THE SHARK: DIET, BEHAVIOR, REPRODUCTION

The shark, one of the many species of cartilaginous fish with predatory behavior that make up the order Selachii (class Chondrichthyes). There are several hundred species of sharks, ranging in size from less than 20 cm to more than 20 m , and which can live in all marine environments around the world. These amazing animals have a fierce reputation and a fascinating biology.

Sharks, along with rays and skates , make up the subclass  Elasmobranchii  of the Chondrichthyes . Sharks are distinguished from other elasmobranchs, however, and resemble ordinary fish, in the shape of their bodies and the location of their gill slits on either side of the head. Although there are exceptions, sharks generally have tough, dull gray skin , roughened by tooth-like scales. They also generally have a muscular, asymmetrical, upturned tail, pointed fins, and a pointed snout that extends forward and over a crescent-shaped mouth? adorned with sharp triangular teeth. Sharks do not have swim bladders and must swim continuously to keep from sinking.

There are over 400 living species of sharks (find some shark species in more detail in more depth), grouped into 14 to 30 families. Several larger species can be dangerous to humans. Many sharks are commercially fished. However, overfishing in the late 20th and early 21st centuries has significantly reduced the populations of some shark species.

1) The Shark in 10 Seconds Top Chrono

• COMMON NAME: Sharks
• SCIENTIFIC NAME: Elasmobranchii
• DIET: Carnivore?
• HABITATS: Marine, coastal and ocean habitats worldwide
• BASIC GROUP OF ANIMALS:   Fish
• AVERAGE LIFESPAN: 20 - 150 years
• SIZE: 20 cm - 20 m
• WEIGHT: Up to 11 tons
• CONSERVATION STATUS: 32% are threatened, 6% are endangered and 26% are globally vulnerable; 24% are near threatened

2) Description of the shark

A- The Diversities of this Marine Animal

Sharks come in a wide variety of shapes, sizes, and even colors. The largest shark and largest fish in the world is the whale shark  (Rhincodon typus), which is estimated to reach a maximum length of 20 m??. The smallest shark is thought to be the dwarf lanternshark ( Etmopterus perryi ), a rare deep-sea species about 20 cm long.

B- The  Body Structure of the Shark

Sharks are cartilaginous fish. This means that they have a body structure made of cartilage , instead of bone. Unlike the fins of bony fish, the fins of cartilaginous fish cannot change shape or fold along their body. Even though sharks do not have a bony skeleton – like many other fish – they are still classified with other vertebrates in the class Elasmobranchii. This class includes about 1,000 species of sharks, rays, and skates (as seen above).

C- The Particularity of Shark Teeth

Shark teeth do not have roots, so they usually fall out after about a week. However, sharks have replacement teeth arranged in rows, and a new one can grow in within a day . Sharks have between five and fifteen rows of teeth? in each jaw, with most having five. Sharks have tough skin that is covered by dermal teeth, which are small enamel-covered plates similar to those found on our teeth.

D- The Colors of Sharks

Shark species are indefinite in color, ranging from gray to cream, brown, yellow, slate, or blue, and are often adorned with spots, bands, marbling, or protuberances. The strangest sharks? are the hammerhead sharks  ( Sphyrna ), whose heads resemble two-headed hammers and have an eye on each stalk. Even stranger than the hammerhead is the bearded carpet shark  (family Orectolobidae ), whose skin flaps and protective coloration closely resemble the sea floor. Common names for sharks often indicate the colors of the living species, such as the blue shark  ( Prionace glauca ), the white shark  (Carcharodon carcharias; also known as the  great white shark ), and the lemon shark  (Negaprion brevirostris).

3) The origin of sharks

The origin of sharks is obscure, but their geological history goes back at least to the Devonian Period  (419.2 million to 358.9 million years ago). Fossil shark-like fish appeared in the Middle Devonian Period and became the dominant vertebrates of the Carboniferous Period  (358.9 million to 298.9 million years ago). Modern sharks appeared in the Early Jurassic Period  (201.3 million to 174.1 million years ago) and in the  Cretaceous Period  (145 million to 66 million years ago), they evolved into the present-day families. Examples include the famous megalodon . Overall, evolution has changed shark morphology very little, except to improve their feeding and swimming mechanisms. Shark teeth vary greatly between fossil and modern species.

4) Distribution Area and Movement of Sharks

Sharks are found in both shallow and deep waters , in coastal, marine and oceanic environments worldwide. Some species live in shallow coastal regions, while others live in deep waters, on the ocean floor and in the open ocean. 

But in general, the geographic ranges of sharks are poorly understood . Their extensive movements are related to breeding or feeding activities or to seasonal environmental changes. Tagging returns of large sharks from the east coast of the United States indicate regular movements between New Jersey and Florida, and blue sharks have been found after crossing the southern Atlantic Ocean.

A tagged spiny dogfish  (Squalus acanthias) was recovered after travelling approximately 1,600 km in 129 days . Advances in tracking technology include the use of satellite tags that continuously send a signal to orbiting satellites as the shark surfaces, revealing north-south movements. Transoceanic movements and wandering and migration patterns are thus beginning to be understood for many species.

Some species of the genus Carcharhinus ,  such as the bull shark ,  move easily in salt, fresh, and brackish waters . River sharks? are small to medium-sized and are exceptionally voracious and bold.

5) The Diet of this Aquatic Voracious

Sharks are carnivores , and they hunt and eat primarily fish, marine mammals such as dolphins and seals, and other sharks. Some species prefer or include in their diet turtles and seagulls, crustaceans and mollusks, as well as plankton and krill.

Feeding habits vary mainly depending on foraging methods and dentition . For example, the whale shark (Rhincodon typus) and the basking shark  (Cetorhinus maximus), both of which can weigh several tons, are harmless giants that live on plankton extracted from the sea through modified gills. The whale shark can grow up to 18 meters long, while the basking shark can reach 14 meters as an adult.

All other sharks feed on small sharks, fish, squid, octopuses, crustaceans, other invertebrates, and in some species, garbage. Among the most predatory species, the largest is the great white shark, a voracious animal of 6 meters , which attacks seals, dolphins, sea turtles, large fish and sometimes humans. The Greenland shark  (Somniosus microcephalus), more lazy in deep and cold waters, feeds on seals, large fish and even swimming reindeer; it can also scavenge whale carcasses.

But in general, sharks feed on fish and often attack in schools . Deep-sea species such as the mackerel shark  (Lamna), mako shark  (Isurus), and thresher shark  (Alopias) frequently feed near the surface and are highly sought after for sport with rod and reel. These deep-sea sharks, with their streamlined lines and powerful fins, feed on fast-moving tuna, marlin, ... Bottom-feeding shark species have robust shapes and blunt heads. They tend to have slower habits. These shellfish eaters often have coarse teeth.

6) Shark Behavior

A- Feeding Behavior of the Shark

Sharks may abstain from feeding for long periods of time , and in captivity they may refuse to feed. Feeding is inhibited in large males during courtship and in pregnant females (carrying an embryo or fetus) when they are on the breeding grounds. Areas selected for pupping are generally free of large sharks.

Sharks circle their prey , appearing disconcertingly out of nowhere and frequently approaching from below. Feeding behavior is stimulated by numbers and rapid swimming when three or more sharks appear in the presence of food. Activity changes from tight circles to rapid crisscrossing movements. Under strong food stimuli, excitement can intensify and develop into a sensory overload that can lead to cannibalistic feeding, or "shark frenzy," in which injured sharks of any size are devoured.

B- How does the shark locate its food?

To locate food, the shark uses primarily its chemical senses, particularly smell ?. Visual acuity ? is adapted to short- and long-range localization and to distinguishing moving objects by reflection rather than color, in dim or bright light. Pit organs located above the body serve as remote touch receptors ?, responding to the motion produced by sound waves. Irregularly pulsed signals below 800 hertz will quickly bring sharks to a given point, suggesting acoustic orientation from considerable distances.

In addition, electroreception, the ability to detect minute electrical activity , works effectively at short range in sharks. Electroreception ⚡ is perhaps the last sensory system used to direct sharks to live prey just before they capture it.

C- The Complex Systems of Sharks 

Sharks have a system of lateral lines along their sides that sense water movement. This system helps the shark find prey and navigate around other objects at night or when water visibility is poor. The lateral line system is a network of fluid-filled channels under the shark's skin. Pressure waves in the seawater around the shark cause this fluid to vibrate. This fluid is in turn transmitted to the jelly in the system, which transmits it to the shark's nerve endings and the message is relayed to the brain.

Sharks need to keep water moving over their gills to get the oxygen they need. But not all sharks need to move constantly. Some sharks have spiracles, a small opening behind their eyes, that force water through their gills so the shark can stay still when it rests.

Sharks that need to swim constantly have periods of activity and rest rather than undergoing deep sleep as we do. They appear to "sleep swim" , with parts of their brain appearing less active as they continue to swim.

Information about sharks and individual and collective actions has provided insight into their behavior. Since large sharks prey on small ones , the habit of segregation by size seems vital to their survival. In a uniform group, dominance among different species is apparent in food competition, suggesting a definite order. All sharks keep away from hammerheads, whose maneuverability, enhanced by the rudder effect of the head ?, gives them a swimming advantage over other sharks.

7) Shark Reproduction, Offspring and Lifespan

A- Reproduction and Offspring of this Legend of the Oceans

Fertilization in sharks is internal . The male introduces the sperm into the female using special copulatory organs (claspers) derived from the pelvic fins. 

Some species of sharks are oviparous , meaning they lay eggs ?. Others are viviparous and give birth to live young. Among these viviparous species, some have a placenta like human babies ?, and others do not. In these cases, shark embryos feed on a yolk sac or unfertilized egg capsules filled with yolk.

Some species can even consume their siblings before they are born .  Like with the sand tiger shark where things are quite competitive. The two largest embryos consume the other embryos? in the litter. 

B- The Longevity of Sharks

Although no one seems to know for sure, it is estimated that the whale shark , the largest species of shark, can live up to 150 years ?‍♂️ , and that many smaller sharks can live between 20 and 30 years.

8) Sharks and Humans

The bad publicity surrounding a few shark species has condemned sharks in general to the misconception that they are vicious man-eaters ❌ . In fact, only 10 shark species out of all the species (about 400) are considered dangerous to humans. All sharks should be treated with respect, however, as they are predators, often with sharp teeth that could cause injury (especially if the shark is provoked or feels threatened).

In Australia, New Zealand, South Africa and elsewhere along coasts where sharks are considered a nuisance or threat to humans , public beaches often have watchtowers, bells, sirens or nets. Since 1937, gillnets have been used off Australian beaches to catch sharks, using gillnets suspended between buoys and anchors, parallel to the beach and beyond the break line. The nets ensnare sharks from all directions and, although they do not touch the surface or bottom and are widely spaced, the nets provide simple and effective control.

The most feared species is the great white shark , whose irregular presence in U.S. coastal waters is associated with infrequent attacks along the California coast and elsewhere. Other sharks implicated in attacks on humans include the tiger shark  (Galeocerdo cuvier), bull shark, oceanic whitetip shark  (C. longimanus), blue shark, and hammerhead shark. Of course, the larger the shark, the more fearsome the attack, but several small specimens can also be dangerous, as confirmed by seasonal attacks off the southeastern coast of the United States.

Not all shark encounters can be called “attacks .” Studies have suggested that a more accurate method of reporting incidents involving sharks and humans would place encounters into one of four categories, rather than calling every encounter an attack. These categories could include:

• Shark sightings  ?
• Encounters with sharks that do not bite, but may come into contact with a kayak, surfboard?‍♀ or other object
• Shark bites resulting in non-fatal injuries?
• Deadly shark bites ☠️

Attacks on humans occur when sharks are hungry, harassed, or in some cases defending their territory, although many interactions with humans seem more likely to be due to mistaken identity . Provocation is increased by the vibrations of people's kicks or punches in the water (which, to sharks, resemble the irregular movements of a wounded fish), the presence of speared fish or bait in the water, or the presence of blood from wounds or menstruation can attract a shark?. Most injuries occur on the lower limbs and buttocks. It is estimated that there are about 100 shark attacks per year worldwide. Less than 25% of these are fatal , mainly as a result of blood loss or shock. It should be noted, however, that shark attacks are much less common than other aquatic mishaps.

9) Sharks are Endangered, Let’s Protect Them!

Paradoxically, humans are a bigger threat to sharks than sharks are to us . Many shark species are threatened by fishing or bycatch, which results in the deaths of millions of sharks ? each year. Compare that to the statistics on shark attacks: while a shark attack is a horrific thing, there are only about ten deaths worldwide each year due to sharks.

Because they are a long-lived species and only have a few young at a time, sharks are vulnerable to overfishing . Many are caught accidentally in fisheries targeting tuna and billfish, and a growing market for shark fins and shark meat for restaurants is also impacting different species. One threat is the wastefulness of shark finning.

10) Shark-Finning

Among the human threats sharks face is shark finning, a cruel practice that involves removing the lateral and dorsal fins and the lower part of the caudal fin of a shark by commercial and other fishing operations around the world. After the shark is caught and its fins removed, its body, which is most likely still alive, is often thrown overboard  – to save weight and cargo space. 

The practice is believed to have originated in China around 1000 CE, primarily as a means of providing fins for shark fin soup served to guests at social occasions – where the dish is symbolic of the host's status. Although most shark fin products are traded through Hong Kong, some are sent to local markets around the world that supply restaurants. The annual global demand for shark fin soup results in tens of millions of sharks being harvested each year .

Campaigns by animal rights groups and environmentalists have discouraged the consumption of shark fin soup. Since 2011, some restaurants around the world have removed the soup from their menus , and since 2012, it has not been served at official Chinese state functions.

Shark fin tissues are known to contain the neurotoxin BMAA (beta-methylamino-L-alanine), produced by cyanobacteria  (blue-green algae). Consumption of water and food contaminated with BMAA has been linked to some forms of neurodegenerative diseases in humans .

11) The Conservation Status of Sharks

The International Union for Conservation of Nature (IUCN) has assessed more than 60 species of pelagic sharks and rays. About 24% of them are classified as Near Threatened, 26% as Vulnerable and 6% as Endangered ? globally. About 10 species are classified as Critically Endangered .

12) How to fight against shark finning?

Although banned by the European Union in 2012, shark finning persists in Asia and other regions of the world. As you may have seen previously, this practice does not leave people indifferent! But fortunately, without giving up everything, going to the other side of the world? and fighting against all this, we can act differently.

First of all, the simplest and most obvious thing is to not consume shark fins or shark fin soup. Then, you can raise awareness among those around you, and also be careful not to consume fish or marine products that could cause the death of dolphins, sharks or other marine creatures in the nets of these large commercial fishing boats. Another solution, quite well known to the general public, is to support the international organization  Sea Shepherd, which aims to preserve the oceans through its various actions, its global presence and its fleet of ships.