Those of us who have been lucky enough to encounter a shark in its natural environment will know that there is something special about these enigmatic creatures. Scientists believe that sharks have inhabited in our oceans for approximately 420 million years, since then they have evolved to become the perfect predators that they are today. There are over 440 known species, spanning a wide spectrum of shapes and sizes that ranges from the 8-inch dwarf lanternshark to the 40-foot whale shark. Despite the difference in their appearances, all sharks possess traits that set them apart from other fish. Here weâ€™ll take a look at some of their most impressive adaptations.
Skeleton and Skin
Unlike most fish, sharks do not have bony skeletons. Instead, a sharkâ€™s skeleton is made entirely from cartilage, just like a human ear or nose. Cartilage is durable and incredibly flexible, allowing the shark to make tight turns when pursuing prey. It is also far lighter than bone, thereby reducing the sharkâ€™s weight so that it can move quicker through the water and expend less energy. Sharksâ€™ skin is also unique. Instead of scales, itâ€™s made of tiny enameled teeth known as dermal denticles, which are incredibly tough and protect the shark from external damage. Because male sharks use their teeth to hold onto a female during sex, female sharks often have thicker denticles on their pectorals to help minimize their injuries. Dermal denticles also reduce surface drag, helping the shark swim faster and more efficiently.
Sharks filter the oxygen they need to survive from the water using their gills; to do so they must keep water flowing over their gills at all times. Some sharks, including large pelagic species like the great white, maintain the flow of water through a process called ram ventilation. As they swim, water is forced through their open mouths and over their gills, which is why they must keep swimming in order to breathe. Most shark species can remain still, however, thanks to an evolutionary characteristic known as a spiracle, a small slit just behind the sharkâ€™s eye through which it can pump water while it rests. This method of ventilation is called buccal breathing, and is particularly useful for bottom-dwelling species like wobbegongs and catsharks.
Ampullae of Lorenzini
Sharks have two extra senses in addition to the five that humans possess, one of which is the ability to detect electric signals. All sharks have thousands of jelly-filled pores on their head and snout, known as ampullae of Lorenzini. Together, these tiny electroreceptors allow the shark to pick up even the faintest electrical fields, including those given off by all living creatures and the Earthâ€™s geomagnetic field. Sharks use their ampullae of Lorenzini to pinpoint prey, and in smaller sharks, to evade predators. Some species, like the giant hammerhead, even use this sense to locate prey buried under the sand. This ability also allows the shark to navigate effectively using the Earthâ€™s magnetic field, which is why those species that undertake long migrations are able to find their way back to the same spot with incredible accuracy.
Sharksâ€™ second extra sense is the ability to detect vibrations via an adaptation known as a lateral line. The lateral line runs along the length of the shark from snout to tail, and consists of a row of small pores connected to a fluid-filled canal lined with tiny sensory hairs. As the water flows across the lateral line, differences in pressure are picked up by these hairs, which then send signals to the sharkâ€™s brain. If a fish is wounded or sick, the vibrations that it emits will be more erratic than those of a healthy fish, alerting the shark to the possibility of easy prey. Sharks can also use their lateral line as a navigation tool in much the same way that bats and dolphins use echolocation to create a visual map of their surroundings.
Sense of Smell
In a world of often-reduced visibility, having a keen sense of smell is important. A sharkâ€™s olfactory lobes account for two-thirds of its total brain weight, so that some species are capable of detecting one part of blood in a million parts of water. A sharkâ€™s sense of smell is directional; for example, the sensory cells in its right nostril will detect a scent that comes from the sharkâ€™s right side before it is picked up by the left nostril. This allows the shark to locate the source of a specific smell in much the same way that we can pinpoint the origin of a certain sound. Incredibly, studies have shown that a sharkâ€™s sense of smell is so acute that it can even differentiate between the scent of a healthy fish and that of an injured or distressed one.