Shark

Sharks are a group of cartilaginous, predatory fish that belong to the subclass Elasmobranchii. Research has shown that they originated more than 400 million years ago. The various shark species that exist today can be easily identified by some physical characteristics, mainly the presence of 5-7 gill slits and eight fins.

Physical Description and Appearance

Size: They vary in size, ranging from the dwarf lanternshark having a length of 6.7 in (17 cm) to the whale shark measuring 40 ft (12 m) or more.

Weight: Their weight varies widely between 0.5-1 oz (14-28 g) and 19-21.5 t (41,887-47,399 lb).

Color: Sharks mostly have a gray, tan, white, or brown body. While some may display a creamy underbelly, others may possess light or slightly dark stripes and spots.

Eyes: These are located on the sides of their head. Each eye has two eyelids that do not fully close since the surrounding seawater cleans their eyes.

Gill Slits: These are the 5-7 slit-like openings found behind the head.

Jaws: Like skates and rays, sharks have a pair of movable jaws that are not attached to their skull.

Fins: Sharks usually have eight fins made of soft filaments of an elastic protein called ceratotrichia.

Tail: The tail fin or the caudal fin is heterocercal in sharks, meaning that the upper portion is visibly larger than the lower part.

Distribution

Sharks are found in all oceans and seas, excepting the waters of Antarctic Peninsula that remain close to freezing temperatures throughout the year. They are distributed off the coasts of North and South America, Africa, Europe, Asia, and Australia. The areas where most shark species are found include the Mesoamerican Reef, Gulf of California, Coral Triangle, Galapagos, and Coastal East Africa.

What kind of habitats are they found in

Although most shark species inhabit the seas, a few exceptions including the river shark and bull shark can swim both in freshwater and seawater. They commonly live at depths of about 2,000 m (7,000 ft) while some can swim even deeper. Even though sharks are not typically found below 3,000 m (10,000 ft), a Portuguese dogfish was spotted at 3,700 m (12,100 ft).

How long do they live

While most sharks live for about 20-30 years, the spiny dogfish and whale shark have lifespans at over 100 years. Greenland sharks are known to live the longest, with an estimated lifespan of about 300-500 years.

What do they eat

Sharks are apex predators in the oceanic ecosystem with their diet consisting of mollusks, crustaceans, and fish. Mammals like porpoises, seals, sea lions, and dolphins, and large fish species including mackerel, tuna, and small sharks make up the diet of larger shark species. Some even have specific preferences, such as the tiger shark particularly likes feeding on turtles, while hammerhead shark prefers rays, and the blue shark likes eating squids.

Behavior

• Sharks are most active at dawn, dusk, and night and prefer hunting at night.
• These aquatic predators remain solitary most of the time, swimming and hunting alone. They meet only during mating season or when searching for food in a particular zone.
• Some shark species, like the hammerhead sharks, socialize and form groups or ‘schools’.
• Since hunting activities expend plenty of energy, large sharks eat only a couple of times each week. Small shark species, however, kill and feed once or twice a day because they do not have to use a considerable amount of energy in capturing small prey.
• Sharks typically migrate in search of food and can travel thousands of miles during a year.

Adaptations

• Unlike bony fishes, sharks have a sizeable oil-filled liver containing natural organic compound squalene, which alongside their cartilaginous skeleton helps in maintaining neutral buoyancy so that they do not float up or sink.
• Their jaws have one or more layers of hexagonal crystalline plates, called tesserae, made of calcium salts that give strength.
• The nasal openings, found under the snout, consist of two olfactory sacks that help them detect the scent from a great distance. Sharks can detect a drop of blood several hundred meters away. Some species like the great white can detect one part-per-million of blood in seawater.
• The cartilage in their snout is flexible and spongy that helps in absorbing impacts during hunting or when attacked by other predators.
• Tapetum lucidum, a tissue present in shark eyes, helps reflect light to the retina and increase their visibility in dark waters.
• The blood and tissues of sharks, unlike bony fish, have large concentrations of trimethylamine N-oxide and urea, which allow them to maintain osmotic balance and survive in seawater.
• Their tail provides thrust, helping sharks to propel forward with speed being dependent on its shape.
• Shark teeth are continually replaced throughout their lives, and to make the process easier and painless, their teeth are embedded in their gums instead of the jaws.
• Sharks that eat crustaceans and mollusks have dense, flattened teeth for crushing. Those that eat fish possess needle-like teeth suitable for gripping, while species that prey on mammals have triangular teeth with serrated edges for cutting flesh.

How do they Reproduce and Mate

The mating season varies across different shark species, such as the tiger shark reproduces once in three years while pelagic species like the oceanic whitetips, threshers, and silkies reproduce throughout the year.

Female sharks give out chemical signals (pheromones) to attract males. The male inserts his clasper (an extension of his pelvic fin) into the female’s oviduct to transfer sperm and fertilize the eggs inside her body.

Most sharks, including the sand tiger shark, whale shark, basking shark, and dogfish are ovoviviparous (eggs develop inside the mother). Some species such as horn shark, swellshark, catshark, and Port Jackson shark are oviparous (the mother lays eggs). Other species like the hammerhead, smooth-hound, and requiem sharks are viviparous (give birth to fully-formed, live pups).

The babies of oviparous sharks emerge after their eggs hatch in approximately 6-12 months while the gestation period for viviparous species ranges between 9 and 22 months.

What do the Baby Sharks look like

After the pups are hatched or born, they look like the miniature version of their parents and do not receive parental care.

Conservation

The IUCN included 64 species of oceanic sharks in its endangered species list in 2009 due to excessive finning and fishing. As of January 2019, 12 US states (Maryland, Massachusetts, California, Delaware, Illinois, Oregon, Hawaii, Rhode Island, Nevada, Washington, Texas, and New York) and 3 US territories (Guam, Northern Mariana Islands, and American Samoa) have made it illegal to sell or possess shark fins. Several countries, such as the Bahamas, Cook Islands, Maldives, Micronesia, Palau, Marshall Islands, and French Polynesia, have now banned shark fishing.

Shark-FAQs

1. How many teeth do sharks have?

The number of teeth varies between 5 and 15 rows in each jaw across species. Great whites, for example, have about 50 teeth at a time.

2. What is a baby shark called?

A pup

3. What predators may eat sharks?

Orcas or killer whales

4. What is a group of sharks called

A gam, frenzy, school, herd, or shiver

5. How many sharks are there in the world?

Over 400 different shark species exist in the world today.

6. Do sharks have backbones?

Since sharks are vertebrates, they have backbones that are made of cartilage instead of bones.

7. Do Sharks Sleep?

They have periods of rest. Some shark species need to swim to breathe, and they perform a specific “sleep swimming” during which their brains are at rest. According to research on spiny dogfish, the swimming movement is coordinated by its spinal cord and not its brain, which explains why the shark can continually swim while sleeping.

8. Can sharks see color?

A study has revealed that some shark species such as the tiger sharks are color blind.

Interesting Facts

• The shortfin mako is the fastest shark that exists today and can reach speeds of up to 50 kph (31 mph). Sharks, in general, swim at 8 kph (5 mph) but can achieve speeds of 19 kph (12 mph).
• Sharks have thousands of electroreceptor organs, called ampullae of Lorenzini, used for detecting electromagnetic fields produced by all living things, thereby helping them to locate prey.
• The Megalodon, an extinct species, is the biggest shark that ever lived measuring 45-60 feet in length and weighing 50-77 tons.
• Sharks are the only fish that cannot swim backward because their pectoral fins cannot curve upwards.