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Sharks Portal (Wikipedia)

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shark : any of numerous mostly marine cartilaginous fishes of medium to large size that have a fusiform body, lateral branchial clefts, and a tough usually dull gray skin roughened by minute tubercles and are typically active predators sometimes dangerous to humans — Webster

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Sharks are a group of elasmobranch 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. 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. 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.

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. 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.

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. Many shark populations are threatened by human activities. — Wikipedia

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Phys.org - latest science and technology news stories Phys.org internet news portal provides the latest news on science including: Physics, Nanotechnology, Life Sciences, Space Science, Earth Science, Environment, Health and Medicine.

  • 'Seeing' tails help sea snakes avoid predators
    on February 15, 2019 at 6:18 am

    New research has revealed the fascinating adaptation of some Australian sea snakes that helps protect their vulnerable paddle-shaped tails from predators. […]

  • Giant 'megalodon' shark extinct earlier than...
    on February 13, 2019 at 11:00 am

    Megalodon—a giant predatory shark that has inspired numerous documentaries, books and blockbuster movies—likely went extinct at least one million years earlier than previously thought, according to new research published Feb. 13 in PeerJ—the Journal of Life and Environmental Sciences. […]

  • New study finds ecosystem changes following loss...
    on February 13, 2019 at 10:00 am

    A new study has documented unexpected consequences following the decline of great white sharks from an area off South Africa. The study found that the disappearance of great whites has led to the emergence of sevengill sharks, a top predator from a different habitat. A living fossil, sevengill sharks closely resemble relatives from the Jurassic period, unique for having seven gills instead of the typical five in most other sharks. […]

  • Marine life typically thrives in the tropics –...
    on February 12, 2019 at 3:29 pm

    Life in the sea isn't easy. Talk to most people about the ocean and they are likely to imagine a tropical scene with a stretch of golden sand and warm, clear water. The reality is often quite different – the marine environment can be a surprisingly cold place. […]

  • Longest-ever eDNA study offers important insights...
    on February 12, 2019 at 1:39 pm

    Tiny genetic 'breadcrumbs' left behind by marine organisms offer unprecedented insights into ocean biodiversity and how it changes over time and in response to our changing climate, new research at Curtin University, in collaboration with CSIRO, has revealed. […]