Sharks have been celebrated as powerful gods by some native cultures.
在一些土著文化中，鲨鱼被当作强大的神而崇拜。
For example, Fijians believe the shark god Dakuwaqa, could protect fisherman from the dangers at sea.
例如斐济文化中所崇拜的鲨鱼神，可以保护渔民免除海上的危险。
And today, sharks are recognized as apex predators of the world's ocean
现在，鲨鱼是世界海洋中的顶尖掠食者，
and include some of the Earth's longest living vertebrates.
包括一些在地球上存活了很久的脊椎动物在内。
What is it that makes these fish worthy of our ancient legends and so successful in the seas?
是什么使得鲨鱼这一古老的传奇在今天的海洋中还能如此成功？
Much of their hunting prowess stems from a unique set of biological traits honed for more than 400 million years.
鲨鱼的很多捕食技能都来源于一整套独特的生理特质，这些生理特质从4亿多年中进化而来。
Their cartilaginous skeletons are less dense than bony ones and require less energy to move.
它们软骨质的骨架比硬骨的骨架更轻，在游动时更加节省能量。
Large oily livers lend buoyancy to their streamlined bodies,
它们巨大的油质肝脏为流线型的身体提供浮力，
and while trunk muscles of bony fishes attach directly to their skeletons, those of sharks also join to their skin.
与其它硬骨质的鱼类不同，鲨鱼的躯干肌肉不仅直接附着在骨架上也连接着他们的皮肤。
This special design transforms them into pressurized tubes whose springy skin can efficiently transmit muscular forces to the tail.
这一特殊的生理机能增加了鲨鱼身体内部的弹性，它们像弹簧一样的皮肤将肌肉的力量有效的传递到尾部。
Shark skin has additional remarkable features.
鲨鱼的皮肤还有别的独到之处。
Despite its smooth external appearance,
虽然从外面看上去十分光滑，
at the micro level, it has a coarse texture thanks to thousands of tiny teeth-like scales called dermal denticles.
但是在显微镜下有着粗糙的纹路，鲨鱼皮肤表面布满了成千上万微小的齿状麟片，称之为盾麟。
Each denticle is coated in a substance called enameloid, which turns the skin into a tough shield.
每一片盾麟都覆盖着一层釉质，这使表面异常坚固。
Plus the structure of denticles varies across the body in such a way
而且这些盾麟的结构因身体部位不同而各异，
as to reduce noise and drag when the shark moves through water.
以减少鲨鱼在水中游动时带来的噪音和阻力。
As for the teeth in their mouths, sharks can produce up to 50,000 in a lifetime.
至于鲨鱼的牙齿，它们一生可以长出五万个牙齿。
On average, they can lose one tooth a week, and each time that happens, it's rapidly replaced.
鲨鱼可以平均一个星期掉一颗牙，而且每次掉一颗牙，很快就会有新的牙齿长出来。
Thanks to a layer of fluoride coating their teeth, sharks also avoid cavities.
鲨鱼牙齿表面的氟化物涂层还可以使它们免除蛀牙的苦恼。

But teeth aren't the same in all sharks. They can vary across species and by diet.
不同的鲨鱼牙齿通常也不同。取决于它们的种类和食物。
Some are dense and flattened, useful for crushing mollusks.
有些是紧密而平整的，用来磨碎软体类的动物。
Others are needle-like for gripping fish.
还有些是针头一样尖的用来抓鱼。
The mouths of Great Whites contain pointy lower teeth for holding prey and triangular serrated upper teeth for slicing.
大白鲨的下牙低而突出用来捉住猎物，它们三角锯齿状的上牙则用来切削。
This variety enables sharks to target prey in a diversity of ocean environments.
这种多样性使得鲨鱼可以在广阔的海洋环境中选择目标猎物。
Many species also have another peculiar trait
很多种类的鲨鱼还有一个独特的本领，
the ability to launch their jaws out of their mouths, open them extra wide, and grab prey by surprise.
它们可以把下颌从嘴里伸出来，把嘴张得巨大，突击捕食猎物。
Over the course of evolution, shark brains have expanded, coupled with the growth of their sensory organs.
在不断的进化进程中，鲨鱼的大脑扩张了，与此同时它们的传感系统器官也更发达。
Modern-day sharks can smell a few drops of blood and hear sounds underwater from 800 meters away.
现代鲨鱼连几滴血液的味道都能嗅到，听觉达到了水下800米以远。
They're particularly well-tuned to low frequencies, including those emitted by dying fish.
它们尤其能够识别低频声波，包括那些濒死的鱼类发出的声波。
And like cats, they have reflective membranes called tapeta lucida at the backs of their eyes
在鲨鱼的后眼角位置有一种叫做tapeta lucida的反光膜，类似于猫眼的反光组织，
that dramatically improve their vision in low light.
极大提高了黑暗中的视力。
As if these heightened abilities weren't enough, sharks have even honed a sixth sense.
似乎这些高大上的特质都还不够，鲨鱼还进化出了第六感。
They're able to hunt using a network of electrosensory cells called ampullae of Lorenzini.
它们能够利用细胞网状系统的电波来捕猎，这个系统被称为鲨鱼的电感受器。
These cells are filled with hypersensitive jelly
这些细胞充盈着高度敏感的胶状物，
which allows them to detect electrical signals from prey, including the slightest twitch of a muscle.
而胶状物使得鲨鱼能够探测到猎物发出的电流信号，即便是极其微小的肌肉颤动。
Some of the most iconic shark species, like Great Whites, Makos, Porbeagles, and Salmon Sharks
一些最具代表性的鲨鱼种类，比如大白鲨、灰鲭鲨、鼠鲨以及鲑鲨，
owe their success to another surprising trait: warm blood inside a cold-blooded creature.
还拥有另一种令人惊讶的特点：在他们冷血动物的头衔下有着恒温的内在。
Inside their bodies, they have bundles of arteries and veins called rete mirabile.
鲨鱼体内有着相互交织的动脉和静脉，称之为细脉网。
Here, venous blood warmed up by the shark's working muscles
在这里，被工作肌群变暖的静脉血
passes right next to arteries carrying cold, oxygen-rich blood from the gills.
与从鳃部获得的富氧的冷动脉血擦肩而过。
This arrangement transfers heat to the blood that gets cycled back to the body's vital organs.
这样就把热量通过血液传递给了身体各部分的重要器官。
Warmer muscles enable faster, more powerful swimming,
温暖的肌肉使鲨鱼游的更快更有力，
while warmer bellies aid digestion, and the more rapid development of young in utero.
温暖的腹部有助于消化，而且鲨鱼宝宝们在子宫里也成长的更快。
And warmer eyes and brains keep the sharks alert in cold waters.
温暖的眼睛和大脑使得鲨鱼在寒冷的水中保持警觉。
With these amazing adaptations, there's more to revere than fear from the 500 shark species roaming our oceans.
这些奇妙的适应性，对于在我们的海洋里游荡着的500种鲨鱼，带来的更多的应该是崇敬而不是畏惧。
Unfortunately, one-third of these species are threatened due to overfishing.
很不幸的是，其中有高达三分之一的种类受到过度捕捞的威胁。
After millions of years in the making, these apex predators may be meeting their greatest challenge yet.
经历了上百万年的洗礼之后，鲨鱼这一世界海洋中的顶尖掠食者也许正在遭遇着最大的挑战。