Why is a shark not a mammal?

Shark – or Selahimorpha according to its scientific name, it is a general term that unites about 440 species distributed in all the seas of the Earth: for example, we find the white shark, giant shark, nurse shark, mako shark, bat, thresher shark or silky shark.

If they all have their own characteristics, they have one thing in common: all sharks belong to the family of fish, not mammals.

Test your knowledge of animal species that have disappeared and are endangered due to human activities. ⋙ Animal quiz: Are these species endangered or extinct because of humans

Shark, cartilaginous fish

The large carnivorous sharks are indeed part of the cartilaginous fish, also called “Chondrichthyes”: this means that their skeleton is mostly cartilage, with no real bone. Here they come closest to another fish, the stingray.

In both cases, the skin of animals is covered with so-called placoid scales. In addition to the prismatic calcified cartilage, another characteristic of cartilaginous fish is the presence in males of the “pelvic clasp”, or “pelvic mixipterygium”: these two elements are evidence that sharks are monophyletic, that is, they are said to have all descended from the same same variety.

Shark gill breathing

It is anatomically easy to distinguish mammals from fish: indeed, if mammals breathe from the water thanks to their lungs, then fish are aquatic vertebrates with gills, thanks to which they breathe underwater. They use their gills to filter oxygen from the water – most sharks have five gill slits on each side of their body, but some species have six or even seven.

On the other hand, marine mammals breathe air (like land mammals): therefore, animals must regularly rise to the surface in order to breathe. Moreover, if mammals are so-called “warm-blooded” animals – they are able to regulate their body temperature and their heat is released regardless of the sun, then fish are “cold-blooded” animals. This means that sharks do not have an internal mechanism to regulate their body temperature.

Shark lays eggs

Another detail that distinguishes fish from mammals: reproduction and related organs. Indeed, female fish lay eggs, and mammals bear cubs. The latter also have breasts that allow them to breastfeed their babies, which cannot be said about fish, and therefore sharks.

Among fish, shark breeding is one of the longest, which poses a serious problem for the survival of the species: in fact, overfishing and accidental captures that sharks fall prey on have more serious consequences than for other fish, since populations are lower.

Shark, not so dangerous, but very dangerous

Despite the image of a bloodthirsty beast in the collective imagination, only four species of sharks are considered dangerous to humans (white shark, ocean shark, bull shark and tiger shark). Most other shark species are harmless little animals that feed only on fish.

However, the shark, a true regulator of marine ecosystems, is a victim of poaching, in part because of the value of its fins and teeth.

Today, 60% of pelagic sharks are endangered, according to the International Union for Conservation of Nature’s global red list of endangered species.

Read also:

⋙ How to react to a shark? Biologist shares his advice

⋙ Fossil reveals the existence of a bizarre 93-million-year-old ‘winged shark’.

⋙ New Zealand: A diver is attacked by a shark and filming a scene

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