Many type of shark live in Bay of Bengal waters. These range from the 30cm pygmy shark to the world’s biggest fish, the gentle whale shark, which grows up to 12m long and is a popular feature of the Bangladesh sea tourism sector. The presence of many species of shark as 'apex predators' – occupying the top level of the food chain – is an indication of a healthy marine environment. Despite their reputation, sharks seldom cause harm to humans, however, when incidents do occur the results can be devastating.
Biology of Sharks:
Sharks are typically slow growing, late maturing, long-lived and produce only a few young per year, making many species particularly vulnerable to overfishing. Like rays and skates, sharks belong to a class of fish called Chondrichthyes which have skeletons made of cartilage not bone. Sharks are covered in tough scales known as dermal denticles making their skin extremely tough and abrasive. Sharks have between five and seven external gills. Some sharks which live on the seabed, such as angel sharks and wobbegongs, have a flattened shape like rays and skates. However, most sharks have a torpedo-shaped body. A large triangular dorsal fin on top of the shark keeps the shark stable. Large, stiff pectoral fins behind the gills are used for steering. The tail or ‘caudal’ fin powers the shark’s swimming. Many sharks must keep swimming in order to breathe.
Sharks like most fish typically ‘cruise’ at two – four body lengths per second or less. Burst speeds of up to 10 body lengths per second or more are possible for a short periods of time to obtain prey or escape predators. Sharks have an continuous supply of teeth, with worn out teeth being replaced as needed. A shark can have up to 3,000 teeth in its mouth at one time and 30,000 plus teeth over its life. Their teeth vary according to diet. White sharks have large serrated teeth for cutting big prey while grey nurse sharks have long, needle-like teeth, used for gripping. Many rays have teeth modified into grinding plates to grind crabs and other invertebrates.
Sharks have excellent eyesight and an incredible sense of smell. Like other fish, sharks have clusters of hair-cells called ‘neuromasts’ located in canals just below their skin along the sides of their bodies and around the head and mouth. As with many species of fish, these can sense vibrations caused by fish movements in the water and sound hundreds of metres away. Through pores in their snouts and heads called ‘ampullae of Lorenzini’, sharks also have the ability to detect the weak electrical signals produced by most living organisms. These pores help them find prey buried in sand or hidden by darkness. It is thought they may also help sharks navigate by responding to fluctuations in the Earth’s magnetic fields.
Upper level predators:
Many people are fearful of sharks, but sharks may have more to fear from humans. Their slow rate of reproduction means many species of sharks are highly vulnerable to fishing and may take a long time to recover if stocks are depleted. Sharks can feed on a wide variety of prey, including fish, squid, octopus, crustaceans, mammals, reptiles and other marine creatures. Many species of sharks are specialist feeders, targeting particular prey. Whale, basking and megamouth sharks and others species filter feed on plankton and have relatively small teeth because they do not need them to feed. Modified extensions on their gills allow the plankton to be filtered from the water. Sharks play an important role in maintaining balanced ecosystems, controlling prey populations and removing weak animals while healthy ones survive to reproduce. Because they are at the top of aquatic food webs, sharks have few predators, apart from humans, other sharks and orcas.
Life cycle of Sharks:
Each shark species has its own expected life span and it is difficult to set an average for sharks as a whole. However, very broadly speaking, most sharks live for between 20 and 30 years. The Spiny Dogfish lives for over a century, and some Whale Sharks have been known to do the same. Unlike most other animals, sharks generally do not care for their young. As soon as pups are born into the water, they swim away and care for themselves. Some sharks give birth to litters of up to 100 babies at a time, while others may have as few as two or three. The Great White Shark gives birth to approximately three to 14 pups at a time. In almost all cases, all of the pups are healthy and well-developed. This is in stark contrast to many other animals (such as several fish species and some turtles), which give birth to large numbers of ill-developed young (in effect, producing quantity over quality). This means that sharks are defined as K-selected reproducers. Mating amongst sharks is somewhat elusive, rarely seen by researchers and scientists. Fertilisation of the female egg occurs internally. The male’s pelvic fin (the fin on the underside of the body, close to the tail) has evolved into claspers, a pair of organs that can best be compared to a penis in mammal species.
Sharks use one of three ways to carry their young – 1) Oviparity, 2) Viviparity and 3) Ovoviviparity.
Yolk-sac viviparity or ovoviparity:
Most sharks are ovoviparous – eggs hatch in the mother’s body, nourished by the egg’s yolk and fluids secreted by the mother. In the lamniforme shark, the first embryos to hatch eat the remaining eggs. In some species, including grey nurse sharks, embryos actually feed upon sibling embryos inside the mother.
A few sharks lay their eggs in water, mostly in an egg case with the consistency of leather, which may be corkscrewed into crevices for protection. The empty cases or “mermaid’s purses” can wash up on shore.
Some shark species have a placenta-like link between the mother and developing young, similar to mammals.
Distribution and habitat of Sharks:
Sharks live in every ocean, estuary and freshwater system. Most species of sharks tend to inhabit shallower continental shelf areas, where rivers deposit nutrients into the oceans, providing the basis of an ocean food web. Megamouth sharks favour the open ocean while bull sharks are often found many kilometres upriver.