Sharks: success of evolution ...

écrit par Tobias Friedrich 25 March 2018

Cut to succeed!

Seeing sharks evolve in their natural environment, around you, allows you to quickly realize the efficiency of their streamlined body. Perfect hydrodynamics, due not only to the shape of their body, but also to the coating of their skin. A perfect form of the most effective, offering the minimum of resistance in the water allowing them to slide with a reduced effort. The silhouette of the animal seems to be ideal in the great white, offering it a hydrodynamics close to that of the tuna!

Shark skin also has an exceptional character: it is covered with tiny scales called “placoids” whose structure is similar to that of teeth! These small “teeth” are curved towards the back of the animal which explains the rough side of the skin.

A Super Predator!

And if this animal has always impressed, it is certainly thanks to its successful integration into many marine ecosystems and even freshwater on the planet (like the bulldog shark that can live in both!).

Better, sharks are even super-predators in force. Most species are at the top of food chains, and thus represent one of the most important links for the good health of ecosystems. As Dr Gruber says, an ocean without sharks is a sick ocean.

And for that, the sharks have formidable weapons. Starting with their jaws: while the most archaic sharks have an upper jaw welded to the skull, thus limiting the choice of their prey, most modern sharks have hyostylic type jaws. Separated from the skull, they have greater mobility, and can then become prominent when attacking a prey to better catch it.

Teeth at will!

Those strong jaws wouldn't be as impressive without some powerful teeth! And these teeth, positioned in several rows, have the particularity of constantly renewing themselves. They originate in the cavity of the maxillary cartilage, and migrate to the outside of the jaw. When they fall out or are broken, they are quickly replaced by the previous tooth which straightens as it migrates. Functional teeth are replaced approximately every 8 to 15 days. Sharks therefore have no risk of not being able to hunt as effectively when their teeth break. (What would become of the beaver if its incisors did not grow continuously !!!)

Since the teeth are the only hard parts in sharks (they have a cartilaginous skeleton), it is these teeth that have helped trace their history. They also allow when an animal attacked by a shark is found, to be able to recognize the species if a tooth has remained in the bite. They indeed have a multitude of shapes and functions: some are long and pointed and can easily catch small fish, others are curved and provided with sharp or denticulate edges… etc, and are adapted to different diets.

Blood for meaning!

Sharks are also endowed with high-performance sensory organs, which set them apart even more from other animals. Starting with the lateral line, present only in fish and some amphibian species. It can be seen on the sides of the animal, and is used in particular in the identification of fish because the shape of the line differs from one species to another. It is made up of tiny holes barely visible to the naked eye, leading to a duct filled with mucus. Hair sensory cells bathed in this liquid are sensitive to the slightest movements of the water, and thus inform the animal about what is happening around it.

Bulbs that light up well!

But in elasmobraches (sharks and rays), this system is supplemented by an even more efficient organ! These are the Lorenzini bulbs, described for the first time by Stefano Lorenzini Fiorentino, in 1678 in torpedo rays. Located at the level of the muzzle, they form pores on the surface of the skin, which are easily seen, leading to channels filled with a gelatinous substance extremely sensitive to electromagnetic fields.

Every living being produces an electric field, even at rest. It is the contraction of the muscles that is the cause, and of course, the heart never stops beating. Finding prey, even buried in the sand, becomes child's play for sharks, and of course rays particularly suited to digging in soft bottoms.

But the function of Lorenzini bulbs does not end there. They are also used for orientation. The marine currents producing their own magnetic fields, they thus make it possible to follow these currents, and to travel long distances without getting lost. This explains why certain species are found each year to reproduce in well-defined areas.

Sniff… sniff… sniff…

Developing their assets allowing good survival… this is how sharks have survived millions of years. But that's without counting on what is certainly the most developed sense in them: the sense of smell!
And the hunting process starts there, and ends with the attack!

The nostrils located at the front of the muzzle, are formed of a bag whose interior is pleated. Each fold is covered with numerous receptor hair cells. The water that enters the nostrils is instantly analyzed. These cells can thus identify an odorous substance diluted ten million times! (They say they spot a drop of blood several miles away. When the substance is spotted, then begins a hunt to trace back to the prey. Perhaps an injured animal? The shark waves to follow its trail ... a hit on the right, a blow to the left! Its receivers inform it in real time on the direction to take ...

Rather… animal sexuality!

Sex, we always say it, is important! This is what will also allow living beings to succeed or not. Here too the sharks have been able to evolve and adapt as well as possible. Especially since the intercourse is rather muscular!

In order to fertilize the female, the male will grab her with his teeth and inflict deep wounds on her. Females are easily noticed after the reproductive periods, the body of which is lacerated. But these have developed a passive defense, having a much thicker skin than males.

Males are easily recognized from females, since they have two pterygopods, which are the copulatory organs. During mating they will only use one to impregnate the female.

Shark mating

Eggs, yes… but not always!

Sharks have a low reproductive rate. Their “k” type strategy consists of producing only a small number of offspring (unlike the “r” type strategy), but which will be able to live independently from birth.

Although considered as fish (cartilaginous), the great peculiarity comes from the diversity of embryo development. As surprising as it may seem, many species of sharks do not lay eggs! There are indeed oviparous sharks of course (including fruit bats), many ovoviviparous species, when the eggs incubate and hatch inside the female (the baby is not connected to its mother). Finally, certain species such as the tiger shark, the lemon shark, the thresher shark, etc., are viviparous. The young are fed by the mother through the umbilical cord. This system closely resembles that of mammals.

Most of the time in viviparous and ovoviviparous sharks, the uterus is divided into different compartments, each containing a small one. But in the bull shark (ovovivivpare), we find an additional peculiarity! All the eggs are contained in the same compartment. The first hatchling will then feed on the other eggs! It's intrauterine cannibalism. The female will then continue to produce unfertilized eggs until the birth of the only two young which will be born (one in each proboscis) and which will already be real predators!

A little more organic ...

Here is a diagram of the morphology of sharks. There are of course variations according to the different orders of the classification. The backbone is not always present, especially in more recent species. The gill slits can be 5, 6, or 7.

Carcharhiniformes, certainly the best known sharks, have a nictitating membrane which protects the eyes.

Large families of sharks

Sharks are a paraphyletic group. As such, it is above all the morphological characteristics that will allow them to be recognized. They belong to the class of Chondrychtiens which includes cartilaginous fish.

Here are some ways to visualize the different orders of sharks:

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