Today on Jonathan Bird’s Blue World, the incredible world of sharks and their diversity! Hi, I’m Jonathan Bird, and welcome to my world! ( music ♪ ) Before any animals existed on land, sharks ruled the oceans. They have been around for over 250 million years. Many people think that because they are ancient animals, they are in some way primitive. But as it turns out, sharks are highly evolved sophisticated animals with amazing adaptations for survival!The fact that sharks have survived so long demonstrates the incredible effectiveness of their anatomy. And lets face it–250 million years worth of evolution has got to count for something! Over 400 species of sharks live in the oceans across many habitats. Some, like the Caribbean Reef shark, live on shallow tropical coral reefs. Others, like the blue shark, are pelagic, living far from land in the open ocean. The Greenland shark lives in the freezing waters of the arctic, while the Tiger shark prefers the tropics. Sharks are fish, but there are several key things that differentiate them from other kinds of fish. Sharks are everywhere, but what are they? Sharks and their close relatives the rays differ from the bony fishes in several ways. First, sharks and rays have a soft flexible skeleton made of cartilage. The cartilaginous skeleton makes the shark more flexible than similarly-sized bony fishes. Also, sharks and rays have no swim bladder. A bony fish uses an organ called a swim bladder to maintain neutral buoyancy, so it can hover like a hot air balloon. The fish adds or removes gas from its swim bladder through its bloodstream to increase or decrease its buoyancy. The shark’s lack of a swim bladder means that, unlike bony fishes, the shark tends to sink. To stay off the bottom, sharks have to keep moving. While the shark uses its tail fin in a back and forth motion to provide forward thrust, its pectoral fins work like airplane wings to provide lift. Like an airplane wing, as long as the fins move forward through the water, they provide lift to keep the shark up. Not too long ago, scientists thought all sharks had to swim at all times, not just to stay off the bottom, but to keep water moving over their gills. For some species like the hammerhead, this is true. Hammerheads must keep swimming at all times to force water through their gills. Ironically, if they stop swimming they’ll drown. But many species of sharks sometimes stop swimming and rest on the bottom, gulping water to ventilate their gills. This is a Lemon shark, common in the Caribbean, taking a break resting on the sand. It gulps water to breathe. Nurse sharks also spend a lot of time resting on the bottom. Bottom-dwelling species of sharks like the Wobbegong actually live their entire lives on the bottom. They are camouflaged to look like a rock covered in algae, and they hunt by being very patient and waiting for an unsuspecting fish to come close. With such a long evolutionary history, sharks have had plenty of time to refine their senses. For example, most sharks have an incredible sense of smell. Extrapolations on experiments on shark smell have suggested that some sharks can detect one drop of blood dissolved in as much as one million gallons of water! Sharks also have senses we can’t even begin to experience. Sharks have an electrosensory system that allows them to detect the extremely minute electrical currents generated by the muscles of a swimming fish. The snout of a shark is covered in tiny pores called Ampullae of Lorenzini, which convert electrical impulses in the water to an electrical signal in the shark’s nerves. Therefore the shark can “feel” extremely tiny electrical currents in the water– a skill it uses to hunt, even in complete darkness. Speaking of darkness, many people believe that sharks have poor eyesight. Not true! They don’t see in color, but they have sharp eyesight, and super powerful night vision. That’s because they have a shiny “mirror” called the tapedum lucidem located behind the retina that reflects light back through the retina a second time, increasing its sensitivity. Many sharks like to hunt at dawn, dusk and night, because they can see their prey much better than the prey can see them. Sharks are also the only fish to have eyelids. They’re called nictitating membranes and sharks use them to protect their eyes when something gets too close. Sharks never need to go to a dentist, since they don’t really care about cavities. They have many rows of teeth. As old teeth break or become too dull, they fall out and new ones rotate into place. For the entire life of a shark, it never runs out of new teeth, and it never worries about the teeth it has. And you may find this hard to believe, but sharks are really important in the ecosystems of the ocean.You’ll never see a fish that needs glasses. That’s because a nearsighted fish would quickly be devoured by a shark. So sharks help keep fish populations healthy by weeding out the sick and the weak. Healthy fish can get away from sharks, so they reproduce and make more fish. As top predators, sharks have an important role to play. And because they do not reproduce quickly, sharks are vulnerable to overfishing. And while many people are afraid of sharks, most do not represent a threat to people at all. The world’s largest shark, the Whale shark, grows larger than a bus, but eats only plankton and small fish. Even mean looking sharks with pointy teeth like the Sand Tiger shark prefer to eat fish and never attack people. The more we learn about sharks, the more we discover how little we know about them. Even the idea of a feeding frenzy isn’t really true. I get up close to sharks all the time while they are feeding and I have never been bitten. This group of over 30 Caribbean reef sharks is competitively feeding on a frozen chum ball. They are all around me, but they are not interested in biting me. They know that there is food available, and I’m not it, so I just sit back and enjoy watching the action. ( music ♪ ) And when all the bait is gone, the action is over, and I’m perfectly fine. Far from being the horrific monsters of legends, sharks are actually highly evolved and sophisticated animals with a range of senses and abilities unmatched in the animal world. The study of sharks may teach us many things about our world and maybe even about ourselves.