It is a cartilaginous fish and hence belongs to the class Chondrichthyes of the kingdom Animalia. They vary in grey, cream, brown, yellow, slate or blue. Sharks feed on small fish and invertebrates. They have a gestation period of 8 to 12 months. They like to live in tropical coral reefs, in the deep sea and under arctic sea ice. They are mainly of 8 types. They are classified based on biological, physical, and behavioural differences.
These types are:
- Carcharhiniformes (Ground shark)
- Heterodontiformes (Bullhead shark)
- Hexanchiformes (Cow and frilled shark)
- Laminiformes (Mackerel shark)
- Orectolobiformes (Carpet shark)
- Pristiophoriformes (Saw shark)
- Squaliformes (Dogfish shark)
- Squatiniformes (Angel shark)
Carcharhiniformes (Ground shark)
Mostly, they live in shallow seas. They are found in the Pacific, Atlantic, and Indian oceans. The oldest member of this order appeared during the middle-late Jurassic, before 145 million years. They have a nitrating membrane that can be pulled over the eye to protect the eye. They have two dorsal fins, anal fins, and five-gill fins. They are the largest order of sharks. There are 270 species. Cat sharks, swell sharks, and requiem sharks are common land sharks. Catsharks resemble the earliest land sharks due to morphological similarities in their teeth and body forms. Their body shape varies from medium to large. They give birth viviparously with a yolk placenta.
Heterodontiformes (Bullhead shark)
Most of these sharks live in the Pacific and Indian oceans and in the eastern Pacific, from California to the Galapagos Islands. The oldest fossils of the modern genus belong to the Late Jurassic period.
They can be recognized by the large cap, heavy eyebrows over the eyes, thick snout, and short muzzle. Most species grow to about 1 meter in length. The largest one is 1.7 meters and is found along the western slopes of North America. They have two dorsal fins, anal fins, and five gills. There are nine species. The Bullhead, the Crescent Bullhead Shark, the Galapagos Shark, the Horn Shark, the Japanese Shark, and the Mexican Shark are some of them. Most species grow up to about 3.3 feet in length. Their breeding season is during the spring and early summer. They give birth by laying eggs.
Hexanchiformes (Cow and frilled shark)
They are the most primitive order of sharks. Sharks in this order live mostly in the Aleutian Islands, from Alaska to Baja California, Mexico. Inhabiting the deep outer continental shelf and shallow waters, they are exclusively marine. They are found in cold waters across the Atlantic, Pacific, and Indian oceans in temperate and tropical regions. The oldest member of these sharks lived 300 million years ago. Apparently very similar to modern seven-gill species, it is known from Jurassic specimens.
They have a single dorsal fin, the anal fin, far behind the body. There are only seven species and two families. They reproduce by ovoviviparous. They grow to nearly 7 feet in length.
Laminiformes (Mackerel shark)
They live in the Atlantic, Pacific, and Indian oceans at depths of 600 to 4000 feet and in tropical to cold temperate waters. The oldest member in this order lived 124–140 million years ago. They have two dorsal fins, an anal fin, five-gill slits, and a mouth that extends past their eyes. There are 15 species of this shark. They breed every year. Their eggs are fertilized inside the female’s body. Some of them are laying eggs, and others are livebearers. They have eyes without membranes. These sharks have large, stout bodies; the snout is pointed, and the large mouth, equipped with large teeth, is set at the bottom of the head. They are very active, powerful, and swift.
Orectolobiformes (Carpet shark)
They named them carpet sharks because they have a mottled appearance with intricate patterns reminiscent of carpet designs. This order of sharks lives in medium-deep waters. They were founded in all the seas of the world, but predominantly in tropical and temperate waters. They first appeared in the fossil record in the early Jurassic. They have two dorsal fins, a relatively short transverse mouth that does not extend behind the eyes, and five short gills. There are 7 families and 43 species. They give birth by laying eggs and are ovoviviparous. The smallest carpet shark is up to about 12 inches long.
Pristiophoriformes (Saw shark)
They are founded in many areas of the world. Common in the Indian and Southern Pacific oceans. Most of these sharks like to live in deep waters. They are about 56 million years old. They have two dorsal fins but lack anal fins and have six gill slits. Saw sharks are characterized by a long, slender, sawlike snout equipped with sharp, toothlike projections on each edge. They have edges with sharp teeth. There are 8 species in this order. They usually grow to about 5 feet long. Females are longer than males. They give birth by ovoviviparous. They have litters of 3–22 pups every two years.
Squaliformes (Dogfish shark)
They live in the deep sea. Most species of the squaliform order live in salt water or brackish water. They are found worldwide, from northern to tropical waters and from shallow coastal seas to the open ocean. The oldest member of this order lived 111–153 million years ago. Their first fossil record was recorded in the late Jurassic era. They have five gills, two dorsal fins, with or without spines, and an anal fin that is absent. They have 6 families, 22 genera, and 119 species. Many species are bioluminescent to some degree. They give birth by ovoviviparous. In this order, males grow up to 6 inches and females about 7.5 inches.
Squatiniformes (Angel shark)
They are called angel sharks because of their body shape. The population extends from southeastern Vancouver Island to the Gulf of California. They live in large areas of the Northeast Atlantic, from Norway, Sweden, Morocco, and the Canary Islands to the Mediterranean and Black Seas. They are about 164–157 years old. There are 19 species in this order. They grow about 5 feet in length. They develop inside eggs within the mother’s body and are born alive. They produced an average of 6 pups annually from March to June.
Written by:
J.G. Kaweesha Sithmi
1st Year Undergraduate,
Biological Science,
Faculty of Science
University of Colombo
References:
- Encyclopedia Britannica. (n.d.). Encyclopedia Britannica. https://www.britannica.com/
- Pbs, N. H. (n.d.). New Hampshire PBS. NHPBS. https://nhpbs.org/
- Shark Research Institute. (n.d.). Shark Research Institute. https://www.sharks.org/
- Washington Department of Fish and Wildlife. (n.d.). Washington Department of Fish and Wildlife. https://wdfw.wa.gov/
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