Sharks


taxonomy - Classification

Kingdom: Animalia
Phylum: Chordata
Class:Chondrichthyes
Infraclass:
Euselachii
Superorder:
Selachimorpha

Sharks are a group of elasmobranch fish (subclass of Chondrichthyes or cartilaginous fish) characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. 

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Carribean reef shark. image: wikipedia/Albert kok

Modern sharks

 Modern sharks are classified within the clade Selachimorpha (or Selachii) and are the sister group to the rays. However, the term "shark" has also been used for extinct members of the subclass Elasmobranchii outside the Selachimorpha, such as Cladoselache and Xenacanthus, as well as other Chondrichthyes such as the holocephalid eugenedontidans. 

    Branching diagram listing distinguishing characteristics, including mouth, snout, fin spines, etc. Identification of the 8 extant shark orders. image: wikipedia/Chris huh


Under this broader definition, the earliest known sharks date back to more than 420 million years ago.  Acanthodians are often referred to as "spiny sharks"; though they are not part of Chondrichthyes proper, they are a paraphyletic assemblage leading to cartilaginous fish as a whole.

Species and sizes are extreme

Megalodon (An ancient extinct shark) (top two, estimated maximum and conservative sizes) with the whale shark, great white shark, and a human for scale. image: wikipedia/Scarlet23 


Since then, sharks have diversified into over 500 species. They range in size from the small dwarf lanternshark (Etmopterus perryi), a deep sea species of only 17 centimetres (6.7 in) in length, to the whale shark (Rhincodon typus), the largest fish in the world, which reaches approximately 12 metres (40 ft) in length. 

There are more than 470 species of sharks split across thirteen orders, including four orders of sharks that have gone extinct:

  • Carcharhiniformes: Commonly known as ground sharks, the order includes the blue, tiger, bull, grey reef, blacktip reef, Caribbean reef, blacktail reef, whitetip reef, and oceanic whitetip sharks (collectively called the requiem sharks) along with the houndsharks, catsharks, and hammerhead sharks. They are distinguished by an elongated snout and a nictitating membrane which protects the eyes during an attack.
  • Heterodontiformes: They are generally referred to as the bullhead or horn sharks.
  • Hexanchiformes: Examples from this group include the cow sharks and frilled sharks, which somewhat resembles a marine snake.
  • Lamniformes: They are commonly known as the mackerel sharks. They include the goblin shark, basking shark, megamouth shark, the thresher sharks, shortfin and longfin mako sharks, and great white shark. They are distinguished by their large jaws and ovoviviparous reproduction. The Lamniformes also include the extinct megalodon, Carcharodon megalodon.
  • Orectolobiformes: They are commonly referred to as the carpet sharks, including zebra sharks, nurse sharks, wobbegongs, and the whale shark.
  • Pristiophoriformes: These are the sawsharks, with an elongated, toothed snout that they use for slashing their prey.
  • Squaliformes: This group includes the dogfish sharks and roughsharks.
  • Squatiniformes: Also known as angel sharks, they are flattened sharks with a strong resemblance to stingrays and skates.


Etmopterus_perryi_SI_cr

dwarf lanternshark (Etmopterus perry). image: wikipedia

Whale_shark_Australia

A whale shark in Western Australia. image: wikicommons/ Brocken Inaglory


Habitat locations

P6111027.jpg

A blacktip reef shark (Carcharhinus melanopterus). image: R.Conan-Davies

Sharks are found in all seas and are common to depths of 2,000 metres (6,600 ft). They generally do not live in freshwater although there are a few known exceptions, such as the bull shark and the river shark, which can survive and be found in both seawater and freshwater. 

Sharks have a covering of dermal denticles that protects their skin from damage and parasites in addition to improving their fluid dynamics. They have numerous sets of replaceable teeth. 

General anatomical features of sharks. image: wikipedia/Chris_huh 

Apex predators

Well-known species such as the great white shark, tiger shark, blue shark, mako shark, thresher shark, and the hammerhead shark are apex predators—organisms at the top of their underwater food chain. 


Senses include smell, sight, hearing, electroreception , vibrations.

Electroreception

The ampullae of Lorenzini are the electroreceptor organs. They number in the hundreds to thousands. Sharks use the ampullae of Lorenzini to detect the electromagnetic fields that all living things produce. This helps sharks (particularly the hammerhead shark) find prey.

Electromagnetic field receptors (ampullae of Lorenzini) and motion detecting canals in the head of a shark. image:Chris_huh/wikipdeia

Smell 

Sharks have keen olfactory senses, located in the short duct (which is not fused, unlike bony fish) between the anterior and posterior nasal openings, with some species able to detect as little as one part per million of blood in seawater

Human impacts

Many shark populations are threatened by human activities. Threats include habitat alteration, damage and loss from coastal development, pollution and the impact of fisheries on the seabed and prey species. The 2007 documentary, Sharkwater exposed how sharks are being hunted to extinction.



Source adapted from: Shark. (2017, August 23). In Wikipedia, The Free Encyclopedia. Retrieved 05:04, August 31, 2017 , from https://en.wikipedia.org/w/index.php?title=Shark&oldid=796838758