More than four hundred million years ago (Paleozoic Era), long before the existence of bony fishes and before dinosaurs roamed the land; primitive sharks hunted the ancient seas much as they do today. From the Paleozoic Era until now, thousands of marine species have become extinct because they were unable to cope with the slowly changing sea or competition with other animals.
The huge prehistoric shark Megalodon was the most dangerous fish to have ever lived.
Sharks have changed very little over the last 100-200 million years and are as successful today as they were so far in the past. One of their secrets to success is their amazing skin.
Believe it or not, the outgrowths from the skin of sharks that form minute scale-like structures are similar to miniature ‘teeth’. It is believed that teeth in the earliest vertebrates arose from these tiny outgrowths.
Shark scales are formed of dentine (like our teeth) and originate from the dermal layer of the skin (bony fish scales originate from the epidermis). The dermal denticles are so hard and so sharp that people have used sharkskin as sandpaper. Japanese sword smiths used shark skin for the hand-grips of swords, making them less likely to slip.
Most denticles point backward so sliding your hand from head to tail feels smooth; sliding your hand in the opposite direction is rough enough to abrade the skin.
Studies have found that the denticles create tiny vortices (whirlpools) behind them. By keeping the water turbulence small and ‘clinging’ to the shark’s surface, drag is reduced and speed is increased. Swimming through the water also generates noise that can warn prey of an approaching predator: the denticles reduce the noise making the shark a deadlier predator.
Most sharks have skin colored to fade into surroundings. Midwater hunters are dark above and pale below. From above a shark looks like the darkness of deep water; from below its belly blends into the sunlight above. Sharks that hunt from ambush on the bottom can have colorful skin that looks like seaweed.
A Great White shark cruises the cool waters of South Australia.
Fins and Tails
The fins of bony fish are supported by extensions of bone that run out into a very thin skin membrane. In sharks the fins are thick and supported by rays (filaments) similar to the keratin in feathers and hair.
The shark’s pectoral fins (equivalent position to our arms) are fairly rigid and designed to provide lift to keep the front of the shark from sinking while swimming.
Sharks have very distinctive tails (caudal fins). The design of the tail is related to its swimming speed, the amount of lift it needs, and how it catches its prey. Sharks have heterocercal caudal fins which mean the top and bottom lobes are different in shape. The larger upper lobe produces a small amount of lift along with forward motion keeping the tail up and balancing the lift from the pectoral fins.
Like the rest of the shark’s body, the fins are covered in dermal denticles. They reduce drag and noise and increase biological efficiency. Energy saved in swimming is converted to growth and size means safety even for these master predators.
A Basking Shark is a harmless species that preys on plankton by filtering tons of water through its gills.