Thanks to their many starring roles in Hollywood films, sharks have become one of the most feared animals in human history
The ominous silent gliding of a shark strikes fear into every ocean dweller and no animal is safe from attack by the powerful gnashing jaws. These super-charged hunters hurl themselves into the air to snatch escaping prey and can even sniff out prey hidden beneath the seabed. These are some of the most fearsome creatures on the planet and will stop at nothing to catch their prey.
The bizarre-looking hammerhead shark deserves the title of an ocean killer as every part of their its body can be used as a weapon. They are some of the most successful hunters of the sea and their grey camouflage blends in with the ocean backdrop, making them almost invisible to their prey until it’s too late. Not only do they have dagger-sharp teeth and lightning reactions on their side, they have methods of sensing their environment of which humans can only dream.
Hammerhead sharks have 360-degree vision. Shark vision is very good. They are able to scan over 180 degrees with a single eye, which means the visual fields of both eyes overlap. This wide spacing of the eyes comes at a cost, however. Hammerhead sharks eyes are spaced so far apart, they have larger blind spots in front of them than other sharks. Light enters the eye through the cornea, and travels to the light-sensitive rod and cone cells through several layers of cells. These cells contain chemicals that change when they are hit by light, and this then sends a signal to the brain via the optic nerve. Behind the retina are mirrored crystals called the tapetum lucidum, and when light hits this layer of crystals it is reflected many times. This means that in low light a hammerhead shark can see 10 times better than a human could.
Behind a shark’s eye is a tiny pinhole, which is the opening to the inner ear. Sound travels faster in water than in air, helping sharks hear prey from 250 metres (800 feet) away. A shark’s ear is made of three curved canals that each contain four sensory cells called maculae that are packed with nerve endings. The inner walls of each canal are coated with tiny hairs, and when water vibrations produced by sound stimulate these hairs the information is sent to the brain. Low pitched sounds are easiest for sharks to pick up, and hammerheads can hear low sounds that humans can’t.
A shark has tiny teeth in its skin called denticles to help streamline the shark and help protect against predators. Is a bigger shark or a toothed whale attacked, these denticles would give the victim a fighting chance of escaping. They are very rough to the touch and a human rubbing these denticles the wrong direction would suffer cuts from the sharp spines. Despite having such well-armoured skin, sharks have nerves capable of detecting touches. They use their noses to investigate an object to determine whether or not it is edible and sometimes simply use their teeth to identify objects.
The hammerhead shark’s sense of smell is one of the most important, and a shark can detect a teaspoon of blood in an Olympic sized swimming pool. Hammerhead sharks have two nasal cavities called nares, which each have an entry and exit openings. Inside the nares are a maze of chambers lined with skin folds covered with scent detectors. The cells that decipher smells send this information to the brain, where the shark decides what course of action to take. Two thirds of a hammerhead’s brain weight is dedicated to decoding smells, making them some of the best sniffers in the sea.
A hammerhead shark doesn’t need to use taste to catch prey, so it is their least developed sense. Sharks do have taste buds, but they can only detect flavour once the item is inside the mouth and items that are foul-tasting will be spat out. This has been demonstrated by sea otter carcasses that are found floating out at sea that exhibit bite marks. Sharks don’t eat the otters, they simply have a taste and discard them. This also explains why many victims of shark attacks survive, as sharks bite before realising that a human is not suitable meal.
The face of a shark is covered with tiny black spots that scientists call the ampullae of Lorenzini, specialised cells that give the shark an extra sense. The ampullae detect electric fields in the water. They are simply open pores filled with electrically conductive material. The ampullae contains a jelly like substance which vibrates within the pore when an electrical signal is detected. The base of the pore is covered with tiny hair-like cilia, which respond to changes in electrical currents. Just like the hairs in a human ear detect the direction and volume of sound, the cilia of the ampullae of Lorenzini can distinguish tiny changes in electric currents. The ampullae can pick up weak electrical signals from muscle contractions of prey. A great hammerhead shark has over 3000 of these receptive pores, making them the masters of electroreception. Hammerheads scan along the ocean floor in search of buried prey, and any movement made by a hidden animal is immediately registered by the shark’s ampullar pores. Hammerhead sharks can sense the earth’s magnetic field along ancient volcanic sites and use them for navigation.
Sharks have lateral lines along the sides of their bodies. This lateral line works by detecting changes in water pressure and direction similar to the way human skin can feel or detect wind or changes in air direction. All fish species have long grooves down their sides, which are made of receptive cells called neuromasts. These cells sense minute changes in water pressure and help fish build up a mental picture of their environment. The lateral line is made up of a series of pores formed by modified scales and hair cells. These open into a canal just below the skin surface. Between the pores is a gelatinous mass called the cupula. As water flows past the cupula it causes it to bend modified hair cells located within a sensory patch under the cupula, which fires off a series of nervous impulses to the brain. Information sent to the hammerhead’s brain tells the shark exactly what is happening around it, even in complete darkness. This is the seventh sense of the hammerhead shark, making it one on the oceans most deadly predators.
Image from flickr.com/photos/barrypeters