Author: Marco Angelozzi
Sharks, large predators... preyed upon
Sharks, especially after the release of films that distorted their behavior, have been considered murderous predators, constantly searching for food, an activity that would lead them to feed on anything they see, without any distinction.
The reality is quite different: these elegant cartilaginous fish are merely large predators, positioned near the top of the food chain in their habitat, and as such possess considerable offensive potential. Each shark species is accustomed to feeding on specific prey, which they hunt with techniques that leave nothing to chance or fate, and the search for food, as in every living organism, occupies a significant portion of these animals' lives. They don't prey on just anything they see, but in 99% of known species, there is a genuine discrimination based on instincts and preferences.
White shark (Carcharodon carcharias)
In fact, humans, not being part of aquatic fauna and probably due to the limited adipose tissue in their body structure, are not among the prey of any shark! Only a few species that reach large sizes can pose a danger to humans, as can all large land predators. (50% of sharks are less than 1 meter in length, 82% are less than 2 meters, and only 4% reach dimensions greater than 4 meters, sufficient to pose a serious threat.)
Every year, in all the world's seas, there are approximately 100 attacks, around thirty of which are fatal. In almost all of these cases, death is caused by the severity of the first and only bite, which the shark uses, along with its other senses, to determine what it encounters, precisely because it is accustomed to selecting its prey and discriminating against anything that isn't part of its diet. The shark doesn't continue the attack or swallow its prey, but instead moves away.
There are also very dangerous situations that can attract these large predators and increase the risk of an attack, such as during a feeding frenzy or with the presence of blood and other bodily fluids in the water.
Even the silhouette of a surfer swimming with his hands outward above his board can be mistaken for that of a marine mammal, a coveted prey for large sharks.
There are known to have been attacks on spearfishermen swimming in waters where large quantities of blood had been spilled as a result of their fishing and where dying prey was dangerously tied to the belt.
All these considerations, combined with the fact that humans capture and kill approximately 100 million sharks every year, mainly to feed the food and commercial markets, may lead us to think that the shark, despite itself, has currently transformed from a great predator into... great prey.
Mako shark (Isurus oxyrinchus) and whitetip sharks (Triaenodon obesus)
Many aspects of shark biology and ethology are still poorly understood, primarily due to the difficulty of studying these fish in their natural habitat and their almost always shy and solitary behavior. Below is some information on the evolutionary history, biology, and morphology of these magnificent cartilaginous fish.
Sharks, along with rays, electric rays, and chimaeras, are cartilaginous fish belonging to the class Chondroichthyes and are present in all the seas of the globe, from the surface to 1500 meters deep, from warm tropical waters to cold Arctic and Antarctic seas.
There are approximately 350 species of sharks, ranging in size from just 25 cm for the pygmy ribbontail catshark (Eridacnis radcliffei) to the approximately 18 m long whale shark (Rhiniodon typus), which at this size is also the largest fish known to date.
(There are around fifty different species of sharks in the Mediterranean, including the great white shark (Carcharodon carcharias), the sand tiger shark (Carcharias taurus), the thresher shark (Alopias vulpinus), the shortfin mako shark (Isurus oxyrinchus), the grey reef shark (Carcharinus plumbeus), the blue shark (Prionace glauca), the hammerhead shark (Sphyrna zygaena), and the basking shark (Cetorhinus maximus).
The oldest fossils of these animals date back to around 400 million years ago, and it is thought that their evolution reached its peak as early as 100 million years ago.
The skeleton of sharks is not bony, like that of common fish, but cartilaginous, that is, made of cartilage, similar to that which makes up the human ear or trachea.
Unlike common fish, sharks lack a swim bladder (an internal buoyancy chamber that can fill with gas), whose buoyancy function is partly replaced by their large liver, which can account for up to 25% of the animal's weight. They are predatory fish, and their diet may include fish, smaller sharks, crustaceans, mollusks, and marine mammals.
Three species of sharks are currently known to feed on plankton, filtering water through their jaws and therefore not active predators. These are the aforementioned whale shark (Rhiniodon typus, up to 18 m), the basking shark (Cetorhinus maximus, up to 13 m), and the greatmouth shark (Megachasma pelagos, up to 5 m).
Sharks possess the same senses as humans, plus two others unknown to us, which are the ability to perceive electric fields and pressure waves diffused in water.
Taste : Sharks' sense of taste is ensured by the presence of human-like taste buds in their mouths, as well as on the surface of their skin. Therefore, sharks can utilize this sense even through simple contact with their body surfaces.
Touch : Along the body of sharks there are sensory cells that are closely related to the taste buds and that let the shark know when physical contact occurs.
Hearing : Sharks have an inner ear, with the ability to perceive especially low-frequency sounds, such as those emitted by wounded animals, and therefore potential prey for this superb animal.
Smell : Sharks' nostrils, always clearly visible on the underside of their snouts, consist of two blind canals, ending with olfactory cells that detect the presence of odorous substances dissolved in the water. Sharks' sense of smell is highly developed; it is thought they can detect 1 part blood in 100 million parts water.
Sight : Sharks' eyesight, contrary to popular belief, is highly developed. The pupil can constrict or dilate depending on the amount of light, and many sharks have a tapetum lucidum behind the retina, a series of reflective plates that amplify light and allow vision even at night. In bright light, the tapetum lucidum darkens to prevent blinding the retina and causing potentially irreversible damage.
Lateral lines : The lateral lines, one on each side of the shark, are made up of sensory cells that give the animal the ability to perceive pressure waves due to water movements.
Ampullae of Lorenzini : These are organs that connect to the outside world through numerous small holes, mostly found in the head region. These holes are filled with a conductive gelatinous substance that communicates with nerve endings. This allows sharks to sense the electrical fields generated by animals (and thus locate prey under the sand) and likely determine their position relative to the Earth's magnetic field.
Sharks' bodies are covered in placoid scales, also known as dermal denticles. They have the same structure as the larger denticles found in the jaws. Placoid scales, in addition to providing effective protection for the shark's body, also reduce resistance to seawater, improving the animal's hydrodynamics.
The teeth found in the upper and lower jaws are very large, modified placoid scales. Their shape and arrangement indicate the sharks' usual prey. Long, sharp teeth, like those of the sand tiger shark (Carcharias taurus), are adapted for spearing and holding small fish and cephalopods, while broader, stockier teeth, like those of the tiger shark (Galeocerdo cuvier), are more resistant and used to crush shells and tear off pieces of food.
Tiger shark (Galeocerdo cuvier) and sand tiger shark (Carcharias taurus) teeth
In shark jaws, teeth are arranged in multiple rows, usually six, and the last ones, toward the inside of the mouth, are slanted, covered by a fold of tissue, and not yet fully developed. As they grow, they are subject to a forward movement due to the continuous formation of the gingival tissue to which they are attached. As they advance, they gradually straighten, due to simple mechanical tension, until they become fully functional. After some time, the teeth in the front rows are destined to fall out, either due to traumatic breakage or spontaneously, and are believed to be replaced individually every 8–15 days, more frequently in younger specimens. Some species are believed to renew an entire row of teeth at a time.
Reproduction of the text and photos in this article, even partial, is strictly prohibited without the author's consent.
