鲨鱼 Shark

鲨鱼（Shark）是一群板鳃类鱼的通称，屬於軟骨魚的一支。这是一种古老的鱼，最早在4億2千萬年前的志留紀就已經出現，至今已經演化出約440個不同的種，大约包括5目20科（由於分类学家的意见不一致）。它們有高度流線、適合游泳的外型，全身覆滿了盾鳞，盾鳞除了保護鯊魚免於受傷或者被寄生，還可以增進它們的流體動力，讓它們游得更快速。鯊魚體側用於呼吸的鳃裂有5-7个。它們有數套可替換的牙齒。鯊魚在水域食物鏈中是高級消費者(有些種類甚至是最頂級的掠食者，如巨牙鯊)，不同的種類有不同的食物，有的肉食，可以吞噬海豹、海龟，有的只濾食浮游生物；大的体长达20米，小的只有10多厘米。

鲨鱼的感觉器官相当灵敏，甚至能嗅出几公里之外的血腥味。它們具有感应电的能力，並可以此发觉隐藏在沙底下的猎物。

Sharks are a group of 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 holocephalideugenedontidans. Under this broader definition, the earliest known sharks date back to more than 420 million years ago.^[1] 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 (39 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.^[2] They breathe through five to seven gill slits. Sharks have a covering ofdermal denticles that protects their skin from damage and parasites in addition to improving their fluid dynamics. They have several sets of replaceable teeth.^[3]

Well-known species such as the great white shark, tiger shark, blue shark, mako 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.