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SMF HEADING JOURNALISM n°2

Sharks: incredible fish, super-predators

 

Between the fear they sometimes inspire and the fascination they exert, who really are the sharks? Present on earth for more than 400 million years, survivors of several mass extinctions, this denomination gathers in fact some 500 species. Exceptional cartilaginous fish (Chondrichthyans, as well as rays and chimeras), they have their own peculiarities. Holder of a key place in the good balance of the ecosystem, it is essential. Now, meet the lord of the seas.

 

An incredible sensory tool

 

The notion of meaning, when it concerns sharks, differs somewhat from ours. Indeed, if they have our five common senses, they are also equipped with an even more powerful tool: the ampullae of Lorenzini. Electro-receptive sensors that allow the animal to detect weak electric fields. A discovery dated 1678, and realized by the Italian Stefano Lorenzini during a study conducted on electric stripes. Visible in the form of small dark dots, mostly concentrated around the eyes and muzzle, they are easily distinguishable from the color of the skin. Heartbeats, muscle contractions and more, are then detectable. A great help for predators to spot prey buried under the sand, or for night hunting. Another exceptional capacity, the ampullae of Lorenzini serve as a compass and help the shark to orient itself through the detection of magnetic fields. In addition to that, they can also, through different currents, detect the terrestrial magnetic field. The ampullae are so sensitive that they can even perceive a variation, if only slightly, of temperature.

 

Great skills

 

Among the many special features of sharks are its scales. As for rays (also Elasmobranchii, subclass of Chondrichtyes mentioned earlier), shark scales are placoid. These denticles, called cutaneous, cover their skin and give it a rough appearance. A special structure that promotes their hydrodynamics. With this natural aid, the mako shark (scientific name: Isurus oxyrinchus) for example, would arrive at peak speeds of 110 km / h (for 50km/h average speed).

Another faculty that allowed sharks to cross the various eras: their adaptation to the temperature of the water. Although this only concerns very few species that have warm blood. This time, we will take as an example the great white shark (Carcharodon carcharias). They can withstand significant temperature variations during their travels and regulate their body temperature by 4 (39.2°F) to 14°C (57.2°F) higher than that of water. A way to stay active on a continual basis.

 

The peculiarities of pregnancy

Sharks have a late sexual maturity. It takes several years, between two and twenty for some species, about 7 years for the zebra shark (Stegostoma fasciatum) and up to 150 years for the Greenland shark (Somniosus microcephalus), which has a life expectancy who would approach 400 years old. A feature that makes them much more vulnerable to overexploitation. Indeed, one of the consequences of overfishing is the impediment for sharks to reach the age of this necessary sexual maturity or the end of a gestation, that is also long. It takes 9 to 12 months, and up to 22 months for spiny dogfish (Squalus acanthias). Sharks present three reproductive methods: oviparous, viviparous placental and viviparous placental. To give a few examples, the little dogfish (Scyliorhinus canicula) is oviparous. In other words, it lays eggs containing embryos fed by their yolk sac. These then harden on contact with seawater. The great hammerhead shark (Sphyrna mokarran) is a viviparous placental (like humans). Its embryos develop thanks to the placenta of the mother, to which they are connected. As for bull sharks (Carcharias taurus), they are viviparous aplacental. In other words, the eggs (then embryos), develop inside the genital tract fed by their yolk sac. So there is no placenta. In the bull shark, the most vigorous cubs eat the other, less vivid embryos: a phenomenon called intrauterine cannibalism or adelphophagy. A kind of natural pre-selection. In bull sharks and other species, oophagy is also observed: embryos in the uterus also feed on unfertilized eggs produced in excess by the mother.

 

The essential place of sharks

 

Essential links in the food chain, sharks occupy an indispensable place. First, they are real regulators. Their decline would lead to a disastrous imbalance in the marine ecosystem. They allow a balance of the food web (all the food chains of an ecosystem). They keep, in fact, the other species under control. Their disappearance would lead to a chain reaction with dramatic consequences. Super-predators, they feed on predators (tunas, marlins …), which themselves feed on small fish. The latter feed on plankton and aquatic vegetation. A missing link would completely disrupt this balance and contribute to both proliferation and the disappearance of other species. Predation of sharks has also forced its prey to develop different defense strategies, which has therefore played a role in biodiversity. Sharks also play an important health role. Although predators, sharks feed first and foremost on weak, wounded or sick animals that are identifiable by their earlier meaning. Also, even if no species of shark is based solely on this practice, it remains for many a real godsend. The result is doubly advantageous: a major energy contribution, against minimal expenses. The same goes for the corpses of various marine animals, thus cleaned.

 

MH.