Sunday article – Vanishing Species: Sharks


Sunday Article by Mohan Pai

 
 
Shark
(Selachinmorpha)

The great predator.
 
 
Virtually unchanged for more than 400 million years, shark’s streamlined bodies and amazing sensory systems fit the mold of a perfect predator.
 
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago, before the time of the dinosaurs. Sharks are found in all seas and are common down to depths of 2,000 metres (6,600 ft), and some live even deeper but they are almost entirely absent below 3,000 metres (9,800 ft). They generally do not live in freshwater, with a few exceptions such as the bull shark and the river shark which can live both in seawater and freshwater. They respire with the use of five to seven gill slits. Sharks have a covering of dermal denticles that protect their skin from damage and parasites and improve fluid dynamics so the shark can move faster. They have several sets of replaceable teeth. Well-known species such as the great white and the hammerhead are apex predators at the top of the underwater food chain. Their extraordinary skills as predators fascinate and frighten us, even as their survival is under serious threat from fishing and other human activities.
 
 Extraordinary Sensory System
Sharks have sensory organs unlike any other creatures. Most sharks can:
* Pick up sound waves from more than 5 kilometers.
*Detect a single drop pf blood in an amount of water contained in an Olympic size swimming pool.
*Register the heightened body tension of a wounded or panic-stricken creature.
* locate prey in total darkness.
 
Unlike bony fish, sharks have no bones; their skeleton is made of cartilage, which is a tough, fibrous substance, not nearly as hard as bone. Sharks also have no swim bladder (unlike bony fish).
 
 Size & Shape
 
There are many different species of sharks that range in size from the size of a person’s hand to bigger than a bus. Fully-grown sharks range in size from 7 inches (18 cm) long (the Spined Pygmy shark), up to 50 feet (15 m) long (the Whale shark). Most sharks are intermediate in size, and are about the same size as people, 5-7 feet (1.5-2.1 m) long. Half of the 368 shark species are under 39 inches (1 m) long.
 
 Sharks have a variety of body shapes. Most sharks have streamlined, torpedo-shaped bodies that glide easily through the water. Some bottom-dwelling sharks (e.g. the angelshark) have flattened bodies that allow them to hide in the sand of the ocean bed. Some sharks have an elongated body shape (e.g., cookiecutter sharks and wobbegongs). Sawsharks have elongated snouts, thresher sharks have a tremendously elongated upper tail fin which they use to stun prey, and hammerheads have extraordinarily wide heads. The goblin shark has a large, pointed protuberance on its head; its purpose is unknown.
 
 There are about 368 different species of sharks, which are divided into 30 families. These different families of sharks are very different in the way they look, live, and eat. They have different shapes, sizes, color, fins, teeth, habitat, diet, personality, method of reproduction, and other attributes. Some types of shark are very rare (like the great white shark and the megamouth) and some are quite common (like the dogfish shark and bull shark). Sharks belong to the group of cartilagenous fish, the Elasmobranchii, that includes the sharks, rays, and skates.
 
Sharks play a vital role in our ecosystem as part of nature’s complex system of checks and balances. Known as apex predators, they are at the top of the food chain. Many sharks prey upon wounded and sick animals, keeping the populations of various species healthy and in balance, while others scavenge the ocean by feeding on dead animals or by filter feeding.
 
According to United Nation’s Food and Agriculture Organization (FAO) approximately 100 million sharks are killed each year. This does not include those caught as bycatch (non-targeted animals caught unintentionally and wasted), which is largely unreported. Many sharks also fall victim to finning, the practice of cutting shark’s dorsal, caudal and pectoral fins, then discarding the still-living shark into the sea to die. Sharks play a vital role in our ecosystem as part of nature’s complex system of checks and balances. Known as apex predators, they are at the top of the food chain. Many sharks prey upon wounded and sick animals, keeping the populations of various species healthy and in balance, while others scavenge the ocean by feeding on dead animals or by filter feeding.
 
Most sharks have no predators, but biological characteristics such as slow growth, late sexual maturity and low number of offsprings make sharks susceptible to almost any fishing pressure. Most species are either fished to capacity or overfished worldwide and for products like shark meat, fins and cartilage contribute to their decline.
 
In India, the bull shark is often called the Sundarbans or Ganges shark and it is found in the Ganges and Brahmaputra rivers of West Bengal and Assam in eastern India and adjoining Bangladesh.Sharks of the family Carcharhinidae are the most important group, dominating the fishery all over the world, and this applies equally in India.
 
 
SHARK ATTACKS
 
When some sharks (like the Great White or the Gray Reef shark) turn aggressive prior to an attack, they arch their back and throw back their head. This places their mouth in a better position for taking a big bite. They also move their tail more acutely (probably in preparation for a chase). Sharks do not normally attack people, and only about 25 species of sharks are known to attack people. Sharks attack fewer than 100 people each year. Many more people are killed by bees or lightning.
The sharks that are the most dangerous to people are the great white shark, the tiger shark, the bull shark, and the oceanic whitetip shark. The bull shark is the most frequent attacker of people as it swims in very shallow waters where people swim and is a very plentiful shark. Some of the other sharks that are known to have attacked people include the gray shark, blue shark, hammerhead shark, mako shark, nurse shark, lemon shark, blacktip reef shark, wobbegongs, sandtiger, spitting sharks, and the porbeagle. Some people believe that sharks mistake people (especially people swimming on surf boards) for seals and sea lions, some of their favorite foods.
 

References: Wikipedia, enchantedlearning.com

 

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