The oceans, which make up more than 90 percent of the earth’s livable space, are full of almost invisible animals.
海洋占地球可居住空間的90%以上，其中充滿各種幾乎不可見的生物。
To illustrate why, Dr. Sonke Johnsen, a professor of biology at Duke University in North Carolina, began a recent talk with a macabre scenario. Suppose just then a gunman burst into the room, shooting at the audience. Naturally, people would scramble for cover behind chairs and walls.
為了證明箇中原理，美國北卡羅來納州杜克大學生物學教授江森最近以一種駭人的場景展開對話。假設一名持槍男子闖進這個房間，向著聽眾開槍；人們當然會躲在椅子與牆壁後尋求掩護。
His point: There would be places to try to hide.
他的論點：總會有那麼些地方可以躲藏。
On land, many animals camouflage themselves amid the foliage and terrain; in coastal waters, sea creatures blend into the sand or find refuge among coral or rocks. But in the deep ocean, creatures floating in the water have nowhere to seek refuge.
在陸地上，許多動物在樹葉與地形之間偽裝自己。在沿海水域，海生動物融入沙子或在珊瑚、岩石之間尋求掩護。然而到了深海裡，浮游在水中的那些動物可就無處可以藏身了。
Transparency is the most obvious strategy and the one Dr. Johnsen first began researching almost 20 years ago.
透明是最明顯的一個策略，也是江森博士將近20年前開始率先研究的重點。
Transparency is not just a lack of pigmentation. Albinos, Dr. Johnsen points out, are not invisible; rather, the entire body must absorb or scatter as little light as possible.
透明不只是沒有天然色素那麼簡單。江森博士指出，白化並不是不可看見。要不被看見，整個身體必須盡可能把吸收或散射的光減到最少的程度。
Scattering is a challenge. When light passes into a material of a different index of refraction, which is often proportional to the density, part of the light reflects and part of it bends. That largely explains why one could search long and wide for a transparent cow or pigeon and not find one: The density of air is so much less than that of flesh that even a see-through terrestrial animal would probably be easily spotted from its reflections.
如何散射是個挑戰。光進入折射指數（通常與密度成正比）不同的物質時，部分的光會反射，另一部分則彎曲。這足以解釋，為什麼一個人就算費盡功夫也找不到一隻透明的牛或鴿子。由於空氣的密度遠低於肌肉，即使真有一隻透明的陸地動物，只怕也會因為反射作用而輕易被人看見。
Water is much denser, and body tissues are roughly the density of water, greatly reducing the amount of scattering. But some organs are denser than others, and the transparent animals pack their insides differently to minimize the reflections.
水的密度可就大多了，而身體組織的密度則與水相當，以致散射的光量大幅降低。然而部分器官的密度大於其他器官，透明的動物會以不同的方式包裝牠們的內臟，將反射降到最低限度。
Dr. Johnsen’s measurements of the see-through creatures that he brought up from the depths found that 20 to 90 percent of the light passed through, undisturbed. “You could read a book through these animals,” he said.
江森博士測量他取自海洋深處的透明動物之後發現，有20%到90%的光線可以不受阻擋直接穿透。他說：「你可以隔著這些動物閱讀一本書。」
But transparency can complicate life, and transparent creatures near the surface could be sunburned, not only on the skin but inside, too. To protect themselves from ultraviolet light, “these guys basically have suntan lotion in their transparent tissues,” Dr. Johnsen said.
然而話說回來，透明也使得這些動物要活命變得更為困難。接近水面的透明動物可能曬傷，不只是皮膚，體內也會。江森博士說，為了讓自己不至於因為紫外線而受傷，「基本上來說，這些傢伙的透明組織裡含有天然防曬液」。
Evolution has come up with two other forms of stealth technology: mirrors and biological light bulbs.
演化造就另外兩種隱形技術：鏡子與生物燈泡。
Some predators find their food by looking for silhouettes above. “You see many animals with upward-looking eyes, and even a squid with one big eye looking up and a ‘normal’ eye looking to the side,” said Steven Haddock, a scientist at the Monterey Bay Aquarium Research Institute in Moss Landing, California.
部分掠食者搜尋在自己上方的側影輪廓，據此找到食物。加州莫斯碼頭蒙特利灣水族研究所的科學家哈多克說：「許多動物具備往上看的眼睛。甚至有一種烏賊的一個大眼睛會往上看，另一個『正常』的眼睛則看著側邊。」
The silvery sides of fish like herring and sardines are systems of mirrors: They reflect the downwelling light, much the way a part of the sky is sometimes reflected by a glass skyscraper. Thus, a predator from below would see the blue water, not a fish, above.
鯡魚、沙丁魚等魚類的銀色光澤側邊有如一排鏡子。它們能夠反射下降的光，就像一棟玻璃帷幕大樓有時候會反射一部分的天空。換言之，位於下方的掠食者只會看到湛藍的海水，卻看不到一條魚。
Eric Denton, a British marine biologist, studied mirrored fish in the 1960s and figured out that the mirrors were vertical, maximizing the illusion.
英國海洋生物學家丹頓1960年代研究有銀色光澤側邊的魚類後發現，這些鏡子呈垂直狀態，導致幻覺最大化。
The third strategy, called counterillumination, also seeks to mimic the downwelling light. But instead of mirrors, the animal generates its own glow, much as fireflies do with light-producing organs known as photophores.
第三種策略名為反照明，同樣意在模仿下降光。然而這些動物不使用鏡子，而是自體發光，就像螢火蟲以發光器官發光。
The animals employing counterillumination make sure the light they are producing is pointed downward.
採用反照明方法的動物會確定牠們製造的光往下。
“They don’t want light leaking out to the side and making them vulnerable, so they have lenses, mirrors and filters on their photophores,” Dr. Haddock said.
哈多克說：「牠們不希望因為光漏向側邊而使自己陷入危境。在這種情況下，牠們的發光器官還配備透鏡、鏡子與濾光器。」
Some animals have evolved ways to defeat the camouflage. Species of squid and shrimp have eyes that can differentiate between the polarizations of light, something that many insects can do, but which people cannot do without polarized sunglasses.
有些動物演化出破解偽裝的方法。有些烏賊與蝦子的眼睛可以分辨光的偏振。許多昆蟲也有這種能耐，人類則必須借助於偏光太陽眼鏡才做得到。
Photons – particles of light – can be thought of as arrows with tail fins representing the oscillating magnetic and electric fields, and the polarization represents the orientation of the fields. To human eyes, the color of reflected light is unchanged. When reflected, the angle of the polarization changes.
我們可以把光子比擬為配置尾翼的箭。這些尾翼代表不斷振盪的磁場與電場，偏振則代表它們的定位。對人類的肉眼而言，反射光的顏色不會改變。一旦反射，偏振的角度就會改變。
As the sun moves across the sky, the polarization of light filtering down to the depths changes, and to an eye that can tell the difference between the polarizations, a mirrored fish suddenly sticks out.
太陽在天空移動時，過濾到海洋深處的光偏振會隨之改變。對能夠分辨偏振差別的眼睛而言，一條反射的魚就會變得明顯可見。
“It turns out while the camouflage is really good, you can really break it with polarization vision,” Dr. Johnsen said.
江森博士說：「偽裝固然很好，偏振的視覺卻可以破解。」
That might, for instance, allow a squid to spot an approaching hungry tuna and flee.
舉例來說，這或許可讓一隻烏賊察覺一隻飢餓的鮪魚正在接近，及時逃走。
“We’re just surrounded by an entirely mysterious world,” Dr. Johnsen said. “And the fact we can’t see it means we ignore it most of the time.”
江森說：「我們被一個完全神秘的世界包圍，看不到它意味我們多數時候忽略了它。」
