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Introduction

Sharks, skates, and rays, which together form a group of about 900-1150 species(1,2) of ocean-dwelling and freshwater-dwelling fish(3,4) called elasmobranchs,(1) are some of the most fascinating creatures of the deep. While they come in many sizes and shapes—from the giant whale shark(5) and the huge manta ray (3) to the dwarf lanternshark(6) and the tiny short-nosed electric ray,(3) and the from the odd-looking hammerhead sharks(4) to the totally bizarre sawfish(3)—all living elasmobranchs share certain key features. First of all, their skeletons are made up of a strong, flexible, and light material called cartilage,(7) rather than bone, making them (along with another fish group called chimaeras(1)) fundamentally different from other fish.(1,3,8) Other important characteristics include their rows of replaceable teeth(2) and the 5-7 gill slits on each side of their body.(3) In addition, although these creatures are ancient—the first elasmobranchs evolved at least 400 million years ago!(1,5,8,9)—they have many highly-developed senses,(4,10) including the amazing ability to perceive tiny changes in electricity around them.(10,11) Sharks, as well as rays and skates (which you can tell apart from sharks by their generally flattened, diamond-shaped bodies(3)), often use this sense for finding prey, as well as for finding their way through the water.(10,11) In at least some elasmobranchs this sense may even be used in various social and mating behaviors.(11) In part thanks to this electric sense, many elasmobranchs are skillful hunters, often serving as the top predators in the food chain and keeping their environments in the proper balance.(9,12,13) These creatures also have an important relationship with humans. Some rays, such as stingrays and electric rays, can cause injury to people.(3,4) And even if you haven’t heard much about the dangers of those fish, you’ve definitely heard of shark attacks, which have given sharks a very dark reputation even though these attacks are actually rare.(4,12,14) In fact, although elasmobranchs can pose dangers to humans, humans pose a much greater danger to them.(13) For over 5000 years shark meat has been eaten by people,(8) and ray meat, skate meat, shark skin, and other elasmobranch products are also sometimes used by humans today.(3,8,13) Overfishing, accidental catching (called “bycatch”),(1,9,13,15,16) higher numbers of people living on the coast, and greater damage to coastal environments(13,15) are all threatening sharks, skates, and rays. And because elasmobranchs generally grow slowly, reproduce late in life, and have only a small number of children, they have trouble recovering from population decline caused by humans.(1,5,8,12,13,15) As a result, many elasmobranchs around the world are endangered.(13,15)

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