Sharks: 7 basic facts

Post by DPM Diving

July 20, 2021

The Shark Week is just over and we want to honor them by sharing some characteristics and facts that make sharks unique. 

They are such special creatures that by the end of this article we promise that you will have many reasons to agree that sharks should be celebrated each and every single day of the year!

In this post you’ll discover these 7 facts about sharks:

Let’s dive into details!


Sharks are older than dinosaurs

With fossil records dating back 400 million years, sharks have outlived the dinosaurs and many other forms of life currently on earth.

Based on fossil scales found in Australia and the United States, scientists hypothesize sharks first appeared in the ocean around 455 million years ago.


Sharks do not have bones

Every shark belongs to the classification Chondrichthyes which identifies them as cartilaginous fish, fish whose internal skeletons are composed of flexible cartilage – the clear gristly stuff that your ears and nose tip are made of – rather than bone. Their cartilaginous skeletons are much lighter than true bone and their large livers are full of low-density oils, both helping them to be buoyant.


Sharks 7 senses

Sharks have the same 5 senses as we do and 2 more! They can sense electrical currents and pressure changes. Pretty cool right?

Shark’s sense of smell

Up to two thirds of the total weight of a shark’s brain is dedicated to smell. They’re super-sensitive to smells that are important to their survival. Including scents produced by potential predators, prey or a mate.

Shark’s sense of sight

With eyes positioned on the side of their head they’re able to see in almost all directions. But, their vision becomes more acute 15m from an object. It’s not until this point that sight becomes their dominant sense. 

Sharks also have a ‘tapetum lucidum’. A reflective layer of shiny cells that lies behind the retina. This improves vision in low light conditions, allowing nocturnal and deep-water species to hunt effectively.

Shark’s sense of sound

Sharks have an acute sense of hearing and are sensitive to low-frequency signals. They’re able to track sounds and are particularly attracted to sounds made by wounded prey.

Shark’s sense of touch

Sharks have many nerve endings under their skin. Some also have barbells around their mouth that can be used to probe the sand for prey. Their teeth also contain many pressure sensitive nerves. Lacking hands to feel, sharks will use their teeth to learn more about an object. 

As well as direct touch, sharks experience distant touch through their lateral line

Shark’s sense of taste

The taste organs of a shark are not as highly adapted as their other senses, because taste doesn’t help them find food.

Electroreception

Sharks have a complex electro-sensory system. Enabled by receptors covering the head and snout area. These receptors sit in jelly-filled sensory organs called the ampullae of Lorenzini.

These tiny pores are extremely sensitive and can detect even the faintest of electrical fields, such as those generated by the Earth’s geomagnetic field or muscle contractions in prey.

Pressure Changes

The lateral line is responsible for alerting a shark to potential prey and predators. It’s made up of a row of small pores that run all the way from the snout to the tail. Surrounding water flows through these pores and special sensory cells sense any pressure changes.


How do sharks breathe?

Sharks use their gills to filter oxygen from the water. Most sharks have to keep swimming to pump water over their gills.

Some species of sharks have a spiracle that allows them to pull water into their respiratory system while at rest. A shark’s spiracle is located just behind the eyes which supplies oxygen directly to the shark’s eyes and brain.

Bottom dwelling sharks, like angel sharks and nurse sharks, use this extra respiratory organ to breathe while at rest on the seafloor. It is also used for respiration when the shark’s mouth is used for eating.


Sharks reproduction

Different shark species reproduce in different ways

Sharks exhibit a great diversity in their reproductive modes. There are oviparous (egg-laying) species, ovoviviparous (the female carries the eggs inside her body) species and viviparous (live-birth) species.


Shark species

Did you know that here are more than 400 shark species!?

All sharks belong to the Chondrichthyes classification, which identifies them as cartilaginous fish, whose internal skeleton is formed by flexible cartilage instead of bone.

Chondrichthyes consists of two groups, Holocephali and Elasmobranchii (sharks and rays belong to this group). Within Elasmobranchii are eight (8) orders of sharks

Carcharhiniformes

This is the largest order of sharks. Their characteristics include five (5) gill slits (openings), moveable eyelids which protect their eyes from injury, two (2) spineless dorsal fins, an anal fin, and a wide mouth filled with sharp teeth located behind the eyes. Bull Sharks, hammerheads and black tip sharks are some of the species that belong to this group. 

sharks species: blacktip shark
Blacktip shark – Photo credit: John Kincaid

Heterodontiformes

This is a small order of sharks with only nine (9) known species. They have five (5) gill slits, a dorsal fin with a strong spine and both sharp and flat rounded teeth in their mouth.

Horn sharks belong to this category.

sharks species: horn shark
Horn Shark

Hexanchiformes

Considered the most primitive order of sharks alive today, these sharks have six (6) or seven (7) gill slits, a single dorsal fin, an anal fin and thorny teeth. Most live in cold, deep water.

sharks species: bluntnose shark
Bluntnose Sixhil Shark

Lamniformes

These sharks have five (5) gill slits, a large mouth with several rows of sharp teeth, two (2) dorsal fins, an anal fin, and are able to maintain a higher body temperature than the water in which they are swimming. Great White shark and Longfin Mako are some of the species in this group.

sharks species: great white shark
Great White Shark – Photo by Gerald Schömbs on Unsplash

Orectolobiformes

Among the most diverse order of sharks, these sharks have five (5) gill slits, two (2) spineless dorsal fins, an anal fin, and spiracles near their eyes. Most have a patterned skin, and some have barbels on their chins. Species as diverse as wobbegongs and whale sharks belong to this group!

sharks species: whaleshark
Whaleshark – Follow us on Instagram for more underwater content

Pristiophoriformes

Also known as sawsharks, these sharks have long saw-like snouts. These sharks have five (5) or six (6) gill slits, two (2) dorsal fins, no anal fin, wide pectoral fins and transverse teeth. Most sharks in this order live in tropical coastal waters.

sawshark scaled 1 - Sharks: 7 basic facts
Sawshark – Picture by Tristan Guttridge

Squaliformes

With an estimated 126 different species, this order of sharks is found in nearly every marine habitat. These sharks have long snouts with a short mouth, five (5) gill slits, two (2) dorsal fins and lack an anal fin. Some deep-water Squaliformes are bioluminescent!

gulper shark scaled 1 - Sharks: 7 basic facts
Gulper shark – Picture by Andy Murch

Squatiniformes

Sharks in this order have flattened bodies, a mouth with dermal flaps in front of a short snout, nasal barbels, eyes and spiracle on the top of their head, and lack an anal fin.

angel shark scaled 1 - Sharks: 7 basic facts
Angel shark – Pic by Lluis Masuet

Sharks conservation status

Shark populations around the world are in rapid decline.

These majestic top predators that are so essential to the natural order of marine ecosystems now face their most severe threat from overfishing. Many species are threatened with extinction

While sharks have been an irreplaceable resource for coastal communities in the developing world for centuries, this unique balance is in danger of being lost forever. The global abundance of ocean sharks and rays has declined by 71 percent since 1970, due to increased fishing and a lack of protections.

Sharks grow relatively slowly, take many years to mature and produce relatively few young. These characteristics make sharks vulnerable to overexploitation.

This vulnerability is exacerbated by the large and growing demand for shark fins and the general lack of management of shark fishing. Populations simply cannot replenish at the same rate as they are caught to meet market demand.

Overfishing is the overwhelming threat, with open ocean longlines using hundreds if not thousands of hooks each catching the greatest volume of sharks globally.

While these fisheries may be primarily targeting tuna and billfishes such as marlin, the sharks caught are an important source of income, particularly their fins

The clearance of mangroves has a negative impact on species whose young use these as nursery grounds. The loss of living coral reefs due to sedimentation and fertilizer run-off from farmland, and climate change, will often reduce the amount of prey for sharks.

Many kinds of food fishes that humans like to eat inhabit reefs, and so reefs are targeted by fishers, using types of fishing that also catch sharks.

If you love sharks as we do, stay tuned because there are plenty of posts about them coming soon! And for those who would like to become experts, take the challenge and enroll in our Shark Ecology Program.

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1 Comment
  1. Avatar

    Great article! Sharks are the best!! It’s very sad we are killing them all 🙁

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