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Lets talk about sharks! #factfriday

Great White Shark
Great white shark, photo credit : Gerald Schombs.

There are more than four hundred and fifty known species of shark in the world, with 98 different species of shark found in South Africa’s ocean. Species include, the world’s largest shark, the Whale shark (Rhinocodon typus), the incredible Great white shark (Carcharodon Carcharias), the unmistakeable Tiger shark (Galeocerdo cuvier), the Bull shark (Carcharhinus leucas) also known as the Zambezi shark, South Africa’s most loved shark Ragged toothed shark (Charcharius taurus), the Oceanic Blacktip shark (Carcharhinus limbatus), the torpedo shaped shortfin mako shark (Isurus oxyrinchus), the Dusky shark (Carcharhinus obscurus) and the impressive Hammerhead shark (Sp. Sphyrna).

‘Sharks are apex predators and play complex and diverse roles in our oceans and are important for the health and function of their ecosystems.

Fossil recordings date back to 400 million years ago, which means sharks outlived dinosaurs. They have not changed much since they evolved into their current forms some what 70 million years ago. Their ability to adapt to different ecological niches as a top predator is the reason for their survival success. Sharks are apex predators and play complex and diverse roles in our oceans and are important for the health and function of their ecosystems. Despite their incredible ability to adapt and survive, humans are putting many shark species at risk of extinction. Their numbers are dwindling because of overfishing, accidental bycatch, the high demand for their meat, skin and fins. Sharks have been feared by humans because of how they are portrayed on films. Beyond their ferocious appearance, the majority of sharks are harmless and we should not become ignorant to the conservation of sharks. Sharks are extremely fascinating and remarkable creatures. With greater understanding of the species, people would fear them less and have greater admiration for them.

Todays #factfriday I will be sharing with you some interesting facts about sharks that you may not have known.

Sharks are not blood-thirsty, human killers

Let’s start of the shark facts by talking about the misconception that sharks are blood-thirsty human eating predators. Sharks are known to investigate potential food sources; they have a keen sense of smell and are highly sensitive to vibrations and electrical pulses in the ocean. If a human is mistaken for prey, it is not surprising that their incredibly shark teeth could inflict serious and sometime fatal injuries to a person through an investigative bite. In fact, Hippos kill more humans (approximately 500 people) a year in Africa. In comparison, there are approximately 5 deaths caused by sharks a year worldwide.

Sharks do not have bones

Sharks are made up of cartilaginous tissue, they are categorized as a special type of fish known as “elasmobranch”. Rays, sawfish, and skates also belong in this category. Having cartilaginous skeletons is an evolutionary adaptation to enable them to be much lighter their other bony fish helping them to become more buoyant. Sharks also differ from other fish because they have eyelids. They are used to protect the eye when closed from abrasions when hunting or if they encounter another shark. These eyelids, however do not close all the way. Sharks that have membrane called the nictitating membrane closes, fully protecting the eye. Sharks that do not have eyelids roll their entire eyes back, exposing the white of their eyes to protect them.

Sharks have electroreceptor organs

These are visible on sharks’ nose, eyes and mouth as small black spots. These are called the ampullae of Lorenzini, the special electroreceptor organs that enhances the shark’s ability to sense their environment. The species organs can sense electromagnetic fields and temperature shifts in the ocean, helping them to find their prey.

Sharks are actually covered in tiny teeth-like scales

Sharks skin is very rough to the touch and is compared to the texture of sandpaper. Sharks are actually covered in tiny teeth-like structures called dermal denticles. They resemble hard, grooved teeth that are called placoid scale and arranged in a mosaic pattern, surrounding the body of a shark like a spiral. As sharks do not have skeletal bones, their muscles are directly connected to the dermal denticles. The denticle spines point in the direction of the tail and help improve the sharks streamline. The structure of the shark’s skin helps them to move faster in the ocean, saving energy and travel further without tiring.

Sharks do not have swim bladders

Bony fish has swim bladders, this is an oval sac lying in the abdomen that is filled y gulping in air helping them to become buoyant in water. Sharks do not have swim bladders. Instead, sharks have a large liver filled with low-density oil that helps with their buoyancy in the water. Some shark species gulp in air into their stomach, and gradually releasing the air to reach their desired depth. This allows the shark to remain neutrally buoyant in the water and they may appear to be hovering motionless.

Vertebrae rings are used to determine the age of a shark

Scientists have discovered how it is possible to calculate the age of sharks by counting layers of calcium carbonate on their vertebrae, just like counting tree rings. However, since the layers stop growing after the shark gets older, scientists have been using new techniques. Scientists have been measuring levels of carbon-14, a naturally occurring radioactive element that also is a by-product of nuclear explosions, found in Whale sharks cartilaginous vertebrae rings. After death the rings can be counted, and one ring is equal to one year. Carbon-14, from the atomic bomb tests conducted during the cold war, saturated the atmosphere and oceans, passing down through the food chain. Knowing the growth rate and age of sharks is an important tool for the conservation of the species.

Whale shark, photo credit: Sebastian Pena Lambarri.

Each spot on a whale shark is unique

Whales sharks spot pattern is unique to each shark, just like our finger prints. This makes it easier for scientist to identify them during conducted studies.

Sharks can only swim forward

Unlike other fish, sharks can only swim forward. Due to their body design and the fact they cannot curve their pectoral fins means they are unable to swim backwards. Sharks need to keep swimming to breathe. The gill slits running on each side of the shark’s head has specialised thin lined blood vessels, they absorb oxygen as the oxygenated water passes over them when swimming. These vessels absorb and transport oxygen similar to our own circulatory system from our lungs. There are a few exceptions to this as some sharks have evolved spiracles that allows them to pull water into their respiratory system whilst at rest. This is mostly common in bottom dwelling sharks that use this highly specialised respiratory organ.

It is important to raise awareness of sharks and help to change the stigma around the specie. They are vital for the oceans ecosystems and with greater understanding of sharks people may show more empathy and support conservation efforts around the world. Together we can protect sharks from the threat of extinction.

Come back next Friday for more interesting facts, #factfriday !

Further reading
  • Ong, J., Meekan, M., Hsu, H., Fanning, L. and Campana, S., 2020. Annual Bands in Vertebrae Validated by Bomb Radiocarbon Assays Provide Estimates of Age and Growth of Whale Sharks. Frontiers in Marine Science, 7.
  • Oceans Africa Diving & Whale Watching. 2020. Sharks Of South Africa – Shark ID, Shark Biology And Behavior. [online] Available at: <https://www.oceansafrica.com/sharks-of-south-africa/> [Accessed 11 September 2020].