My Science School

While the shark in Jaws was inspired by a great white shark in New Jersey, the legendary fish is far less fearsome in reality. As scientific research on these elusive predators increases, their image as mindless killing machines is beginning to fade.

Found in cool, coastal waters around the world, great whites are the largest predatory fish on Earth. They grow to an average of 15 feet in length, though specimens exceeding 20 feet and weighing up to 5,000 pounds have been recorded.

They have slate-gray upper bodies to blend in with the rocky coastal sea floor, but they get their name from their white underbellies. They're streamlined, torpedo-shaped swimmers with powerful tails that can propel them through the water at speeds of up to 15 miles per hour. They can even leave the water completely, breaching like whales when attacking prey from underneath.

Credit : National Geographic

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Great whites are torpedo-shaped with powerful tails that can propel them through the water at up to 15 miles per hour.

The great white shark can dive to the depth of 1, 200 m (3, 900 ft) in the ocean spending most of their lives that deep in the ocean. That is the average depth of the Atlantic Ocean.

Great white sharks are built for survival and kill which means they would do anything to achieve this. They stay underneath and hunt prey from that depth of the ocean sneaking up on their prey.

Deep diving exposes sharks to colder water so they would be having a good reason before they would be going that deep. Great white sharks are warm-blooded, to be able to digest food their temperature would have to be above that of the ocean. 

Human divers begin to experience twice the pressure they felt because of the air on the surface when they dive up to 10 meters of the ocean. At 10 meters depth, the ocean already absorbs 80% sunlight.

Credit : Shark Keeper

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When great white sharks are young, they feed on smaller prey, like fish and rays. As they grow larger, they feed more exclusively on marine mammals, such as sea lions, seals and small whales.

The great white is at the top of the food chain and has few threats in the ocean. Only orcas and larger sharks can pose a risk. The only other risk to the great white shark is human interaction. They are sometimes caught by accident in fishing nets or intentionally sought out by sport fisherman. Their jaws and fins are sold for considerable amounts of money.

Not much is known about the mating habits of great white sharks. What is known is that after mating the female develops several eggs which hatch in her womb. The newly-hatched shark pups feed on unfertilized eggs in the womb as they develop before being born. In general, the mother gives birth to a litter of two to ten pups, each of which average 1.5 m (5 ft.) in length. Male great whites reach maturity at 9-10 years of age. Females mature even later, between 14 and 16 years of age. Female sharks are thought to give birth once every couple years, but even that is uncertain.

Credit : National Geographic Kids

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Baby sharks, which are known as pups, can be born in three quite different ways. First, some sharks lay eggs. We call this oviparous. The “mommy shark” lays an egg case in a good spot and swims away. The egg case, which is sometimes called a “mermaid’s purse,” can be perfectly camouflaged to blend in with the sea floor or algae. The egg includes all of the nutrition the pup will need to grow from a fertilized embryo to a fully functioning shark pup. When the pup is ready, it emerges from the egg case and is totally independent.

Second, some sharks grow from eggs–but inside the mother shark’s body. This is called ovoviviparous. In this type of reproduction, there is no placenta to link the “mommy shark” and the “baby shark.” The shark pup gets all of its nutrition from its own egg yolk, other egg yolks, or (yikes!) from eating its fellow fertilized eggs or other pups. Ovoviviparous sharks give live birth to a fully independent pup. This is how sand tiger sharks, like the ones you can see at the Greater Cleveland Aquarium, reproduce.

And third, some sharks do have a placenta and directly support the shark embryo until it is ready to be born as a pup. This is called viviparous, and is also how humans are born. When the shark pup has matured enough, it is born and swims away. This is how sandbar sharks, which you can also see at the Aquarium, have pups.

Credit : Great clevel and aquarium 

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Like almost all shark species, Great White Sharks have a highly distinctive appearance with large, torpedo-shaped bodies and a pointed snout. They have very tough skin that is covered in tiny teeth called denticles that is slate-grey to black in colour on the top of their bodies which helps them to remain camouflaged into the rocky, coastal sea floors where they are most commonly found. The underside of the Great White Shark is white and is what has led to their name. Great White Sharks have powerful, crescent-shaped tail fins that help to propel them through the water at a tremendous speed, and are aided by their pectoral (side) fins that are held out in fixed wings to prevent the Great White Shark from sinking. The large and highly characteristic dorsal (back) fin of the Great White Shark is used to help them to steer through the water, along with diving and helping them to balance. One of the most characteristic features of the Great White Shark is their jaw. Their mouths are filled with up to 300 serrated, triangular teeth that are arranged in rows and are replaced continuously throughout their lives. Each tooth can grow to around 6cm in length providing Great White Sharks with a formidable bite when they are attacking their prey.

Great White Sharks are fearsome carnivores that primarily hunt large marine mammals in order to gain their nutrition. Seals, sea lions, porpoises, dolphins and smaller whales are among their most commonly hunted prey species around the world. Great White Sharks have poor eyesight in comparison to their other senses and use both their sense of smell and ability to detect vibrations caused by animals in the water to detect their prey. Once located, Great White Sharks fiercely attack with great speed and force before retreating and leaving their wounded prey to weaken before returning to feed once it is safe to do so. Although they are largely solitary, Great White Sharks can be seen in pairs or small groups to feed on a large whale carcass. In these circumstances, larger and more dominant individuals feed first with varying swimming display patterns thought to contribute to establishing their dominance hierarchy.

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Sharks do not have a swim bladder. Instead, they have a large liver filled with oil. Because oil is less dense than water, all of this oil helps sharks float in the ocean.

Sharks have livers different from other animals. It takes up much more internal space and serves more functions than simply helping with digestion; the oil inside the liver helps sharks stay buoyant under water.

While digestion is an important function of a shark's liver, it's not the reason the liver is so big or why it's filled with oil. The oil, called squalene, is lighter than the water. A shark's body is naturally heavier than water, and he doesn't have a swim bladder to fill with air like some other fish. The oil lightens the shark's body, providing buoyancy so he won't sink. Sharks must keep swimming to push water past their gills to breathe, and buoyancy is key to keeping on the go and staying off the ocean floor. Sharks use their pectoral fins to help them change directions in the water, but without the oil, sharks would expend too much energy swimming and staying buoyant than they could replace with their food.

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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.

Some sharks can detect the blood of prey from a huge distance - one part of blood to one million parts of water. That's equal to one teaspoon in an average sized swimming pool.

Light doesn’t travel well through water. So sharks need to maximise the amount available to help them see. 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.

Like cats, 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. It also gives their eyes a green glow in the dark.

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.

Their ears are located on either side of their head, behind the eyes. From the outside these are visible as 2 small holes. These are made up of 3 cartilage tubes filled with fluid and lined with hair cells. Sound waves cause these tiny hairs to vibrate and the brain then interprets the sound.

Credit : Shark Trust

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The skin of the shark is unique in many ways. It is characterised by its grey colour and the contrast between its slippery appearance and its rough tactile texture.

These teeth are what makes the shark feel rough and what has led to the use of shark skin as sandpaper. The skin is so rough, in fact, that softer animals brushing against it can be badly injured. These denticles’ spines point towards the tail, so the rough effect is best felt by moving one’s hand over the body from the tail to the head.

They are arranged in a mosaic pattern which surrounds the body of the shark like a helix. This acts as a supportive corset. Because the sharks have no skeletal bones, the muscles used for swimming are attached directly to the inside of this corset. This saves energy, allowing them to swim faster and further without tiring.

Shark skin is usually grey in colour, with the top of the body being darker than the underneath. This means that the shark’s body camouflages with the dark depths of the ocean when viewed from above as well as the lighter surface of the water when viewed from below.

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As you might suspect, the number of teeth a shark has depends upon exactly what type of shark it is. There are many, many different types of sharks in the world's oceans, and the number of teeth they have can vary greatly.

For example, great white sharks have approximately 50 teeth at any one time (their “working" teeth). Like most sharks, however, they have multiple rows of teeth in development that are ready to take the place of teeth that fall out. Some sharks have over 300 teeth in various stages of development within their mouths at any one time!

Unlike human teeth, shark teeth are not very strong and tend to fall out easily. It's not a big deal for sharks, because they are constantly producing new teeth to replace the ones they lose. In fact, some sharks can have over 50,000 teeth over the course of their lives!

Many sharks have teeth in layered rows. Both their upper and lower jaws can have 2-3 or as many as 15 such rows. Their teeth do not have roots, so they break off easily and may last as little as a week. New teeth can move forward to replace lost teeth within a day to a couple of weeks. The skin within a shark's mouth moves new teeth into position like a conveyor belt.

Although it might sound like a bad thing to be constantly losing teeth all the time, this system works well for sharks. Over history, sharks have developed extremely strong jaws that give them the powerful bite that makes them the apex predators of the oceans.

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The dugong, commonly known as sea cow, is a marine mammal found in the Gulf of Mannar, Palk Bay, Gulf of Kutch and the Andaman and Nicobar islands in India. In a first for the country, Tamil Nadu will establish a conservation reserve for the animal. Why is such a reserve necessary? Come let's find out.

Listed as vulnerable" on the IUCN Red List of Threatened Species, the sea cow's range is spread across 4S countries within the tropical and subtropical coastal and island waters between east Africa and Vanuatu in Oceana". Largely a herbivore, it feeds primarily on sea grass, and this helps maintain a healthy coastal ecosystem. How? By constantly browsing sea grass, the mammal helps in grass regrowth, ensuring the habitat Bair remains healthy and welcoming of several other marine creatures.

However, dugong populations across the globe have been diminishing due to several reasons such as habitat loss. fatalities caused by gill nets pollution, and illegal hunting Adding to the pressure, they also have a slow rate of reproduction, which could be pushing them further towards extinction. Abundant in Indian waters once, the dugong population is said to be a mere 200 today. And the Tamil Nadu region has a good number of these mammals, which is why the idea of setting up a conservation reserve has come up. The reserve will be established in the Gulf of Mannar and Palk Bay area, spanning 500 sq km.

Did you know?

 The dugong and the three species of manatees are related. They also had another cousin called the Steller's sea cow, now extinct. Said to have been massive at about 30 feet, weighing over 10 tonnes , and larger than many modern whales, it was discovered in 1741 in the sub-Arctic waters by German biologist Georg Wilhelm Steller’s after whom it was named. "Within just 27 years of being formally described, humans had completely eradicated a marine mammal" - hunted to extinction for fur.

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When it comes to seahorses, it is the male that gets pregnant and gives birth to baby seahorses. The female swims to the surface of the water with the interested male in tow. It transfers its orange eggs into the pouch of the male which then adds its sperm and seals the opening. The growing babies will remain in their dad's pouch till they develop. Once they are born the babies are on their own.

After completing an elaborate courtship dance that may go on for hours or days, the female seahorse transfers her mature eggs into the male’s brood pouch, where they are fertilized. At the end of a gestation period usually lasting from two to four weeks, the pregnant male’s abdominal area begins to undulate rhythmically, and strong muscular contractions eject from a few dozen to as many as 1,000 fully formed baby seahorses into the surrounding water. After that, the offspring must fend for themselves. Large litters are necessary because only about 0.5 percent will survive to adulthood.

Many, if not all, of the 47 known seahorse species—14 of which were identified only in the 21st century—are in decline worldwide.

Because seahorses generally live in shallow, near-coastal waters, human activities including development, pollution, fisheries, and traditional medicine have reduced their numbers. At the same time, their universal appeal has worked against them; until recently, wild seahorses were often captured for the aquarium trade. The delicate creatures tend to fare poorly in aquaria, however. In recent years, captive-bred seahorses have shown promise as hardier tank-dwellers than their wild relatives.

Credit : National Ocean Service

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There's this fast-swimming zooplankton (a small organism) that lives in groups. And it is tiny, growing to about 6 cm. But lying at the base of the food web in the Southern Ocean, the krill is what links the ecosystem together. Its population is on the decline, and this can have an unimaginable impact on marine life in the long run. How?

A new study published recently says krill population in its Antarctic habitat is likely to decline about 30% this century due to both natural climate variability and human activity-induced climate change. The study also adds that it will be difficult to "tell the difference between natural and human-caused effects on krill until late in the 21st century". This is worrisome due to two reasons. One is that unless we know how exactly human activity affects krill population, it may be difficult to initiate relevant steps to improve the situation. Two, it is said that "natural climate variability can obscure human-driven trends". Neither augurs well for the krill population.

As mentioned, krill are tiny creatures. But they are one of the most abundant too. For several sea creatures including penguins, seals, fish, and whales - in the ocean surrounding Antarctica, krill are the main food. Not just that. Many sea animals feed on the creatures that consume krill, which is why the krill has an important role to play in the aquatic food chain, especially in the Southern Ocean. If the krill population keeps coming down, it could affect several other creatures, causing a severe imbalance in the food chain. Krill are adapted to surviving harsh winters. But one wonders how well they can adapt to climate change.

Did you know?

The largest animal on Earth, the blue whale feeds almost exclusively on krill. Apparently, it needs to eat a few tonnes of these tiny crustaceans every day!

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Just as a microphone collects sound in the air, a hydrophone detects acoustic signals under the water. Most hydrophones are based on a special property of certain ceramics that produces a small electrical current when subjected to changes in underwater pressure. When submerged in the ocean, a ceramic hydrophone produces small-voltage signals over a wide range of frequencies as it is exposed to underwater sounds emanating from any direction.

By amplifying and recording these electrical signals, hydrophones measure ocean sounds with great precision. While a single hydrophone can record sounds from any direction, several hydrophones simultaneously positioned in an array, often thousands of miles apart, result in signals that can be manipulated to “listen” with greater sensitivity than a single device. Omni-directional and hemi-directional hydrophones pick up sound from a particular direction and can be used to track fish movements.

In addition to NOAA's National Marine Sanctuaries, NOAA’s Pacific Marine Environmental Laboratory (PMEL) also frequently uses hydrophones. PMEL acquires long-term data sets of the global ocean acoustics environment to identify and assess acoustic impacts from both human activities and natural processes, such as underwater volcanoes, earthquakes, and icequakes on the marine environment.

Credit : Ocean Service

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Yes, they do, says research. Since turtles don't have vocal cords, it was thought that they did not communicate through sounds. However, scientists have found that turtles communicate at such low frequency that sounds like "clicks, clucks and hoots" can be heard only through a hydrophone. What's surprising is that they talk when they are still inside their eggs just before hatching.

One of the most common sounds which a turtle may produce is the hissing sound. Turtles hiss by quickly expelling air from their lungs. They do this by tucking their heads into their shells.

A turtle in a new environment will hiss a lot as it is not used to its environment. Once the turtle gets used to its environment, you can expect it to hiss less. Snappers) hiss when approached.

They do this because they are stressed and feel threatened. Hissing coupled with heavy breathing produces a menacing sound. A threatened snapping turtle will attack when approached or provoked. Their bites are very powerful and are capable of crushing human bones.

When a turtle hisses, it’s best to leave it alone.

Credit : All Turtles

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Parrotfish are bright-coloured fish found in and around coral reefs. They have fused teeth resembling a parrot beak which they use to scrape algae and seaweeds from the surface of corals and rocks, thereby helping the reefs stay healthy. They grind up the calcium carbonate reef material and excrete it as fine white sand.

Parrot fishes range to a length of about 1.2 metres (4 feet) and weight of about 20 kilograms (45 pounds), or occasionally larger. They are variable in colour, the male of a species often differing considerably from the female, and the young may differ from the adult. Parrot fishes are protogynous hermaphrodites; that is, they first function as females and later transform into males.

Parrot fishes are edible but are not, as a group, of great economic importance. The surf, or rivulated, parrot fish (Callyodon fasciatus) is an Indo-Pacific representative of the family; it grows to 46 centimetres (18 inches) or more, and the male is green and orange or red, the female blue and yellow. Atlantic species include the rainbow parrot fish, which grows to about 90 centimetres and is bright orange and green with a blue beak, and the queen parrot fish (Scarus vetula), which grows to about 50 centimetres and is blue with green, red, and orange if male but reddish or purplish with a white stripe if female.

Credit : Britannica

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