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
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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10 Best Places to Swim With Sharks (Professional Association of Diving Instructors, Condé Nast Traveler)
25 Best Destinations for Sharks and Adventure (Travis Marshall, Scuba Diving)
Shark Diving Tips and Advice (Shart Trust)
10 Shark Diving Tips (Dive Magazine)
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.
- Ecuador expands sea life protections around...on January 15, 2022 at 8:35 am
Ecuador created a massive new marine reserve Friday north of its Galapagos islands, forming a Pacific corridor up to Costa Rica's Cocos Island National Park to preserve species of migratory fauna, such as sharks.
- Tiger shark migrations altered by climate change,...on January 13, 2022 at 10:00 am
A new study led by scientists at the University of Miami (UM) Rosenstiel School of Marine and Atmospheric Science revealed that the locations and timing of tiger shark movement in the western North Atlantic Ocean have changed from rising ocean temperatures. These climate-driven changes have subsequently shifted tiger shark movements outside of protected areas, leaving the sharks more vulnerable to commercial fishing.
- Under a moon spell: Shark attacks related to...on January 12, 2022 at 5:49 pm
New research from LSU and the University of Florida suggests that more shark attacks occur during fuller phases of the moon. While the exact cause remains unclear, the researchers found that more shark attacks than average occur during periods of higher lunar illumination and fewer attacks than average occur during periods of lower illumination. Many different types of animals show behaviors that are linked to moon phases yet few studies to date have looked at the connections between lunar […]
- Thar she blows! An expert's guide to whale...on January 4, 2022 at 12:10 pm
There's no doubt about it, seeing a whale rise from the depths of the ocean is something special. Whether it's the first time you've seen a whale or you've seen many like I have, it's the thrill of hearing and sometimes smelling whale breath that gets you back for more! It's time to dust off your binoculars because we're about to dive into learning to whale watch.
- Medical scan reveals the secrets of New Zealand's...on January 3, 2022 at 1:46 pm
New Zealand's fossil record of land dinosaurs is poor, with just a few bones, but the collection of ancient extinct marine reptiles is remarkable, including shark-like mosasaurs and long-necked plesiosaurs.
- Farmed fish breeding with wild fish is changing...on December 23, 2021 at 2:20 pm
A team of researchers from the Norwegian Institute for Nature Research and Rådgivende Biologer, has found that interbreeding between farmed salmon and wild salmon is changing the life cycle of the wild salmon. In their paper published in the journal Science Advances, the group describes their study of scale growth patterns in thousands of salmon taken from rivers in Norway over the years 2010 to 2017.
- How do our organs know when to stop growing?on December 22, 2021 at 4:16 pm
The smallest fish in the world, the Paedocypris, measures only 7 millimeters. This is nothing compared to the 9 meters of the whale shark. The small fish shares many of the same genes and the same anatomy with the shark, but the dorsal and caudal fins, gills, stomach and heart, are thousands of times smaller! How do organs and tissues of this miniature fish stop growing very quickly, unlike those of their giant cousin? A multidisciplinary team led by scientists from the University of Geneva […]
- The real reason to worry about sharks in...on December 22, 2021 at 2:20 pm
If you're heading to the beach this summer, the thought of sharks might cross your mind. I don't mean wondering whether a shark will take you for dinner (that's very, very unlikely) but rather, how these remarkable creatures are faring in the marine ecosystem.
- Study finds that not even the largest lakes in...on December 21, 2021 at 6:20 pm
Tourist towns along the Lake Michigan shoreline love to proclaim the giant body of water "Unsalted and Shark-Free." The slogan is plastered on t-shirts, magnets and bumper stickers—but, according to a new study, only one of those claims holds water.
- Giant rubber whale helps Kiwi rescuers battle...on December 20, 2021 at 9:20 am
A two-tonne rubber whale can take on a life of its own in the frigid shallows of a windswept New Zealand beach, particularly when a bedraggled group of would-be rescuers is trying to wrestle it into a harness.
Here are links to pages about closely related subjects.
Life Cell, Gene, Tree of Life
Plant Flower, Tree
Invertebrate Cuttlefish, Octopus, Ant, Bee, Butterfly, Spider, Lobster
Vertebrate Fish, Seahorse, Ray, Shark, Frog, Turtle, Tortoise, Dinosaur
Bird, Ostrich, Owl, Crow, Parrot
Mammal Bat, Rabbit, Giraffe, Camel, Horse, Elephant, Mammoth
Whale, Dolphin, Walrus, Seal, Polar Bear, Bear, Cat, Tiger, Lion, Dog, Wolf
Monkey, Chimpanzee, Human