The Shark: Feeding, Behaviour, Reproduction

The shark, one of the numerous species of cartilaginous fish with predatory behavior which constitute the order Selachii (class Chondrichthyan). There are several hundred species of sharks, varying 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 fascinating biology.

Sharks, along with rays and skates , constitute the Elasmobranchii subclass of the Chondrichthyans . Sharks, however, are distinguished from other elasmobranchs and resemble ordinary fish in the shape of their bodies and the location of their gill slits on each side of the head. Although there are exceptions, sharks generally have tough, dull gray skin , roughened by tooth-like scales. They also typically 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 a swim bladder and must swim constantly to avoid sinking to the bottom.

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

1) The Shark in 10 Seconds Top Chrono

• COMMON NAME: Sharks
• SCIENTIFIC NAME: Elasmobranchii
• DIET: Carnivorous 🐟
• HABITATS: Marine, coastal and ocean habitats around the world
• BASIC ANIMAL GROUP: Fish
• AVERAGE LIFESPAN: 20 - 150 years
• SIZE: 20 cm - 20 m
• WEIGHT: Up to 11 tonnes
• 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 said to reach a maximum length of 20 m 🚌🚌. The smallest shark is believed to be the dwarf lantern shark ( Etmopterus perryi ), a rare deep-sea species around 20cm long.

B- The Body Structure of the Shark

Sharks are cartilaginous fish. They therefore have a body structure formed 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 approximately 1,000 species of sharks, rays and skates (as seen previously).

C- The particularity of shark teeth

Shark teeth don't have roots, so they usually fall out after about a week. However, sharks have replacement teeth arranged in rows and a new one can replace the old one within a day . Sharks have between five and fifteen rows of teeth 🦷 in each jaw, with most having five. The shark has 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, varying from gray to cream, brown, yellow, slate or blue, and are often adorned with spots, bands, mottling or protrusions. The strangest sharks 😲 are hammerhead sharks ( Sphyrna ), whose heads resemble two-headed hammerheads and have one eye on each stalk. Even stranger than the hammerhead shark is the bearded carpet shark (family Orectolobidae ), whose skin flaps and protective coloration closely resemble the sea floor. Common shark names often indicate the colors of living species, such as the blue shark ( Prionace glauca ), white shark (Carcharodon carcharias; also known as the great white shark ), and lemon shark (Negaprion brevirostris).

3) The origin of sharks

The origin of sharks is obscure, but their geological history dates back at least to the Devonian (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 🦖 (201.3 million to 174.1 million years ago) and the Cretaceous (145 million to 66 million years ago), they developed to form today's families. Like for example, the famous megalodon . Overall, evolution has changed shark morphology very little, other than to improve their feeding and swimming mechanisms. Shark teeth are very different depending on the species, whether fossil or modern.

4) Range and Movement of Sharks

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

But in general, the geographic ranges of sharks are poorly known . Their significant movements are linked to breeding or feeding activities or 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 traveling approximately 1600 km in 129 days . Advances in tracking technologies include the use of satellite tags that send a continuous signal to orbiting satellites 🛰 when the shark surfaces and thus reveal north-south movements. Transoceanic movements and patterns of wandering and migration are thus beginning to be understood for many species.

A few species in the genus Carcharhinus , such as the bull shark , move easily through salt, fresh, and brackish water . River Sharks 🏞 are small to medium in size and are exceptionally voracious and bold.

5) The Aquatic Ravenous Diet

Sharks are carnivores , and they primarily hunt and eat fish, marine mammals like 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 mostly based on foraging methods and dentition . For example, the whale shark (Rhincodon typus) and basking shark (Cetorhinus maximus), both of which can weigh several tons, are harmless giants that live on plankton extracted from the sea 🌊 by modified gills. The whale shark can reach 18 meters in length, while the basking shark can reach 14 meters in adulthood.

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 white shark, a voracious animal measuring 6 meters , which attacks seals, dolphins, sea turtles, large fish and sometimes men. The Greenland shark (Somniosus microcephalus), lazier in deep, cold waters, feeds on seals, large fish and even swimming reindeer; he can also search whale carcasses 🐋.

But in general, sharks feed on fish and often attack in schools . Deep-sea species such as mackerel shark (Lamna), mako shark (Isurus) and thresher shark (Alopias) frequently feed near the surface and are highly sought after for sport with a rod and reel 🎣. These deep-sea sharks, with their slender lines and powerful fins, feed on fast-moving tunas, marlins, etc. The species of sharks that feed on the bottom of the water have robust shapes and blunt heads. They tend to have slower habits. These shellfish eaters often have coarse teeth.

6) Shark Behavior

A- Shark Eating Behavior

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 gravid females ( carrying an embryo or fetus) when on the breeding grounds. Areas selected for calving are generally free of large sharks 🚨.

Sharks circle their prey , disconcertingly appearing 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. The activity moves from tight circles to rapid criss-cross passages 🔀. Under strong feeding stimuli, arousal can intensify and develop into sensory overload that can result in cannibalistic feeding, or "shark frenzy", in which injured sharks, regardless of size, are devoured.

B- How does the Shark Locate its Food?

To locate its food, the shark primarily uses its chemical senses, particularly smell 👃. Visual acuity 👀 is suitable for short and long distance location and distinguishing moving objects by reflection rather than color, in low or bright light. The organs in the pits above the body serve as remote touch receptors, responding to the displacement produced by sound waves. Irregularly pulsating signals below 800 hertz will quickly bring sharks to a given point, suggesting acoustic orientation from considerable distances.

Also, electroreception, and the ability to detect minute electrical activities , works effectively at close 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 consists of a network of fluid-filled channels under the shark's skin. Pressure waves in the seawater around the shark cause this liquid to vibrate 🧪. This liquid is in turn passed to the jelly in the system, which passes 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 receive the necessary oxygen . But not all sharks need to be constantly moving. Some sharks have spiracles, a small opening behind their eyes, which force water through their gills so the shark can stay still when resting 💤.

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

Information on sharks and individual and collective actions have provided a better understanding of their behavior. As large sharks prey on small ones , the habit of size segregation appears vital to their survival. In a uniform group, dominance between different species is apparent in food competition, suggesting a definite order. All sharks steer clear of hammerhead sharks, whose maneuverability, enhanced by the rudder effect of the head 🔨, gives them a swimming advantage over other sharks.

7) Reproduction, Offspring and Lifespan of the Shark

A- Reproduction and Progeny 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, the 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 🍳 from 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 old 🧙‍♂️ , and many smaller sharks can live between 20 and 30 years old.

8) Sharks and Humans

The bad publicity surrounding some species of sharks has condemned sharks in general to the misconception that they are vicious man-eaters ❌ . In fact, only 10 species of sharks out of all 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 inflict injury (especially if the shark is provoked or feels threatened).

In Australia, New Zealand, South Africa and elsewhere along the coasts where sharks are considered a nuisance or threat to humans , public beaches 🏖 often feature watchtowers, bells, sirens or nets. Since 1937, netting has been used off Australian beaches to catch sharks, using gillnets suspended between buoys and anchors, parallel to the beach and across the break line. The nets mesh sharks from all directions and although they don't touch the surface or the bottom and are well spaced out, the nets allow for simple and effective control.

The most feared species is the white shark , whose irregular presence in US coastal waters is associated with infrequent attacks along the California coast and elsewhere. Other sharks involved in attacks on humans are the tiger shark (Galeocerdo cuvier), the bull shark, the oceanic whitetip shark (C. longimanus), the blue shark and the hammerhead shark. Of course, the bigger the shark, the more fearsome the attack, but multiple small specimens can also be dangerous, confirmed by seasonal attacks off the southeast 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 put 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 . The provocation is increased by the kicking or punching vibrations people make in the water (which, to sharks, resemble the irregular movements of an injured fish), the presence of harpooned fish or bait in the water, or the presence of blood from wounds or menstruation can attract a shark 😳. Most injuries occur to the lower limbs and buttocks. It is estimated that there are around 100 shark attacks per year worldwide. Less than 25% of them are fatal , mainly following hemorrhage or shock. It should be noted, however, that shark attacks are much less frequent than other aquatic misadventures.

9) Sharks are under threat, let’s protect them!

Paradoxically, humans are a greater threat to sharks than sharks are to us . Many species of sharks are threatened by fishing or bycatch, which results in the death of millions of sharks 👻 every year. Compare that to shark attack statistics: while a shark attack is a horrific thing, there are only about 10 deaths worldwide each year from 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 incidentally in fisheries targeting tuna and marlin, and a growing market for shark fins and meat for restaurants is also impacting different species. One of the threats is the wasteful practice of shark-finning.

10) Shark-Finning

Among the human threats faced by sharks is shark-finning, an English term for a cruel practice of removing the side and dorsal fins and the lower part of the caudal fin from a shark through the operations of commercial fishing and others around the world. After the shark has been captured and its fins removed, its body, which is most likely still alive, is often thrown overboard 🗑 to save weight and cargo space.

This practice is believed to have originated in China around 1000 CE, primarily for the purpose of providing fins for shark fin soup served to guests on social occasions 🤵 where the dish is symbolic of the status of the host. 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 the harvesting of tens of millions of sharks 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 menu and since 2012, it is no longer served at official Chinese state receptions.

Shark fin tissue is known to contain the neurotoxin BMAA (beta-methylamino-L-alanine), produced by cyanobacteria (blue algae). Consumption of BMAA-contaminated water and food has been linked to certain 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 Threatened with Extinction 🚨 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 could see previously, this practice does not leave you 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 not to eat shark fins or fin soup. Then, you can educate those around you, and also be careful not to consume fish or marine products that could cause the death of dolphins, sharks or other sea creatures in the nets of these large commercial fishing boats. Another solution, well known to the general public, is to support the international organization Sea Shepherd , which aims to preserve the oceans through these various actions, its global presence and its fleet of ships.