By Rachel Jacobson
Sharks have been swimming the world’s oceans for millions of years. The oldest sharks date back 420 million years ago to the Silurian period and true sharks evolved during the Devonian period, also known as the “Age of Fishes”, 400 million years ago. 360-286 million years ago marked the Carboniferious period in which occured the appearance of sharks that possessed characteristics similar to the sharks of today. This “Golden Age of Sharks” was home to many bizarre sharks and there was a higher diversity of shark families than what exists today. Unfortunately, many of these families were lost during a mass extinction event that occurred approximately 245 million years ago.
After the mass extinction, the oceans witnessed the rise of new sharks including sharks that possessed extendable jaws and gave rise to todays modern sharks. The sharks that arose during this time, 200-65 million years ago, needed to be streamlined, fast, and large to compete with the marine dinosaurs that were the apex predators of the prehistoric seas. The oldest of these sharks still exist today are the cow (Hexanchiformes) and bullhead (Heterodontiformes) sharks. A second mass extinction event occurred laying waste to the dinosaurs as well as some marine species but 80 percent of the Chondrichthyan families survived this event resulting in the sharks that can be seen swimming in the oceans today.
Today, there are over 470 species of sharks swimming in the planet’s oceans. They range in size with the smallest being the dwarf lantern shark at 17 cm (6.7 in) long and the largest being the whale shark at 12 m (39 ft) long. Sharks are amazing animals and there is more to them than a torpedo shaped body and a mouth full of teeth. Something that all sharks have in common are the senses that they have developed over millions of years of evolution.
1. Smell- The most notorious sense that sharks have is their sense of smell also known as olfaction. It has been said many times that a shark has the ability to detect a drop of blood in an olympic sized swimming pool. This is a slight exaggeration but allows us to visualize how sensitive a shark’s nose is. So how does smell work in a shark? Sharks are able to smell the world around them through scents in the water. As a shark swims around, water carrying scents will flow into a sharks nostrils, or nares, and over plates containing olfactory receptors. These receptors detect the scent and communicate it to the olfactory bulb of the sharks brain. When you watch a shark swimming you may notice that it swims in an S formation, this helps the shark “scan” the water for scents that could lead it to prey. Some species, such as the nurse shark, will use the odor difference between their left and right nares to guide them to a scent source. Other species, like the lemon shark, will simply move up current once a scent is detected.
2. Sight- Vision was once thought to be unimportant in sharks but is now better understood. It is now known that sharks have sharp vision and can even see in color. Sharks eyes contain rods and cones just like a human eye which allows the shark to adapt to different light and detect color. It is thought that sharks may be ten times more sensitive to light than humans allowing them to see in low light situations. To increase their sensitivity to light, a sharks eye contains a membrane, called a tapetum lucidum, which will reflect light back to the sharks retina for increased sensitivity, similar to the glow from a cats eyes at night. Since vision is important for sharks it is necessary for a shark to protect it’s eyes from damage. There are two ways that sharks can do this. One way is through the use of a nictitating membrane which is like an eyelid that extends from the bottom of the eye when feeding. Two species of sharks that have this membrane are hammerheads and tiger sharks. Other species of sharks, such as great whites, will roll their eyes back into the sockets when feeding.
3. Hearing- When dunk your head underwater at the beach you will notice that the ocean is not a quiet place. Sound is carried farther and faster in water than it is in air and sharks use sound to their advantage. When you look at a shark you will notice that they do not have an external ear like humans do, rather they possess an internal ear. This inner ear allows for the detection of low frequency sounds that would indicate an animal splashing around or a wounded fish. Sharks are able to pick up these sounds from more than 270 yards away and will use their hearing as well as other senses to find the source of the sound.
The last two senses that humans and sharks have in common are touch and taste, these are senses that people do not commonly associate with sharks.
4. Taste- Sharks do not have tongues like humans and the extent that sharks taste is not well understood. From the little research that has been conducted, it appears that sharks use taste to identify whether or not an item is or is not food.
5. Touch (Lateral Line)- All sharks, and fish, have a sensory system called a lateral line running horizontally along their body. The lateral line allows them to detect movement in the water giving them the ability to “feel” their environment. The lateral line also allows sharks to detect prey, avoid obstacles, orient into a current to find food by following a scent, and move in schools like scalloped hammerheads do around Cocos Island.
Sharks have two specially adapted senses which include the lateral line mentioned above and the Ampullae of Lorenzini.
Ampullae of Lorenzini- The ampullae of Lorenzini are little black dots easily seen on the underside of a sharks snout which allow sharks to detect the electrical fields given off by all living things. This is a near-field sense that allows sharks to detect creatures hidden in the sand within a few feet as well as hunt at night. It is thought that the bizarre head, or cephalofoil, that hammerheads have increases this sharks ability to detect prey buried in the sand. The ampullae of Lorenzini also help to orient sharks to magnetic fields giving them the ability to navigate.
This information only scratches the surface of the interesting life of sharks. Their biology makes them fascinating subjects to observe and study; some sharks can venture into fresh water and some sharks are predators before they are even born. All sharks interactions with the ecosystem make them important players in the health of the sea.
More of these fascinating facts will be covered in later articles but the information presented here helps to give an understanding of where sharks came from and how they survive and interact in their environment.
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