From the enormous whale sharks(1) and manta rays(2) to the tiny dwarf lanternsharks(3) and short-nosed electric rays,(2) the elasmobranchs (the sharks, skates, and rays(4)) are a diverse group of fishes with some 900-1150 species(4,5) living all over the world (6,7) in both marine and freshwater habitats.(2,8) The elasmobranchs and one other group, the chimaeras, make up the class of cartilaginous fish or chondrichthyans;(4) as these fish have skeletons made up of a strong, flexible, and light material called cartilage,(9) rather than bone, they are fundamentally different from other fish.(2,4,7) Living elasmobranchs also share, among other physical features, rows of replaceable teeth(5) and 5-7 gill slits on each side of their body.(2) Although these creatures are ancient—the first elasmobranchs evolved at least 400 million years ago(1,4,6,7)—they have many sophisticated senses,(8,10) including the ability to perceive very small shifts in electricity around them.(10,11) Sharks, as well as rays and skates (distinguishable from sharks by, among other characteristics, their generally flattened, diamond-shaped bodies(2)), frequently use this sense for finding prey, as well as for navigation;(10,11) in at least some elasmobranchs this sense may even be connected with various social and mating behaviors.(11) As the skillful hunters that many of them are, elasmobranchs are crucial parts of their environments, often serving as the top predators in the food chain and keeping the ecosystem in balance.(6,12,13) These animals also have an important relationship with humans. Some rays, such as stingrays and electric rays, can cause injury to people.(2,8) Much more present in the public imagination are shark attacks, which, while in fact rare, have given sharks a very dark reputation.(8,12,14) But although elasmobranchs can pose dangers to humans, humans pose a much greater danger to them.(13) For over 5000 years shark meat has been used as a food source and in some countries today it is considered a delicacy;(7) ray and skate meat are also sometimes consumed (2) and shark skin is an expensive product in some markets.(7) Because elasmobranchs generally grow slowly, reproduce late in life, and produce only a small number of offspring, they are particularly sensitive to human exploitation,(1,4,7,12,13,15) and as a result of overfishing and accidental catching (“bycatch”)(4,6,13,15,16)—as well as increasing human presence on and damage to the coastal environment(13,15)—many elasmobranchs worldwide are in decline.(13)
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