of the great white shark are perfect cutting tools for biting large chunks of flesh from a blubbery marine mammal, whilst many bottom dwelling sharks have thick conical or flattened teeth in the back of their mouths that are used for crushing crabs and molluscs, for example the nurse shark and angel shark. The teeth are arranged in rows, just as tooth shape differs between species, so does the number of rows a shark has. The bull shark has about 12 or 13 rows of teeth, whilst the basking shark has more than 200 in each jaw! When one tooth is damaged or lost, it is replaced by another. As a front tooth is broken or worn down, it falls out and is replaced by a tooth in the next row. The sets of teeth rotate forward and a new tooth forms in the rear.
The most recognised part of a shark is the dorsal fin, often seen neatly slicing the surface of the water is shark horror movies, however sharks rarely come to the surface high enough for the dorsal fin to break. Sharks also have a caudal fin (tail fin), an anal fin (some species) a second dorsal fin and a pair of pectoral fins and pelvic fins. The pectoral, pelvic, dorsal and anal fins help the shark to manoeuvre, turn, swim straight and move up and down in the water column. The fins are fixed and remain hard at all times, but there are some muscles that allow for subtle movements. Keratin based elements, ceratotrichia supports the fins and is what shark fin soup is made from, it looks like dry spaghetti running through the fin. The caudal fin provides the thrust needed to propel the shark through the water. The vertebral column passes into the upper lobe of the tail, this is a heterocercal tail. The upper lobe is usually much longer than the lower lobe, however in fast swimming species like the Mako and the Great white, the lower lobe is enlarged too but the tail is still internally heterocercal. The caudal peduncal is where the narrowing of the body meets the tail. All the force generated by the sharks muscles is translated to the tail via the caudal peduncal. In general, the body shape of a shark will determine their lifestyle. Fast swimming, open ocean species like the Mako and Great white have a conical head, large thick body, large pectoral fins, a narrow caudal peduncal with keels and symmetrical caudal fin. This is the most efficient body shape for moving quickly though the water. More moderate swimmers like Blue sharks and Tiger sharks have a more flattened head, thinner body, thicker caudal peduncal with little or no keels, large pectoral fins and an upper lobe which is larger than the lower lobe. These sharks have the broadest range of swimming speeds. Slow swimming sharks like the nurse and leopard sharks have large heads with blunt snouts, pelvic fins positioned more forward, dorsal fins set farther back and a highly asymmetrical caudal fin with no lower lobe. Sharks that live on the bottom such as angel sharks, have a flattened body, large pectoral and pelvic fins and a reduced tail section.
Like other fish, sharks have gills to aid in respiration. Sharks have between 5 and 7 gill slits located on the side of ‘the head. Water has to be constantly passed over the gills slits to enable gas exchange to occur. Water flows into the shark’s mouth as the shark swims forward, passes into the pharynx, over the gills and finally leaves through the gills slits. Oxygen in the water is absorbed into tiny blood vessels near the gills, which then carry oxygen around the body. This means that most sharks need to continuously move through the water column to breathe, although some sharks have the ability to rest and suck water into their mouth and pump it over their gills.
Coloration and Camouflage
The color of a shark is very important and unique, it helps them to avoid predators and aids them when capturing prey. The dorsal (top) side of the shark is darker than the ventral (bottom) side. This ‘counter shading’ is a type of camouflage. When seen from above, the shark blends in to the murky depths of the ocean and when seen from below, the shark blends in with the ocean’s surface. This enables the sharks to surprise their prey and hide from predators.
Sharks have a diverse range of skin coloration and some are true masters of disguise. Wobbegongs ( image shows tasselled wobbegong) and angel sharks use their amazing camouflage while resting on the bottom. Potential prey items get close without seeing the shark and quickly become lunch.
Bony fish have a swim bladder, an organ that contains gas and helps fish to remain buoyant. Sharks don’t have a swim bladder; they have other adaptations to prevent them from sinking or floating. Their cartilage skeleton is much lighter than bone and they have a large fatty liver. The liver can take up to approximately 25% of the total body weight. Fatty reserves are stored in the liver, causing it to be an important energy store. In addition, the liver stores oils that are lighter than water, the density of the shark’s body is lighter, thus enabling the shark to remain buoyant.