The bull shark is also known as the zambezi shark. It should not be confused with the sand tiger shark (Carcharhinus taurus), which is also called bull shark in other languages.

Many people consider the bull shark one of the most dangerous sharks in the world since this species has many attacks on humans registered, and according to the Internation Shark Attack File, it is the third species with most attacks on humans, only after the great white shark and the tiger shark. Besides, its aggressiveness and its ability to live in saltwater as well as freshwater, add further concern.

It is a member of the order Carcharhiniformes, Carcharhinidae, and Carcharhinus.

DESCRIPTION

The reason why it is called “bull shark” refers to the short, sturdy form of its body that resembles a bull, and perhaps it is also due to its hostile behavior. It has a blunt snout, rounded and wide but not very long. It has two dorsal fins with a triangular shape, of which the second is visibly smaller but the tips of both are dark in young specimens. They do not have an interdorsal ridge.

The reason why it is called “bull shark” refers to the short, sturdy form of its body.

Their eyes are quite small, which gives a clue about their limited visual sense and their preference for waters near the coasts where the prey are abundant. Instead, its movable jaws contain several triangular teeth that measure about 3 inches long.

The color of its skin is light gray, and its belly is white. There is sexual dimorphism: the female is larger than the male, as the male commonly has a length of 2.13 meters and weighs 90-95 kilograms, while the female reaches a length of 2.3-3.4 meters and weighs 129-230 kilograms.

DISTRIBUTION AND HABITAT

Bull shark distribution is along the coastal waters of tropical and subtropical seas around the world. In the Atlantic Ocean, it is located from the United States to Brazil and from Morocco to Angola. In the Indian Ocean, it is present in waters ranging from South Africa to Kenya and from India and Vietnam to Australia.

It is one of the few species of sharks prepared to inhabit saltwater and freshwater, in the latter for a long time. It dwells in waters ranging from 150 to 30 meters deep of oceans, seas, bays and harbors and even ventures into lakes and rivers.

Bull Shark – Carcharhinus leucas.

FEEDING

The bull shark is an opportunistic carnivorous predator but can eat almost anything it finds in its way including fish, rays, sea turtles, mollusks, echinoderms, young sharks (including the same species), mammals and seabirds. In the stomachs of some individuals have been found remains of hippos and humans but these are not a regular part of their diet. Their favorite fish are chelons, mullets, mackerels, and snappers. However, their food consumption adapts when they dwell in freshwater areas, and then feeds on turtles, shrimp and other species of that habitat.

It hunts only in murky waters and compensates its limited sense of sight with a keen sense of smell. If they find a suitable prey, they swim directly toward it reaching speeds of up to 19 kilometers per hour; it hits the victim, and then sink their sharp teeth into the flesh. The rest is already known.

REPRODUCTIVE HABITS

The bull shark is viviparous, and the age to reach sexual maturity depends on the geographic region in which it is located but usually the age is ten years old for males and 10.9 years old for females. Some individuals can mature when reaching eight years of age.

Mating occurs in late spring and the summer. The mating scars on the females’ body indicate that during the process, the male bites his mate.

The gestation period lasts 10 to 11 months. The female has between 1 and 13 offspring, often in brackish water or in freshwater lakes.

Probably their most notable behavior is that they can tolerate freshwater.

BEHAVIOR

The bull shark prefers loneliness than the company of other sharks of its species, so it hunts by itself. It is not migratory, but some individuals from South America travel thousands of kilometers to the Atlantic Ocean, and many often go to fresh and brackish waters to give birth.

Probably their most notable behavior is that they can tolerate freshwater. While their body adapts to this change (it reduces the amount of salt and urea to suit different salinities), it is surprising that they are not afraid of water systems that are less salty and more shallow than their natural habitat. They avoid regions with low temperatures.

It shows certain aggressiveness and tendency to live near populated areas, a fact that makes it an animal of care.

THREATS AND CONSERVATION

Commercial fishing usually does not try to catch bull sharks, but this species is classified as “Near Threatened” on the IUCN Red List without compelling reasons.

Over time it has been captured to obtain its meat, its skin and its oil for several purposes. Recreational or sports fishing is also responsible for the situation. In current times many aquariums demand the presence of the bull shark in their facilities since it is a species that adapts well to life in a closed environment and catches people’s attention. This practice still does not seriously harm shark populations but is a potential danger if demand increases.

On the other hand, roaming areas close to human populations makes it an animal more vulnerable to the effects of civilization, such as pollution and incidental fishing.

At the moment there are no specific programs, plans or actions for the conservation of the bull shark.

Sources:

http://www.iucnredlist.org/details/39372/0

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

http://www.arkive.org/bull-shark/carcharhinus-leucas/

https://www.flmnh.ufl.edu/fish/discover/species-profiles/carcharhinus-leucas

http://animaldiversity.org/accounts/Carcharhinus_leucas/

BioExpedition Publishing © 2017.

Diving with Lemon Sharks

The shallow waters of the oceans disguise this shark characterized by the yellowish color of its back. It was first described in 1868 and was called Hypoprion brevirostris, but later Hypoprion was changed to Negaprion.

It is a member of the order Carcharhiniformes, of the Carcharhinidae family and the genus Negaprion.

DESCRIPTION

It is a big and robust shark with a nozzle shorter than the width of its mouth. It reaches a length of 2.4 to 3.6 meters and a weight of 90-183 kilograms being the female slightly bigger than the male.

It has a flat head, two large dorsal fins of similar size, one convex pectoral fin and one pelvic fin with concave back edges. The upper lobe of the caudal fin is larger than the lower. Its small eyes have a retina that has in the center a horizontal band useful to provide clear visibility under the water.

Its common name comes from the yellow or brown coloration of the back of its body. Thanks to this feature can camouflage perfectly with the bottom of the ocean in shallow waters because its color is similar to the sand of the sea floor.

The largest populations of this species inhabit regions of the Atlantic Ocean.

DISTRIBUTION AND HABITAT

The largest populations of this species inhabit regions of the Atlantic Ocean, but smaller concentrations also dwell in the Pacific Ocean. In the Nearctic realm of the Atlantic, they are distributed from the coast of New Jersey in the United States to the south of Brazil, passing through the Gulf of Mexico and the Caribbean Sea. To the east, they reside around the African coasts of Senegal and Ivory Coast. In the eastern Pacific, they are located from Baja California in Mexico to Ecuador.

This shark likes to inhabit coastal shallow waters. The usual depth in which they live is around 90 meters. They dwell areas with mangrove, coral reefs and can live in enclosed bays and even near the docks.

Although it is a saltwater species, it is possible that it wanders near the mouths of rivers. Its adaptability allows it to withstand deep water with little oxygen.

FEEDING

All sharks are thought to be opportunistic predators, but the lemon shark challenges this belief. This carnivore proves to have a preference for a particular prey and with a specific size, so its food is selective. In summary, it usually “chooses” animals not fast and medium-sized.

Their hunting technique consists in harassing the prey after detecting it. Quickly approaching it, and before making any physical contact, it stops abruptly. Then it hits the victim, holds it tightly around the head, and shakes it repeatedly until it can tear off pieces of flesh.

Their favorite preys are bony fish, mollusks, and crustaceans. Occasionally they catch seabirds and even other small sharks.

BEHAVIOR

The lemon shark can be solitary or show a social behavior by grouping with up to 20 members according to sex and size. Social interaction and group life seem to improve the processes of courtship, communication, predation and protection. Similarly, offspring have a higher chance of survival when living alongside other small sharks.

They exhibit a learning capacity from such interactions.

Research of the Negaprion brevirostris brain, show that the species has an apparent preference for social interaction, cooperation and the establishment of hierarchies of dominance and stable social relations. They also exhibit a learning capacity from such interactions.

It is an active animal during the day although it prefers to rest in silence in the bottom of the ocean. It sounds strange, but this activity requires more energy than swimming as it takes a lot of effort to pump water through the gills and avoid sinking. Their activity increases during the nights and mornings.

REPRODUCTIVE HABITS

This viviparous shark reaches sexual maturity at around 6.5 years old. The embryos develop inside the mother and feed on the nutrients from the yolk sac.

Before gestation, sharks mate during the spring and summer months. The fertilization is internal, and the wounds on the female’s pectoral fins indicate that the male bites her to hold it and insert its clasper. Apparently, the female stores the sperm of several males inside so there is sperm competition. That is why a single litter can have multiple paternity.

The gestation period lasts between 10 and 12 months. The female gives birth to 4-17 offspring and then waits one year to having offspring again. Meanwhile, those newborn stay in breeding areas until they reach a length of 90 centimeters.

THREATS AND CONSERVATION

It shares some threats with other shark species; its meat, liver, skin, and fins are highly valued in the gastronomic and medicinal fields. That is why there is an intense commercial and recreational fishing that negatively affects the survival of the species. Besides, it is in danger because its habitat of mangroves and shallow waters are devastated by human activities, mainly by constructions in the coasts.

Its conservation status is “Near Threatened” on the Red List of the International Union for the Conservation of Nature (IUCN). In this regard, there are no specific plans or programs for its conservation, but it has some protection from the United Nations International Plan of Action for the Conservation and Management of Sharks.

Sources:

https://www.flmnh.ufl.edu/fish/discover/species-profiles/negaprion-brevirostris

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

http://www.iucnredlist.org/details/39380/0

http://www.arkive.org/lemon-shark/negaprion-brevirostris/

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.