Sharks Essays - Elasmobranchii, Carcharhinidae, Sharks, Ichthyology

Sharks 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.