Evolution has produced some of the most bizarre looking predators in the marine kingdom some of which have gone extinct due to mass extinctions and human activities. However, one group of marine life has remained resilient through the centuries, that is sharks. Sharks evolved in the late Devonian era in the Paleozoic period 370 million years ago. A couple of years later though a large variety of sharks existed in our waters creating a diverseness that has never been seen before. During the carboniferous era their existed mind-blowing species of sharks such as the Stethacanthus and the Helicoprion. The Stethacanthus had an anvil shaped dorsal fin while the Helicoprion had a set of circular teeth in its lower jaw.
stethacanthus above
Helicoprion
THE HAMMER
Fast forward to 20 million years ago evolution presented us with yet another evolutionary master class in the hammerhead shark. With the weird shape of its head, its one of the most iconic species of sharks. The “hammer” called a cephalofoil varies in length in the different nine species of hammer heads. The cephalofoil is not flesh alone but an extended skull of the shark. The smallest is the Bonnet hammerhead while the largest is the wing hammerhead shark whose head is 50% of its whole body length.
The wing hammerhead is the largest of the nine identified species of this shark. It can grow up to 6 meters (20 feet) in length and weigh up to 453 kg (1,000 pounds), although smaller sizes are more common.
HABITAT
Found in temperate and tropical waters worldwide, far offshore and near shorelines, hammerheads are often seen in mass summer migrations seeking cooler water. They are gray-brown to olive-green on top with off-white undersides, and they have heavily serrated, triangular teeth. Their extra-tall, pointed dorsal fins are easily identifiable.
The question then begs why then did evolution design a shark with such a big bulky head. You might think that due to the large size of the head it reduces its chances of catching prey but that is far from the truth. In fact, the large cephalofoil makes the hammer head a formidable hunter. Here’s why.
GREATER VISION
The eye position of many predators is usually in front of the head rather than at the sides for prey animals. This means predators have a greater binocular vision than the prey. However, the hammerhead shark is a predator yet it has eyes at the far end of its cephalofoil, how come?
Scientist decided to compare the vision of three species of hammer heads to sharks with a more streamlined head to find out why. The study showed that all sharks had a 360 degree vertical visual field.
When comparing the horizontal field however they noticed that the hammer head sharks had a greater degree of binocular vision meaning better vision. The winghead had an astonishing 48 degrees of binocular vision. That allows it to have a greater field of view and depth perception and makes it easier to see prey. In conclusion the bigger the head the greater the degree of binocular vision.
HYDRODYNAMICS
Scientists believe that the cephalofoil may serve a hydrodynamic function by increasing maneuverability and/or providing hydrodynamic lift. Because they lack a swim bladder, elasmobranchs (sharks, skates and rays) must rely on other mechanisms for buoyancy regulation. One long-standing supposition (often stated as fact) is that the cephalofoil acts as a “wing” producing lift forces that aid the shark in maintaining vertical station in the water column. Indeed, each lobe of the cephalofoil resembles an airplane wing. A cambered wing is more convex on the upper surface, which causes air or water to accelerate as it passes across the dorsal surface. The increase in speed results in lowered pressure on the upper surface and lift can be produced even when the angle at which the wing is held relative to flow is zero. This then helps the shark to change the angle of attack rapidly since most of its prey are not fast but agile.
SIXTH SENSE
The Ampullae of Lorenzini are special sensing organs called electroreceptors, forming a network of mucus-filled pores that are visible on the tip of the head of all sharks (as seen below)
The ampullae of Lorenzini allows sharks to detect electric signals from potential prey from their muscle movement and heart beat. This means they can still hunt effectively even when the other five senses can’t be used. Since the hammerhead has such a large head the number of pores are more and therefore it is more sensitive to electric impulses. Their senses are 500 times greater than ours. For example a new born Bonnet hammerhead shark can sense an electric impulse from a battery that is 1500km away.
The ampullae of Lorenzini has also been observed to have many functions, including sensory receptors for touch, pressure, salinity and temperature. By being able to sense these magnetic and electric fields, the sensory ability also aids the shark's ability to navigate, as well as affecting social behaviors such as mating and giving them advantages for hunting
INTERACTION WITH HUMANS
Most hammerhead species are fairly small and are considered harmless to humans. However, the great hammerhead's enormous size and fierceness make it potentially dangerous, though few attacks have been recorded.
CONSERVATION STATUS
The IUCN, an international body that assesses the conservation status of wildlife, assessed great and scalloped hammerheads as critically endangered and smooth hammerheads as vulnerable. Great and scalloped hammerheads are already listed as threatened species and it is illegal to fish for them in some waters.
There is concern that the numbers of hammerhead sharks killed in fishing is under-reported in fishing records, so it is unknown how severe the issue really is – it could be worse than we imagine. Many fishery reports have historically only reported the number of ‘hammerheads’ caught, rather than giving the numbers of each species caught. This means there is limited information on the actual number of each of these threatened species caught.