There isn’t much or reliable information about the social structures of sharks, given the complexity of studying their individual interactions and behavior. However, in recent years the relationships between fish once considered simple and socially primitive have been better understood.

Thus, there is evidence that some shark species manifest a diverse social structure. The traditional image of a solitary shark tends to be half true because not everyone prefers to swim, hunt and feed without any company, on the contrary: they can be territorial and establish hierarchical positions, more often around the feeding areas.

Regarding the above, many have the need to cooperate and share several activities so that from time to time they show some sociability and form large groups, made up of about 100 or more members. This kind of association is the case of hammerhead sharks (Sphyrnidae), accustomed to gather in vast numbers around areas with abundant food.

There is evidence that some shark species manifest a diverse social structure.

The information available on social structure is equivalent to knowing the habits of segregation of some species, not about all of them since it is impossible to generalize but it is possible to formulate hypotheses based on such observations. The following are examples of social behaviors discovered, which are far from the supposed brief and simple relationships between sharks:

Most white sharks (Carcharodon carcharias) are solitary, but couples sometimes swim together for a long time. Apparently, the link does not break immediately after mating. Females are at a higher hierarchical level than males, but body size is a major factor in determining who dominates whom.

They are perhaps the shark species that shows the most complex social structure. Each year couples or small groups of individuals meet whose members are almost always the same age, the same size, and the same gender.

When conflicts arise within a group, members resolve the situation through rituals and demonstrations of power; There are rarely strong physical attacks.

Blacknose sharks (Carcharhinus acronotus) can establish and maintain relationships. They form groups but are very selective, so they tend to build links only with individuals of the same size. Also, they have the habit of harassing or scare smaller Blacknose sharks. The rules within a group are usually clear. The big sharks eat before the smaller ones, which wait until they finish up under penalty of being involved in the frenzy of the moment and also become the food of its companions.

The Scalloped Hammerhead (Sphyrna lewini) usually do not gather with other animals of the same species.

The Gray reef sharks (Carcharhinus amblyrhynchos) only meet with female gray reef sharks when they have to mate.

The sand tiger sharks (Carcharias taurus) congregate in groups of 5 or 6 individuals off the Atlantic coast of the United States and stay stacked on top of each other until it is their turn to mate.

Socially active sharks have higher survival rates than solitary sharks, but it is not known with certainty why some species form groups, the factors that affect the constituents, the mechanisms of recognition among their members, the types of interactions within them nor the effects they have on their organization, structure and complexity.

Of course, these issues provide a perspective on future research, and a broader picture is expected to highlight the importance of sharks in shaping ecosystems and oceanic food chains.

Sharks don’t have to rely on each other to survive.

SOCIAL STRUCTURE OF SHARKS

Survival of the fittest is the best way to describe the social relationship of sharks.

Sharks don’t live in groups but tend to live on their own. Even the offspring have to take care of themselves from the very minute that they are born. Since they don’t spend much time together, they do not have a social hierarchy.

SHARKS ARE LONERS

Sharks don’t have to rely on each other to survive. However, some species of sharks cooperate when hunting. They can take down larger prey this way. They will hunt alone though as long as there are adequate food supplies available.

Many experts believe that this individuality has helped sharks to survive for millions of years.

Sources:

https://seaworld.org/en/animal-info/animal-infobooks/sharks-and-rays/behavior

Jeffrey C. Carrier, John A. Musick, Michael R. Heithaus. Biology of Sharks and Their Relatives, Second Edition. 2012

http://voices.nationalgeographic.com/2013/03/11/shark-have-social-networks-learn-from-friends/

http://animaldiversity.org/search/?q=shark&feature=INFORMATION

BioExpedition Publishing © 2017.

Each shark has a tail adapted to its physiology, its surroundings, and its lifestyle; Therefore, there is a great diversity of types of tails. Each one consists of a precaudal pit and an upper and lower lobe. In some cases, the spine extends to the tip of the superior fin, so the tail is called “heterocercal.”

In the body, the tail functions as a propeller. Swinging it forward and backward to advance in the water, it pushes the water around the fins creating a movement that produces elevation.

The thrust, velocity, and acceleration of the swim depend on the shape of the caudal fin, but some adaptations of the tail have purposes other than those related to swimming. For example, the Cookiecutter shark (Isistius brasiliensis) has bioluminescence in the upper and lower lobes of the tail that serve to attract the prey.

The caudal fin forms can be classified as follows:

– Typical sharks.
The upper lobe of its tails is longer than the lower lobe and turns upward at a moderate angle. Sharks of the Carcharhinidae family own these tails as well as thresher sharks (genus Alopias), which have turned their tail into a powerful prey stun weapon.

– Fast-swim sharks.
The tails of these sharks that swim in open waters are crescent shaped, and their upper and lower lobes are similar in size. Also, lateral keels extend in the precaudal pit. These characteristics favor the swimming efficiency of sharks, such as the whale shark (Rhincodon typus), the basking shark (Cetorhinus maximus) and the lamniform sharks.

– Benthic Sharks.
The upper lobe of the tails is positioned at a small angle related to the body and is quite long in comparison with the lower part. Therefore, the speed of their maneuvers and in general of their swimming is relatively slow. Its style of swimming resembles that of the eels, characterized by undulations.

– Cirrhigaleus and Squalus genera.
The upper part of its tails is longer than the lower, but this does not prevent only low velocities because the spine extends through the top lobe at a lower angle than the lobe itself.

– Angel Sharks.
In the case of these sharks, the lower part is larger than the upper one, and because of this, they can take off quickly when attacking some prey from below.

Sharks that have the upper lobe larger than the lower one have the advantage of getting more energy in situations that require rapid or sudden speed bursts.

Despite the differences, there are practically no extensive studies that address the morphology or evolution of shark tails concerning their lifestyle.

In this image, you can see the tails of various species of sharks, and according to the form, it is possible to get a general idea about the characteristics of their swimming.

Sources:

https://seaworld.org/en/animal-info/animal-infobooks/sharks-and-rays/physical-characteristics

https://en.wikipedia.org/wiki/Shark_anatomy

https://en.wikipedia.org/wiki/Shark#Tails

http://www.nmfs.noaa.gov/ia/species/sharks/fin_guide.pdf

http://www.sharksavers.org/en/education/biology/shark-biology/

BioExpedition Publishing © 2017.

Fossil evidence shows that sharks populated the planet before the dinosaurs existed, that is, more than 400 million years ago. Since the skeleton of the shark is composed of cartilage, the only thing preserved from prehistoric sharks are fossil samples of the dermal denticles and teeth, but there are some exceptional cases of preserved skeletons.

The first fish appeared on Earth about 510 million years ago. They were small, jaw-less beings called ostracoderms. Evolution did its job, and that’s when the first prehistoric sharks first appeared.

The way in which we know them today is believed to be the results of evolution that started about 100 million years ago.

Of course, not all of the sharks that once were on Earth exist today. Fossils prove that many species have become extinct.

The first fish appeared on Earth about 510 million years ago.

ANCESTRAL SHARKS DOMAIN.

The oldest fossil of a prehistoric shark dates back 455 million years and consists only of dermal denticles. This animal lived during the Ordovician period in an environment devoid of terrestrial plants and vertebrates. However, for some paleontologists, the origin of the fossil is not clear, and it is preferred to rely on another whose age is estimated at 420 million years, therefore, it was an inhabitant of the Earth during the Silurian period.

Shark teeth of the Devonian period were found in modern Europe and lived there more than 400 million years ago, but it is the only fossil known about them, and it comes from the remains of their teeth. These are small and measure approximately 4 millimeters wide, so the owners had a length not more than 30 centimeters.

Perhaps the most recent prehistoric sharks are those that belonged to the extinct genus Cladoselache, who lived about 370 million years ago. Their fossils were found in various areas of the United States. The appearance was not ferocious: they were about 1 meter in length, had triangular fins and light jaws, as well as a caudal fin resembling that of present white sharks.

Other paleontologists point to the Mcmurdodus, whose fossil record indicates its presence 390 million years ago. At that time, sharks were predators living near the shores. From the Carboniferous period, sharks diversified and scientists call this stage “the golden age of sharks.” There were sharks of varied shapes and sizes, to the point of being entirely different to the modern image of them. For example, the Stethacanthus had small teeth in the head!

During the Jurassic and Cretaceous periods, species of sharks called Hybodonts and Xenacanthis flourished. Some scientists argue that modern sharks evolved from the Hybodonts, but so far there is not scientific confirmation. The period of diversification was over, and the Sharks remained for several million years without drastic evolutionary changes.

At the end of the Jurassic, there were species with jaws more flexible than those of the Hybodonts, and this was a great advantage when feeding. As a result, the Hybodonts group began to shrink, and sharks similar to modern sharks began to spread.

THE MODERN SHARKS ARRIVE.

The origin of modern sharks dates back 100 million years when many species had already become efficient predators and fast-swimming animals. In fact, most of the families of modern sharks arose during this period.

Towards the end of the Cretaceous, about 65 million years ago, a worldwide catastrophe (probably the fall of a meteorite on Mexican coasts) eliminated most of the species that populated the world at that time, including dinosaurs and mammals. Some shark species survived, and are roaming the oceans. Among the survivors are the cow sharks (Hexanchiformes) and the frilled sharks (Chlamydoselachiformes), which still bear much resemblance to their extinct relatives.

The origin of modern sharks dates back 100 million years.

One of the best known modern sharks is the Megalodon (Carcharodon megalodon), extinct 1.6 million years ago. Fossil records indicate that it probably evolved from the Cretolamna, which lived about 60-100 million years ago. The Megalodon was a shark of titanic proportions (with a length of 16 to 20.3 meters) and large jaws and teeth. At present, some think that the white shark (Carcharodon carcharias) and the Megalodon share some distant relative, but the Carcharodon carcharias is not a direct descendant of the Megalodon shark.

In total, sharks have survived five mass extinctions over the last 439 million years. They are survivors!

SHARKS ADAPTATIONS

Some of the early forms of sharks were quite small. There is evidence to suggest they looked more like fish. Some others were similar to eels; their faces featured noses that were round rather than pointed. Some others were huge with extraordinary dimensions, much larger than the largest shark available today.

Sharks did not have a brain as large as they have today. More than 2/3 of the size of their brain is for the senses, and this is one of the areas where sharks have significantly been able to evolve and to become a more intelligent species.

Their teeth were also different than they are today. Instead of being razor sharp they were smooth. However, these early ancestors also had the rows of teeth that replaced the missing ones like they do today. This kind of teeth could indicate that prehistoric sharks fed on plants instead of being meat eaters as some are today.

Sources:

https://en.wikipedia.org/wiki/Shark#Evolution

http://www.sharksavers.org/en/education/biology/450-million-years-of-sharks1/

http://sharkopedia.discovery.com/shark-topics/prehistoric-sharks/

www.sharks.org/blogs/science-blog/400-million-years-and-counting

BioExpedition Publishing © 2017.

Most sharks make most of their activities during the afternoons and prefer to hunt at night. They are usually solitary: they swim and hunt alone most of the time, but they meet with other sharks in some special cases, like the period of mating or the zones abundant in food.

Some species show a social structure based on groups called “schools” without a strong reason for this because sharks do not need special protection against predators. The sharks that socialize usually segregate according to the size of the individuals.

Some species show a social structure based on groups called “schools”.

Hammerhead sharks are an obvious example of social sharks with a clear hierarchy, and it is common to find them gathered around islands and other areas with an abundant food source. Other species try to be careful with them, as their excellent maneuverability given by the shape of their heads is advantageous compared to other sharks.

However, some species hunt in groups when it comes to obtaining a large prey. Sharks form large groups, and all cooperate first by surrounding the prey, and then, when they are already captive, they gradually approach it until one of the predators takes the first bite. It is then that the others begin to attack and end with the prey. This cooperative behavior is extremely rare.

The methods of hunting in this predators are different and make the element of surprise their primary weapon. Benthic sharks, for example, camouflage themselves beneath the bottom of the ocean and if they detect a possible prey, they stay still, waiting for the animal to get close enough to attack it from below and occasionally, to swallow it whole.

In contrast, pelagic predators must cautiously approach the prey and hurl themselves at it before it knows what is happening. Sometimes, before eating, they perform an entire display of exaggerated movements that they intend to threaten. If it is a large animal, sharks wait for it to bleed to death and then eat it.

Hunting activities require a lot of energy, and as a consequence, large predatory sharks feed only a few times a week and are not always eating, as is often believed. Small species, on the other hand, eat a couple of occasions during the day since their prey are also small.

Their feeding behavior changes according to the presence of prey and the competition for them. If sharks stimulation is intense, it is possible that the excitement increases and they are victims of a frenzy that results in cannibalism.

Migration is a fundamental activity for some sharks, whose motive is the search for food as their prey also migrate and to reach the areas where they mate and spawn. Their journeys can take them through all the ocean basins traveling thousands of kilometers during a year. Sharks remember the migration patterns of their favorite prey and do not hesitate to follow them, but their migratory patterns become more complex than those of birds.

Sharks remember the migration patterns of their favorite prey.

Occasionally they congregate around fishing boats because they remember that the fishermen throw discarded animals.

Several species can travel at the same time, and until recently, some thought that most stayed in one place, but that is not accurate. For example, blue sharks (Prionace glauca) can migrate from the United States to Brazil, which is a significant distance. Another famous migrant is the whale shark (Rhincodon typus), which goes to specific areas to spawn in corals.

It is worth remembering that not all sharks are aggressive and they do not attack for pleasure. Many species are characterized by their docility and even by their timidity. As an example, the shark whale mentioned above is very peaceful, quiet and does not react with fear or aggression to the proximity of humans.

Sources:

https://en.wikipedia.org/wiki/Shark#Behavior

http://www.defenders.org/sharks/basic-facts

https://seaworld.org/en/animal-info/animal-infobooks/sharks-and-rays/behavior

http://animaldiversity.org/search/?q=shark&feature=INFORMATION

http://www.arkive.org/

BioExpedition Publishing © 2017.

There are more than 400 shark species in the ocean, but they all have the same basic anatomy. This astonishing anatomy masterfully designed by evolution is what sets sharks apart from other types of aquatic life including whales and dolphins.

Understanding the shark anatomy will give you a glimpse of the evolutive adaptations that have made sharks the flawless predators they are today.

Despite there are hundreds of shark species, it is possible to describe some general characteristics that distinguish them from the rays and chimeras, their closest relatives.

While it is true that most sharks have a fusiform body, not all are like this. The body of sharks comes in a variety of sizes, shapes, and colors and it is sometimes difficult to recognize a shark from other similar species.

All sharks have cartilage skeletons instead of bones.

There are seven anatomic characteristics of sharks:
1. Cartilage Skeleton.
2. Skin covered with dermal denticles.
3. Powerful Jaws.
4. Efficient Liver.
5. Anatomic Tail.
6. Dynamic Fins.
7. Highly Developed Senses.

Their highly developed senses are the most important anatomic structures of sharks.

SHARK SENSES
1. Lateral Line.
2. Ampullae of Lorenzini.
3. A powerful sense of smell.
4. Good Eyesight.
5. Great Hearing.

CARTILAGE INSTEAD OF BONES.
The skeleton of the sharks is mainly cartilaginous tissue, that is, it is made up of cartilage. Thanks to this condition they stay buoyant and can float. Otherwise, they would sink because of the lack of swimming bladder.

All sharks have cartilage skeletons instead of bones; this is very different from humans and most types of land animals. This cartilage skeleton is what allows them to move at unbelievable speeds through the water.

Anatomy of hammerhead shark.

SKIN WITH DENTAL DERMICLES.
Since sharks do not have bones, they rely on the dermal denticles that cover their skin to help them keep the right shape.

The texture of their skin is rough since it has small scales similar to teeth, called dermal denticles. The aligned structure of these denticles is useful for channeling water over the body and minimizing frictional resistance. The denticles grow throughout the life of the sharks as they grow.

Dermal denticles also reduce friction to water making easier to swim and save energy; these structures also help to protect the shark skin from injuries and several elements in the water.

Sharks don’t have the jaws attached to the skull.

JAWS & TEETH
Sharks don’t have the jaws attached to the skull. Instead, they move as separate parts. The upper and the lower jaw can work independently without the other. This versatility provides sharks with a very powerful pull and latch onto what they want fiercely.

Sharks have many rows of teeth, and its shape depends on the shark’s diet.

Shark teeth are extremely sharp as most sharks are meat eaters. They can rip through flesh and bones instantly without any struggle at all. Sharks lose their teeth all the time, and one from the row behind moves forward to replace it, so they are always geared with a full army of them to attack. They can lose their front row of teeth every couple of weeks to a month. Therefore, a shark will have around 30,000 teeth in its lifetime.

EFFICIENT LIVER.
Just like humans and many other animals sharks have a liver. They use it differently, though. Shark liver can store oil in it for a very long time.

The oil stored helps sharks to eat less often. Sharks can survive with this oil reserve for weeks, months, or even a year before they have to feed again. Once the level of oil in their liver gets decreases, they get back to hunting. A shark never depletes the stored oil in its liver.

TAIL
Each species of shark has a unique tail design; this has to do with the overall size of the shark as well as the purpose of the tail. In many instances the tail allows sharks to move very fast in the water. Other tails help balance their movements so that they can glide along gracefully.

Tails give them the ability to make flexible movements, so they take advantage of their cartilage skeleton.

FINS
All species of sharks have fins. They help them to move along in the water as well as to pick up vibrations. There is a dorsal fin on the back, sometimes two of them depending on the species, that helps with balancing. The anal fin on the underside of the shark serves the same purpose.

The anatomy of a shark resembles the design of an airplane; they have a tail and the wings to help balance everything and to give it speed and while there is no evidence that the dynamic design of airplanes came from the shark anatomy it does make sense.

Leopard Shark – Triakis semifasciata.

BREATHING.
Sharks breathe by extracting dissolved oxygen in the water. The process is like this: when the water enters the mouth, it passes through the gills and exits through the gill slits, which in most cases, are visible.

The body of sharks has three parts: head, trunk, and tail. Let’s see their characteristics:

The body of sharks has three parts: head, trunk, and tail.

Morphology of sharks.

HEAD
1.-Nassal openings.
Located under the snout, they contain a pair of olfactory sacks necessary for the detection of odors. When a shark swims, water flows into the nostrils and enters the olfactory sacs, so they can detect smells at a great distance.

2.- The Ampullae of Lorenzini.
They are groups of sensory pores that cover most areas around the mouth and nostrils, which can detect the electric fields produced by the movement of other animals.

3.- Eyes.
Sharks have a pair of eyes located on the sides of the head to see forward, backward, upward and downward. Each eye has an upper and lower eyelid, but they have no protective function. In contrast, some species have a nictitating membrane to protect their eyes.

4. Gill slits.
They are openings located behind the head. The gill slits extract oxygen from the water and release that water with carbon dioxide. The number of gill slits is variable, but they have between 5 and 7.

5.-Spiracles.
They are holes behind each eye of the shark, which are used to supply oxygenated blood to the eyes and brain. Some species lack them or exhibit reduced spiracles.

6.-Jaws.
Sharks have two movable jaws attached to the skull by muscles and ligaments. Both can move forward and backward.

7.-Teeth.
Almost all sharks have several rows of teeth in each jaw, but they are not fused to it so throughout their life the teeth fall and re-emerge. A fallen tooth is usually replaced within 24 hours.

Facts on video

TRUNK
8. Pectoral fins.
Cartilage rods support all fins. This pair of fins is behind or below the gill slits. They provide lift and are useful when the shark turns during swimming.

9.-Dorsal fins.
Most sharks have two fins along the dorsal line, the first larger than the second, which stabilize their movements. If they have 2, they are called the first and second dorsal fins, but not all sharks have the second.

Most sharks have two fins along the dorsal line, which stabilize their movements.

10.-Pelvic fins.
This pair is located on each side of the back of the body and also work to stabilize the shark’s swim. The males have an extension at the inner edge of the pelvic fins, which make up the claspers.

11.-Anal fin.
It is not present in all species. This single fin is small and is located on the back of the shark, on the ventral surface and behind the anus. It also provides stability when swimming.

TAIL
12.- Precaudal pit.
It is the posterior narrow zone that joins the caudal fin with the rest of the body of the shark.

12.-Caudal keel.
It is a lateral crest located on the peduncle that supports the caudal fin. A shark may have 2 or more keels.

13.-Caudal fin.
Helps propel the shark. In this link, you can know in depth the information on the different types of fins.

Sources:

https://en.wikipedia.org/wiki/Shark#Anatomy

https://seaworld.org/en/animal-info/animal-infobooks/sharks-and-rays/adaptations

https://seaworld.org/en/animal-info/animal-infobooks/sharks-and-rays/anatomy-and-physiology

World of Animals Magazine. Issue 1. Imagine publishing.

http://faculty.bennington.edu/~sherman/the%20ocean%20project/shark’s%20electric%20sense.pdf

https://en.wikipedia.org/wiki/Ampullae_of_Lorenzini

BioExpedition Publishing © 2017.

THE POWER OF SENSES.

In the case of sharks, they are known to have shown curiosity and tendency to play and can communicate to defend or share food by using their senses of sight, hearing, smell and electro-perception. The most significant reason they have to do this is their need to get a partner.

There is a problem that prevents the observation of sharks to study their communicative habits and is that many individuals are solitary creatures. Most can detect low sound frequencies and weak electrical signals detected by their Lorenzini ampoules.

Sharks live in groups called “schools, ” and it is common for them to communicate with each other either by sight or by arching their body. This kind of communication is common in the gray reef shark (Carcharhinus amblyrhynchos), which also perceives its surroundings thanks to its excellent smell sense and a sharp ear.

Olfactory communication channels are necessary for reproduction and the location of prey. As their nostrils are distant from each other, they take advantage of the time it takes the smell to reach each olfactory canal to examine where is the origin of the scent they perceive.

AGGRESSION-COMMUNICATION

One of the most common communicative strategies among sharks is aggression towards other individuals at the time of feeding, driven by the frenzy of the act. They are more likely to physically attack during “Lunchtime” than when defending their territory.

Most can detect low sound frequencies and weak electrical signals.

They also send non-verbal messages by performing some actions. For example, when the males are about to mate, they hold the female tightly and can bite it on the back, as if to say, “Stop, not yet.” Other species such as the Draughtsboard shark (Cephaloscyllium isabellum) can inflate its body when faced by a predator.

Although there is information indicating that these animals use their body movements to externalize their aggressive intentions, there is not much information about other methods of communication.

THE SILENCE OF SHARKS

Sharks can produce sounds but no vocalizations because they lack the necessary organs to emit sounds from the throat. Some reports from New Zealand say that the whale shark (Rhincodon typus) “barks like a dog” but there is not much information about this, and there is not a scientific confirmation either.

Sources:

https://seaworld.org/en/animal-info/animal-infobooks/sharks-and-rays/behavior

http://www.sharkproject.org/haiothek/index_e.php?site=funktion_1

http://animaldiversity.org/search/?q=shark&feature=INFORMATION

http://www.pbs.org/wgbh/nova/sharks/masters/myths2.html

BioExpedition Publishing © 2017.

Talking about the distribution of sharks is simple. These animals are present in all the oceans of the planet and all the seas. They usually live in salt water, but some species inhabit temporarily freshwater lakes or rivers, such as the bull shark (Carcharhinus leucas) and the Ganges shark (Glyphis gangeticus).

Its presence in any part of the world is proof of its adaptability and is indicative of the vast variety of species that exist. They can live in a wide range of habitats at different depths, temperatures, light conditions and salinities, from warm waters to polar seas. Some live in shallow areas or near the coast, but others prefer to live in the open sea, in deep waters and even on the ocean floor.

Sharks and in general any species inhabiting the upper parts of the ocean are pelagic species such as the white shark (Carcharodon carcharias) and the peregrine shark (Cetorhinus maximus). The sharks inhabiting regions close to the ocean floor are called benthic.

Usually, sharks roam at depths no more than 2,000 meters, and it is very unlikely to see them more than 3,000 meters below the surface. The maximum depth recorded, reached by an individual is about 3,700 meters. They prefer areas near the surface because there are a lot of nutrients that come from the rivers. These nutrients favor small species, from which other larger animals are fed and so on until they reach the sharks, located at the top of the aquatic food chain.

Most predatory sharks like warm tropical waters, such as the oceanic whitetip shark (Carcharhinus longimanus) and the blue shark (Prionace glauca). The latter stays swimming in the open sea during all its life.

Obviously, habitats are different for each species and are often related to the behavior of sharks regarding feeding, breeding grounds, cleaning stations and migrations. For example, the abundance of food sources (according to the species) can modify the migration patterns or the fixed habitat of individuals.

Clear examples of the above are the following cases:
– When young seals begin to enter the sea, the abundance of white sharks increases in the areas where seals swim.

– The presence of zooplankton, raised by the Gulf Stream, make the basking shark (Cetorhinus maximus) roam the areas bordering the largest concentrations from March to April as it feeds on tiny organisms.

One final important thing to mention is that sharks are not usually static beings but tend to move around different habitats. Therefore, not seen them in a particular area for some time, does not mean that they are not there or that they can not be there.

Many species of sharks migrate for mating, so you will find them in different locations when it is time for reproduction. Some migrate to warmer waters for this, which is evident every year on the coasts of Florida and California during the mating season.

Some species of sharks now dwell in places where they didn’t live before. Scientists think that this is because the food availability in their typical habitat is getting scarce because overfishing and they have to follow their prey and probably also because of the effect of global warming on prey distribution.

Sharks aren’t territorial like many other animals are. The females do not care for their young, so newborn sharks care for themselves since they are born.

Sharks are excellent adapting to new situations and environments which is the main reason why they have survived in the oceans for millions of years. Most species of sharks live in the oceans. However, some of them are known to live in bodies of freshwater.

Sources:

https://seaworld.org/en/animal-info/animal-infobooks/sharks-and-rays/habitat-and-distribution

http://www.bbc.com/earth/story/20150611-sharks-epic-migration-revealed

https://en.wikipedia.org/wiki/Shark

http://sharkopedia.discovery.com/shark-topics/habitat-location/

BioExpedition Publishing © 2017.

They aren’t territorial like so many types of animals are. So they are free to roam the waters. The females don’t stay to care for their young so for those species who lay eggs, they have not trouble placing them in a hidden location and then being on their way. They young sharks care for themselves from the very moment they hatch.

Sharks easily Adapt

Sharks are very good at adapting to new situations and environments which is why they have survived in the water for millions of years.

The habitat of a shark depends on the species though. Sharks are cold blooded, and that would usually imply that they would search for areas where the temperatures are very cold. However that is not true as there are sharks living in warm, temperate and cold waters.

It is understood that sharks are going to live where the water is deep enough that they can be well hidden. This allows them the ability to swim freely around in the body of water. Of course they need to reside where they can find enough food to survive on. Since sharks have very few enemies they are able to move freely around in the water as they desire. They are loners for the most part so they don’t travel together.

However, many sharks live closer to the surface in order to find the sources of food they are after. They often don’t eat daily though so they can dwell deeper in the water in between feedings. They will follow the trail of food when they need to so they can travel hundreds of miles to find what they need in order to survive. Generally the more food they can live on in a general area though they more they will remain in one spot.

Most species of sharks reside in saltwater which means they are found in the oceans. However, some of them are known to live in bodies of fresh water.

There are many who worry that humans are destroying the natural habitats of sharks. We continue to pollute the waters and to remove various sources of food from the water for our own use. As a result sharks have to move to locations that were once free of them if they wish to continue surviving.

Some sharks now reside in fake habitats along with other types of aquatic live. This is to preserve them. Most are in exhibits at very expensive locations found around the world.

Millions of people pay to go see them up close though as they walk through tunnels with the sharks on both sides swimming around. Others are carefully observed by researchers in order to learn more about certain species of sharks. Not all species do well in captivity though so the process is very selective.

Even with specific locations where sharks are known to live, it is important to always keep an eye out for them when you are in open bodies of water. Sometimes they travel to other destinations. While you don’t want to be afraid of the water due to the risk of a shark being present, you do want to take some precautions.

Sources:

https://seaworld.org/en/animal-info/animal-infobooks/sharks-and-rays/habitat-and-distribution

http://www.bbc.com/earth/story/20150611-sharks-epic-migration-revealed

https://en.wikipedia.org/wiki/Shark#Range_and_habitat

http://www.iucnredlist.org/

BioExpedition Publishing © 2017.

The reproductive habits of sharks determined by a series of biological characteristics cause a low reproductive potential of some species. General aspects of their reproduction are slow growth and late sexual maturity, very long reproductive cycles, low fertility and relatively extended longevity.

These factors explain why the survival of many species of sharks is in great danger; which combined with human practices such as indiscriminate hunting, show the urgency to end with a problem that in the long run, will affect more species of animals.

One of the main differences between sharks and other animals is that the males have claspers that they use to get the sperm into the females. Sharks have two claspers one on each side of their body.

Broadly speaking, reproduction in sharks may be sexual or asexual depending on the species. It is very different from the reproduction of bony fish since eggs (in particular cases) tend to be larger and in much less quantity.

SEXUAL REPRODUCTION

In these cases the fertilization is internal. At the back of the pelvic fins are the male reproductive organs called claspers, which transfer the sperm to the female.

Is common that the male bites the female shark to get its interest or to immobilize it.

The mating usually consists of the insertion of the clasper within the oviduct. In some species, male sharks hold firmly to the female but in other species both swim together while mating. Courtship processes are also varied, but it is common that the male bites the female shark to get its interest or to immobilize it.

The females emit various types of chemicals into the water during the mating season. Since they are usually living apart from males, these chemicals attract them together.

The males often bite females during the intercourse to help keep the two sharks entwined throughout the process. The females aren’t harmed by this biting, although their skin shows the marks from the bites.

Both the number of offspring and the length of the gestation periods vary widely according to the species. It is possible that a female of some species may have only two offspring, but some other species may have between 40 and 80. It is also possible that the gestation period lasts from 3 to 4 months or up to more than two years. Large sharks often have a gestation period longer than the small species and reproduce every two years, while the little ones have offspring each year.

Shark eggs.

The shark reproduction can be:

LAYING EGGS (OVIPARITY)
After mating, the female deposits the eggs in rocks or algae, and occasionally in some sacks known as the mermaid bags. The embryos develop inside the eggs and feed on the yolk.

In total, about 30 percent of sharks are oviparous.

GIVING BIRTH TO LIVE SHARKS (VIVIPARITY).
The female gives birth to live offspring, just like mammals, and they are born fully developed. There is a placental link to the eggs since they feed on the placenta. Specifically, the yolk sac develops within a placenta that is attached to the wall of the uterus and gives the offspring the nutrients necessary for their growth.

Examples of viviparous are hammerhead sharks (Sphyrnidae family), the bull shark (Carcharhinus leucas) and the blue shark (Prionace glauca).

These species give birth to live sharks which they abandon after the delivery. Newborn sharks instinctively know how to survive alone. Shark mothers do not take care of their young.

EGGS DEVELOP INSIDE THE MOTHER (OVOVIVIPARITY)
Most sharks have ovoviviparous embryonic development, as the eggs hatch within the oviduct of the female although she does not provide any food directly; the young feed on the egg yolk of the yolk sac and the fluids that the walls of the oviduct secrete. When they are ready, they hatch inside the mother, and the pups born alive and fully developed.

Some species practice a form of cannibalism known as oophagy, which means that they will eat the remaining eggs that have not hatched to end their development.

ASEXUAL REPRODUCTION

This type of breeding does not seem to be common since there are very few documented cases of species that reproduced in this way. It is strange because the asexual reproduction is not frequent in nature and usually appears in organisms less developed than the sharks. It is likely to occur when no male is available.

There are no further details about this process, but observations showed that females could conceive themselves through parthenogenesis, conclusion achieved when tests revealed that the offspring of a female had no genetic information from any male. Therefore sperm transfer was never performed.

Asexual reproduction appears to reinforce defenses against threats to the same species by diminishing the genetic diversity.

Sources:

https://en.wikipedia.org/wiki/Shark#Life_history

https://seaworld.org/en/animal-info/animal-infobooks/sharks-and-rays/reproduction

https://seaworld.org/en/animal-info/animal-infobooks/sharks-and-rays/birth-and-care-of-young

http://animaldiversity.org/

http://www.sharksavers.org/en/education/biology/shark-and-ray-reproduction/

BioExpedition Publishing © 2017.

Sharks owe much of their effectiveness as predators to their good sensory system. Their senses are not integrated but depend on each other to provide reliable acuity. Sharks can smell thousands of times better than any human being and can detect at long distances chemicals in the water.

The purpose of their senses is mainly survival and, primarily, on the quest for food. While their senses are more sensitive than those of humans, they do not have only five, but one more.

Vision

Not all sharks have the same vision capability. Some are more sensitive to light while others have poor sight. Most have a relatively broad field of vision, as in the case of the hammerhead shark (believed to be able to see 360 °). The eyes are not unusual because they are similar to those of other vertebrate animals. They have the “tapetum lucidum” a layer of tissue that protects eyes from the marine environment, and since it is behind the retina, it reflects the light favorably and allows the sharks to have good visibility in dark conditions.

Some species that live close to the surface do not need a particularly sharp vision as their habitat reaches a lot of sunlight. On the contrary, benthic sharks have larger eyes and better vision. By the way, sharks can not close their eyelids, and some can probably see the colors.

Sharks can not close their eyelids, and some can probably see the colors.

Smell

Sharks have nostrils that they only use to smell. When they swim, water flows through the nostrils, is introduced into the nasal duct and moves the last folds of the skin that has sensory cells. Some sharks, such as the lemon shark (Negaprion brevirostris), can detect a small amount of blood in the water.

The sense of smell is directional, this means that if the aroma comes from the left side of the shark, the shark first perceives it precisely in the left fossa rather than the right and can detect the location of the source of the smelling.

It is necessary to emphasize that they are very sensitive to odors and can differentiate a prey from another shark and even from a predator and respond in a decisive way to the smell produced by a wounded or fearful prey.

Ear

Their sense of hearing is also sharp although they do not have ears, but small openings in the sides of the head and inner ears similar to those of some mammals.

Under the sea, sharks have the ability to hear sounds in a range wider than the human ear and detect the sound source from many kilometers away. This capability is especially useful when hunting because if they listen to the low-frequency sounds produced by injured animals, they do not hesitate to follow the source until they reach the prey and feed on it.

Touch

Thousands of dermal denticles cover the skin of sharks, but this feature does not prevent the loss of sensitivity. In fact, under the skin there are numerous nerve endings important for detecting surfaces and some species have a kind of beards around the mouth, necessary to stir the sand in the search for food.

Their teeth are another tool because their stretchy jaws allow a touch of identification. As their teeth have pressure-sensitive nerves, sharks often bite to test and determine whether or not what is touched is edible.

Facts on video

Taste

The sense of taste is not as developed as the other senses because it is not necessary to detect prey. However, sharks have taste buds all over the inside of the mouth and throat and not just on the tongue. Before they decide to eat something, they bite a piece and from the taste accept to eat it or to reject it if it is not part of its conventional diet.

This sense is very sensitive to the fat contained in the meat tissues, since many sharks maintain a high-fat diet.

This sixth sense is unique and allows sharks to detect small electric fields.

Electroreception

This sixth sense is unique and allows sharks to detect small electric fields generated by living beings, even if they are hidden or semi-buried in the sand.

The organs responsible for electro-perception are the Ampullae of Lorenzini and each shark has hundreds and thousands of them scattered around the muzzle. The Ampullae connect to the pores of the skin by thin tubes filled with a gelatin-like substance.

Lateral line

It is a system that endows sharks with the ability to detect changes in water pressure. It consists of a set of tubes full of liquid located on each side of the body from the head to the tail. If something approaches the animal, the water enters the pores and flows through the lateral line stimulating the sensory cells and at the same time, alerting about possible dangers.

Sources:

http://www.science.fau.edu/sharklab/pdfs/kh02.pdf

https://en.wikipedia.org/wiki/Electroreception

http://news.bbc.co.uk/earth/hi/earth_news/newsid_8376000/8376740.stm

www.discovery.com/tv-shows/shark-week/about-this-show/sharks-electroreception/

http://sharkopedia.discovery.com/shark-topics/shark-senses/

BioExpedition Publishing © 2017.