Sharks come in a variety of shapes and sizes. Some are small enough for aquariums and some are as large as whales. Sharks are in almost every large body of water. Most of which are saltwater sharks and a few are found in freshwater. Some sharks can even thrive between both saltwater and freshwater, such as the bull shark.
When a person thinks shark they probably think of the menacing great white shark or the odd shaped hammerhead shark. What many people don't realize is that there are more than 450 different species of sharks in the world, and this does not include the sharks that are known or thought to be extinct. These sharks are then classified into eight different orders.
The sharks in this order are considered the most primitive. In fact, aside from the cow shark and the frill shark many of the sharks in this order are now extinct. The sharks left in this order have six to seven gills and only one dorsal fin. What the sharks in this order are lacking is nictitating membrane in the eyes.
Bramble shark, prickly shark, dogfish shark, and roughshark are a few of the 80 species of sharks listed in this order. These sharks are known have five pairs of gills and two dorsal fins. However, they do not have an anal nor do they have nictitating membranes in their eyes.
Eight species of saw sharks occupy this order. They have five to six pairs of gills and two dorsal fins. They have notably large nasal barbells on the ventricle side. These sharks do not have an anal fin.
Angel sharks make up the sharks in this order. With nineteen species of angel sharks, squatiniformes have five pairs of gills on the ventricle side and no anal fin. There are two dorsal fins and the lower lobe of the caudal fin is larger than the upper lobe.
Heterodontiformes consist of nine species of bullhead sharks. These sharks are smaller than most sharks and are considered bottom feeders. The largest species in this order only grows to a length of 59 inches which are located in tropical and subtropical waters. Theses sharks have two dorsal fins and an anal fin. This is also another order where the sharks do not have nictitating membranes in their eyes. The sharks of this order have five pairs of gills.
They are also unique because they have a different type of teeth than other sharks. They have molar like teeth for crushing and chewing prey. Other shark groups have some form teeth in the front row used for grabbing prey but the heterodontiforme sharks do not have this feature.
Commonly referred to as carpet sharks, orectolobiformes have thirty-five species in this order. Some of these species include zebra sharks, nurse sharks, and even whale sharks. Each shark has two spineless dorsal fins. Most orectolobiformes have five pairs of gills with the fourth gill overlapping the fifth gill. The whale shark is an exception to the overlapping gills. Most of the sharks in this group are small except for the whale shark. In fact, the whale shark is the considered largest fish in the ocean.
This is the largest order of sharks with over 279 species classified in this group. Some of the sharks in this group include the blue shark, bull shark, tiger shark, whitetip sharks, and hammerhead sharks. These sharks have five pairs of gills and nictitating membrane in the eyes. They are characterized with two dorsal fins and an anal fin.
More commonly known as mackerel sharks, there are 15 species in this order. Some of the species more commonly known are the great white shark and the mako shark, but it also includes lesser known sharks such as the megamouth shark. These sharks have distinguishing characteristics such as two dorsal fins and an anal fin. They have five pairs of gills and do not have nictitating membranes.
As you can see, there are many similarities between the different groups. However, scientists have grouped sharks together so that we make study them a little easier. Sometimes scientist move species to a different group when they have learned more and realize that the shark belongs in another order. Don't think this is an easy task; there are many debates before changing orders happen.
© 2014 Linda Sarhan