Is a shark a fish or a mammal?

In this blog post, we will answer the question “Is a shark a fish or a mammal?” and learn more about the whole physiology of sharks, including why they are known as the terror of the seas.

Is a shark a fish or a mammal?

Sharks are FISH. Sharks belong to a group called cartilaginous fish. They receive this name because they do not have bones forming their skeleton, which is predominantly made up of cartilage. Sharks, stingrays and chimeras are representatives of this group of fish. Most cartilaginous fish are members of marine species.

Another characteristic of this group is the presence of tooth-like plaques that cover the body of these animals, the so-called placoid scales. In sharks and stingrays, placoid scales cover a large part of their bodies. In chimeras, however, they are present in specific body portions.

Sharks are distributed throughout the majority of oceanic environments, from the shallow coastal to great depths and different temperatures.

General characteristics of sharks

Several features helped sharks gain sovereignty in the oceans, some of them are:

–       Great mobility;

–       Predatory specialization;

–       Sophisticated reproduction mechanisms.

Great mobility

Evolutionarily, sharks have succeeded throughout history due to their great mobility. Features such as extremely strong caudal fins, pectoral fins, and extremely hydrodynamic body shape.

Predatory specialization

When we think about sharks, a huge amount of teeth comes into our minds. This is due to the rows of teeth they own and their rapid speed of teeth replacement. Replacement occurs every 8 days and these are attached and connected in a bundle of ligaments present in their jaws.

Some species can have approximately 60 teeth. But in the case of the great white sharks, this number can be 50 times higher, reaching up to 3,000 triangular and very sharp teeth. It

is estimated that a shark can lose up to 30,000 teeth over its lifetime. In addition, the shape of their teeth is related to the types of prey they eat. 

Sharks can detect movement in the water, such as an animal struggling around and vibrations caused by natural movement nearly 1,000 meters away. There is also the shark’s sixth sense that we will see later in this post.

Sophisticated reproduction mechanisms

The reproduction of sharks is quite peculiar when compared to other fish species. This is because the type of fertilization is internal with copula and males have anatomical structures called claspers, used in mating. Sharks have several spawning strategies, including whether to produce eggs or not.

Do sharks lay eggs?

The answer is somewhat complex, some sharks do lay eggs, but others do not. It depends on the shark species, each of them has specific reproductive strategies. 

To try to unravel this answer, we first need to understand that sharks have been on our planet for a very long time, they surged even before dinosaurs. This permanence of sharks on Earth is related to their diverse reproductive strategies.

Oviparous sharks

Oviparous sharks have embryonic development within an egg, laid eggs have a thick layer that is resistant to predators. They are usually left in safe places by the mother. A female can lay up to 100 eggs after fertilization.

If you are thinking of that traditional egg shape, as in the case of birds, forget it! Shark eggs own much more peculiar shapes. They can be rectangular with sharp corners at the ends, which may also be known as “mermaid bag”, but may also have spiral shapes and can come in different colours, according to the species.

Viviparous sharks

There are species of sharks that do not generate eggs, some of them are viviparous. Viviparous sharks are those that undergo embryonic development within the maternal body. During this reproductive process, the offspring are nourished by the mother’s body.

A female shark can mate with multiple males and generate up to 12 embryos from different fathers, but few develop, usually two at the most, due to embryonic or intrauterine cannibalism. The babies devour each other inside the uterus of the mother. After giving birth, the female does not perform any parental behaviour. Thus the newborns are abandoned right after being born. The gestation period can last from 7 months to 3 years.

Ovoviviparous sharks

Ovoviviparous sharks complete embryonic development within an egg retained in the female’s uterine duct. Because they reproduce in this way, they can be confused with oviparous sharks, but the difference is that ovoviviparous sharks come out of their eggs in the mother’s uterus and when they are born they are identical to adults, especially males.

As they develop inside the eggs, these species feed on the nutritional reserves contained in the egg and not on the maternal body material, as occurs with other species. This nutrient reserve is a secretion also known as uterine milk, which is rich in protein.

Shark’s Sixth Sense

Sharks have a sixth sense that helps them belong to the group of top marine predators.

Ampullae of Lorenzini

These are special sensory organs, formed by a network of channels with electroreceptors pores covered by a gelatinous substance. In cartilaginous fish, such as sharks, Lorenzini ampullae are important organs capable of detecting variations in temperature, salinity and electric currents. This organ is widely used in hunting activities, associated with the olfactive, nostrils are capable of perceiving the smell of blood from prey hundreds of meters away.

What are the best-known species of shark?

Hammer shark

This group of sharks has a flat-head and is very similar to a hammer, which favours it in swimming, prey sighting and smelling, and detecting electromagnetic fields. The hammerhead sharks feed mainly on fish, squid and crustaceans.

The hammerhead shark has a very widespread distribution and can be found in the most diverse regions. Its size varies according to the species and can reach up to 4 metres long. This group of sharks is known to be frequent in shipwrecks and have very improved social behaviour.

Tiger shark

This species is found all over the globe, mainly in tropical and subtropical regions. Tiger sharks can inhabit high seas and estuarine environments.

The largest individual registered to date was about 7 metres in length. It feeds on fish, crustaceans, cephalopods, birds, among other animals.

White shark

It is the most feared species of shark as it is a very large predator, reaching up to 6 metres long.

Although it has a cosmopolitan distribution, it is found mainly in temperate waters and several marine regions, and it can go from the shallow coastal areas to over 250 m deep oceanic areas. This species feeds on other fish, turtles, squid and octopus, sea birds, among other large animals.

Whale shark

The whale shark is characterized by its size, reaching up to 20 metres in length and 42 tons.

It lives in coastal and oceanic regions, in several parts of the globe, being more observed in regions where its preys are in abundance, generally in small fish spawning regions and zooplankton hotspots.

Shark attacks

The fear of sharks exists worldwide, but few species pose danger to humans, and attacks are relatively rare.

The top four countries in the world in terms of shark attacks are Australia, the United States, South Africa and Brazil. Most incidents are with tiger sharks, white sharks and flathead sharks.

Shark preservation

While many people are afraid of shark attacks, it is the sharks that are being slaughtered by humans. This population decline that has been occurring is caused due to overfishing, often to remove by-products, such as fins (in fin-fishing), which have a high commercial value and are used in oriental cuisine.

The destruction of coastal regions, heavily used by some marine species for reproduction, for example, is also an aggravating factor for this decline in shark populations.

Conclusion

This post responded if sharks are mammals or fish, and explained their physiology, their general characteristics, curiosities and reproduction strategies.

Frequently Asked Questions (FAQs): Is a shark a fish or a mammal?

What do sharks have instead of bones?

Sharks are part of the fish subclass called elasmobranchs, which are fish with skeletons made up of cartilage instead of bones.

What are the advantages of sharks in the aquatic ecosystem?

Sharks are of great importance for both the ecosystem and the human being. As they are large predators, they are at the top of the food chain. They also contribute by controlling the populations of their prey. In addition, they often feed on sick and old animals, maintaining the populations’ health.

When did sharks appear?

Sharks originated around 400 million years ago. A fish that lived during that period has been identified as a remote ancestor of the sharks.

What is the economic importance of sharks?

Top chain predators are responsible for maintaining the balance in the marine ecosystem. They feed on large fish and invertebrates that are less adapted to surviving, ensuring the health of their prey stocks around the world.

How do sharks move?

To move properly, sharks make powerful movements on their trunk and caudal fin. In addition to the caudal fin, sharks have dorsal, pectoral and pelvic fins, which also assist in effective moving.

Why do sharks attack?

Attacks usually occur because sharks mistake the victim for their usual prey, especially in areas where the waters are turbid. Accidents can also occur as a result of the shark’s need to protect its territory, a common attitude in several fish species.

What attracts sharks?

Sharks often follow ships in search of food scraps and waste is thrown overboard. They can even be attracted by the sound of the ship engine. Another factor contributing to the occurrence of attacks is the sewage dumping on the coast, which attracts small fish that sharks prey on.

References 

Leigh, S. C., Papastamatiou, Y., & German, D. P. (2017). The nutritional physiology of sharks. Reviews in Fish Biology and Fisheries, 27(3), 561-585.

Carrier, J. C., Musick, J. A., & Heithaus, M. R. (Eds.). (2012). Biology of sharks and their relatives. CRC press.

Top 10 Betta Fish Plants for Your Aquarium 

Carrier, J. C., Pratt, H. L., & Castro, J. I. (2004). Reproductive biology of elasmobranchs. Biology of sharks and their relatives, 10, 269-286.

Sims, D. W. (2005). Differences in habitat selection and reproductive strategies of male and female sharks. Sexual segregation in vertebrates, 127-147.

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