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Today on Jonathan Bird's Blue World, the incredible world of sharks and their diversity!
今天，乔纳森·伯德的蓝色世界为您带来的是具有多样性鲨鱼的神奇世界!
Hi, I'm Jonathan Bird, and welcome to my world!
大家好，我是乔纳森·伯德，欢迎来到我的世界!
Before any animals existed on land, sharks ruled the oceans.
在陆地动物出现之前，鲨鱼统治着海洋。
They have been around for over 250 million years.
它们已经存在了2.5亿年。
Many people think that because they are ancient animals, they are in some way primitive.
许多人认为，因为鲨鱼是古老的动物，所以在某种程度上它们是原始的动物。
But as it turns out, sharks are highly evolved sophisticated animals with amazing adaptations for survival!
但事实证明，鲨鱼是高度进化的复杂动物，它的适应能力令人惊讶!
The fact that sharks have survived so long demonstrates the incredible effectiveness of their anatomy.
鲨鱼能存活如此之久，证明了其解剖结构有着不可思议的效果。
And lets face it--250 million years worth of evolution has got to count for something!
我们承认吧——2.5亿年前的进化是有意义的!
Over 400 species of sharks live in the oceans across many habitats.
400多种鲨鱼生活在海洋中的许多栖息地。
Some, like the Caribbean Reef shark, live on shallow tropical coral reefs.
有些鲨鱼，如加勒比礁鲨，生活在浅层热带珊瑚礁。
Others, like the blue shark, are pelagic, living far from land in the open ocean.
其他的鲨鱼，比如蓝鲨，属于远洋动物，生活在远离陆地的开阔海域。
The Greenland shark lives in the freezing waters of the arctic, while the Tiger shark prefers the tropics.
格陵兰鲨生活在北极冰冷的海水中，而虎鲨则更喜欢生活在热带地区。
Sharks are fish, but there are several key things that differentiate them from other kinds of fish.
鲨鱼属于鱼类，但是几个关键点使它们区别于其他种类的鱼。
Sharks are everywhere, but what are they?
鲨鱼无处不在，但这几个关键点是什么呢?
Sharks and their close relatives the rays differ from the bony fishes in several ways.
鲨鱼和它们的近亲鳐鱼在几个方面区别于硬骨鱼类。
First, sharks and rays have a soft flexible skeleton made of cartilage.
首先，鲨鱼和鳐鱼拥有由软骨构成的柔软而灵活的骨架。
The cartilaginous skeleton makes the shark more flexible than similarly-sized bony fishes.
软骨骨骼使得鲨鱼比同样大小的硬骨鱼类更灵活。
Also, sharks and rays have no swim bladder.
此外，鲨鱼和鳐鱼没有鱼鳔。
A bony fish uses an organ called a swim bladder to maintain neutral buoyancy, so it can hover like a hot air balloon.
硬骨鱼使用一种叫做鱼鳔的器官来保持中性浮力，这样它就可以像热气球一样悬浮。
The fish adds or removes gas from its swim bladder through its bloodstream to increase or decrease its buoyancy.
硬骨鱼通过血流从膀胱中增加或排出气体的方式来增加或减少浮力。
The shark's lack of a swim bladder means that, unlike bony fishes, the shark tends to sink.
鲨鱼没有鱼鳔，这就意味着，与硬骨鱼类不同的是，鲨鱼往往会下沉。
To stay off the bottom, sharks have to keep moving.
为了远离海底，鲨鱼必须不停地运动。
While the shark uses its tail fin in a back and forth motion to provide forward thrust, its pectoral fins work like airplane wings to provide lift.
当鲨鱼用它的尾鳍前后运动来提供向前的推力时，它的胸鳍就像飞机的机翼一样提供升力。
Like an airplane wing, as long as the fins move forward through the water, they provide lift to keep the shark up.
和飞机的机翼一样，只要鱼鳍在水中向前移动，鲨鱼就能获得升力来保持上升。
Not too long ago, scientists thought all sharks had to swim at all times, not just to stay off the bottom, but to keep water moving over their gills.
不久以前，科学家们认为所有的鲨鱼必须一直游泳，不仅仅是为了远离海底，也是为了让水在它们的鳃上流动。
For some species like the hammerhead, this is true.
对于像双髻鲨 这样的物种来说，是这样的。
Hammerheads must keep swimming at all times to force water through their gills.
双髻鲨必须不停地游来游去，以迫使水通过它们的鳃。
Ironically, if they stop swimming they'll drown.
讽刺的是，如果它们停止游泳，就会被淹死。
But many species of sharks sometimes stop swimming and rest on the bottom, gulping water to ventilate their gills.
但许多种类的鲨鱼有时会停止游泳，在水底休息，吞水以使鳃通气。
This is a Lemon shark, common in the Caribbean, taking a break resting on the sand. It gulps water to breathe.
这是一只柠檬鲨，柠檬鲨在加勒比海很常见，它正在沙滩上休息，吞水来呼吸。
Nurse sharks also spend a lot of time resting on the bottom.
铰口鲨也会花很多时间在海底休息。
Bottom-dwelling species of sharks like the Wobbegong actually live their entire lives on the bottom.
生活在海底的鲨鱼，如沃伯根鲨，实际上一生都生活在海底。
They are camouflaged to look like a rock covered in algae, and they hunt by being very patient and waiting for an unsuspecting fish to come close.
它们伪装成覆盖着海藻的岩石，耐心地等待着一条毫无防备的鱼靠近。
With such a long evolutionary history, sharks have had plenty of time to refine their senses.
在如此漫长的进化史中，鲨鱼有足够的时间来优化自己的感官。
For example, most sharks have an incredible sense of smell.
例如，大多数鲨鱼都有惊人的嗅觉。
Extrapolations on experiments on shark smell have suggested that some sharks can detect one drop of blood dissolved in as much as one million gallons of water!
鲨鱼气味实验表明，一些鲨鱼能探测到一滴溶解在一百万加仑水中的血！
Sharks also have senses we can't even begin to experience.
鲨鱼也拥有我们甚至无法开始体验的感官。
Sharks have an electrosensory system that allows them to detect the extremely minute electrical currents generated by the muscles of a swimming fish.
鲨鱼有一个电感系统，可以探测到鱼游动时肌肉产生的极其微小的电流。
