Реферат на тему Sharks Essay Research Paper Although sharks belong

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Sharks Essay, Research Paper

Although sharks belong to the class Chondrichtyes, there are many

different types. Sharks arose about 350 million years ago and have remained

virtually unchanged for the past 70 million years and still comprise a dominant

group. It is thought that sharks almost certainly evolved from placoderms, a

group of primitive jawed fishes. It took a long series of successful and

unsuccessful mutations with fin, jaw positions etc to give us all the different

designs of sharks around today. When asked to draw a shark, most people would

draw a shape along the lines of the whaler shark family, tigers or a mackeral

shark such as a porbeagle. However many people do not realize the sheer

diversity in the shape of sharks, or that rays are really sharks. Seldom does

such an animal inspire such a variety of emotions reflecting a mixture of

fascination, awe and fear. Sharks have occasionally exacted a terrible price

from humans who have trespassed on their territory. No better understood than

the ocean that they inhabit, these creatures should be regarded in the same way

as lions, tigers, and bears: as dangerous, predatory but nonetheless magnificent

animals. Different Types of Sharks Living sharks are divided into eight major

orders, each easily recognizable by certain external characteristics. Each order

contains one or more smaller groups, or families. In all there are 30 families

of sharks and they contain the 350 or more different kinds or species of sharks.

The eight major orders of sharks include the Squantiformes, Pristiophormes,

Squaliformes, Hexanchiformes, Carcharhiniformes, Lamniformes, Orectolobiformes,

and the Heterodotiformes. The orders have distinguishing characteristics that

fit in each. The Squantiformes normally have flat bodies that are ray-like with

mottled dorsal surfaces. These sharks have a short terminal mouth, which is

armed with small impaling teeth. They also have a caudal fin, which has a lower

lobe that is longer than the upper lobe. Their pectoral fins extend forward over

the ventrally directed gills. The Pristiophormes have more of an elongated

snout, which is saw-like and edged with slender, needle-sharp lateral teeth.

They have two dorsal fins and no anal fin. They use short transverse mouths and

small cuspidate holding teeth in both jaws. Squaliformes have no anal fin as

well, but their snout is not elongated, but is somewhat long. Many have powerful

cutting teeth in both jaws. In some species these razor sharp teeth are in the

lower jaw only and the upper teeth serve to hold the food. Hexanchiformes have

six or seven gill slits. They are sharks with a single spineless dorsal fin, and

an anal fin. The typical Carcharhiniforme has an elongated snout, a long mouth

that reaches behind the eyes, an anal fin and two spineless dorsal fins. The

eyes have movable, nictitating lower eyelids worked by unique muscles. Teeth

vary from small and cuspidate or flattened to large and bladelike.

Carcharhiniformes have no enlarged rear crushing teeth. Along with this they

have a spiral scroll intestinal valve. A Lamniforme shark has an elongated

snout. Most have long mouths that reach behind the eyes, an anal fin and two

spineless dorsal fins. They also have a ring intestinal valve. The

Orectolobiformes have pig-like snouts and short mouths that in most species are

connected to the nostrils by grooves. There is an anal fin but no fin spines on

the two dorsal fins. They have uniquely formed barbells at the inside edges of

the nostrils. Heterodotiformes are the only living shark that combines fin

spines on their two dorsal fins and anal fin. They only have five-gill slits. In

each order there are specific types of sharks. Each shark belongs to a family

with different species. The Angel shark (Squantiforme) is just one of the many.

It has a single family of about thirteen species. They are all ovoviviparous

livebearers and most do not exceed 1.5 meters. Saw sharks (Pristiophoriformes)

are harmless bottom sharks. They are also a single family but with five species.

They are also ovoviviparous livebearers. Four sharks that belong to the order

Sqauliforme are the Bramble, Dogfish, and Rough sharks. They have three families

with eighty-two species. They too, are ovoviviparous livebearers. They have more

cylindrical bodies. Frilled sharks, Six, and Seven gill sharks (Hexanchiformes)

have two families and five species. Once again they are also ovoviviparous

livebearers. Usually, these guys are found in deep waters. The Catsharks,

Finback Catshark, False Catshark, Barbelled Houndshark, Weasel, Houndshark,

Hammerhead, and Requiem sharks (Carcharhiniformes) have one hundred and

ninety-seven known species. Most of these sharks are known to be dangerous. They

are both oviparous and ovoviviparous livebearers. This is not the type of shark

you would like to have grace you presence. From the order of Landformes is the

Sand, Basking, Goblin, Crocodile, Megamouth, Thresher, and Mackerel sharks. They

come complete with seven families and fifteen or sixteen species. All of them

are ovoviviparous livebearers. These sharks are found in all seas except Arctic

and Antarctic. The last group of sharks would be the Collared Carpet sharks,

Blind, Wobbegongs, Zebra, Longtailed Carpet Sharks, Whale, and Nurse sharks.

They all belong to the order Orectolobiformes and have seven families and

thirty-three species. These sharks prefer the warmer water and are both

ovoviviparous and oviparous livebearers. Obviously these sharks come in many

different sizes and some are more dangerous than others. At least eighteen

species in four families and nine genera have been implicated in attacks on

humans. Obviously a small shark such as the Pygmy is harmless, but they still

must be treated as a predator especially the bigger ones. The smallest of all

sharks is the Pygmy Ribbontail Catshark, which is about 0.24 meters. Next in

line from smallest to largest would be the Port Jackson Shark, which is about

1.65 meters. After them would be the Ornate Wobbegong (2.88m) and then the Bull

shark (3.4m). The average sizes go drastically up from there to the Great White

shark, which is incredibly larger, its about 6.4 meters. The two greatest sizes

are the Basking shark (7.8m) and the Whale shark (13.7m). These sharks listed

here are definitely not all the sharks in the world, they were just meant to

give an average range of size for all sharks. Some of the most dangerous sharks

range from about 2 to 8 meters. The Hammerhead, Great White, Tiger, Blue, and

the Bull shark name a few. There are many types of sharks lurking around in

today?s ocean. In every one is unique in its own way. Some are different by

size, shape, eating habits, or even the way they breed. Although with all these

differences they are all very similar and that is why the shark is one of the

most amazing creatures of our time. Summary Although sharks belong to the class

Chondrichtyes, there are many different types. Sharks are divided into 8 major

orders. Each order contains 1 or more smaller group. There are 350 or more

different kinds of species of sharks. The 8 orders are named the Squantiformes,

Pristiophormes, Squaliformes, Hexanchiformes, Carcharhiniformes, Lamniformes,

Orectolobiformes, and the Heterodotiformes. These orders group sharks according

to certain distinguishing characteristics. The Angel shark, Saw shark, Frilled

shark, Hammerhead shark, Sand shark, Wobbegongs, and more all belong to a

specific order due to their characteristics. Each one of these sharks come in

different shapes and sizes. Some are more dangerous than others. The more

dangerous sharks range from about 2 to 8 meters. It is obvious that sharks are

one of the most amazing creatures of our time.


J. 1975. Shark frenzy. Grosset & Dunlap Publishers, New York NY. 106 pp.

Clark, E. 1981. Sharks, magnificent and misunderstood. National Geographic

160:138-186 (Aug. 1991) Compagno, L. J. V. 1984. FAO species catalogue. Vol. 4. Sharks of the world. An annotated and illustrated

catalogue of shark species known to date. Part 2. Carcharhiniformes FAO Fish.

Synop. (125) Vol. 4, Pt. 2: 251-655. Conniff R. 1993. From jaws to laws – now

the big bad shark needs protection from us. Smithsonian 24: 32-43 (Number 2,

May1993). Burgess, R. F. 1970. The sharks. Doubleday & Company, Inc., Garden

City NY. 159 pp

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