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NARRATOR:Listen to part of a lecture in a biology class.
FEMALE PROFESSOR:Okay, now I want to talk about an animal that has a fascinating set of defense mechanisms, and that’s the octopus... one of the unusual creatures that live in the sea.The octopus is prey to many species, including humans, so how does it escape its predators?Well, let me back up here a second… anyone ever hear of um Proteus?
Proteus was a god in Greek mythology who could change form.He could make himself look like a lion, or a stone, or a tree, anything you want, and he could go through a whole series of changes very quickly.Well, the octopus is the real-world version of Proteus; just like Proteus, the octopus can go through all kinds of incredible transformations...And it does this in three ways: By changing color, by changing its texture, and by changing its size and shape.
For me, the most fascinating transformation is when it changes its color...Its “normal” skin color--the one it generally presents--is, uh, either red or brown, or even gray, and it’s speckled with dark spots.But when it wants to blend in with its environment, to hide from its enemies, it can take on the color of its immediate surroundings--the ocean floor, a rock,a piece of coral, whatever.Charles?
MALE STUDENT:Do we know how that works, I mean, how they change colors?
FEMALE PROFESSOR:Well, we know that the reaction that takes place is not chemical in nature.The color changes are executed by two different kinds of cells in the octopus’s skin, mainly by color cells on the skin’s surface, called chromatophores.
Chromatophores consist of tiny sacs filled with colored dye.There might be a couple hundred of these color sacs per square millimeter of the octopus’s skin, and, depending on the species, they can come in as many as five different colors.Each one of these sacs is controlled by muscles.If the muscles are relaxed, the sac shrinks, and all you see is a little white point.But if the muscles contract, then the sac expands, and you can see the colors.And by expanding different combinations of these color sacs to different degrees, the octopus can create all sorts of colors.Ah yes, Elizabeth?
FEMALE STUDENT:And just with various combinations of those five colors, they can recreate any color in their environment?
FEMALE STUDENT:Well they can no doubt create a lot with just those five colors, but you’re right, maybe they can’t mimic every color around them.So that’s where the second kind of cell comes in.Just below the chromatophores is a layer of cells that reflect light from the environment, and these cells help the octopus create a precise match with the colors that surround them.The colors from the color sacs are supplemented with colors that are reflected from the environment, and that’s how they’re able to mimic colors with such precision.
So that’s how octopuses mimic colors.But they don’t just mimic the colors in their environment; they can also mimic the texture of objects in their environment.They have these little projections on their skin that allow them to resemble various textures.The projections are called papillae.
If the octopus wants to have a rough texture, it raises the papillae; if it wants to have a smooth texture, it flattens out the papillae...So it can acquire a smooth texture to blend in with the sandy bottom of the sea.So the octopus has the ability to mimic both the color and the texture of its environment, and it’s truly amazing how well it can blend in with its surroundings.You can easily swim within a few feet of an octopus and never see it.
MALE STUDENT:I read that they often hide from predators by squirting out a cloud of ink or something like that?
FEMALE PROFESSOR:Yes, the octopus can release a cloud of ink if it feels threatened, ah but it doesn’t hide behind it, as is generally believed.Um the ink cloud is... it serves to distract a predator while the octopus makes its escape.Um now there’s a third way that octopuses can transform themselves to blend in with or mimic their environment, and that’s by changing their shape and size, at least, their apparent size.The muscular system of the octopus enables it to be very flexible, to assume all sorts of shapes and postures.So it can contract into the shape of a little round stone, and sit perfectly still on the sea floor... or it can nestle up in the middle of a plant and take the shape of one of the leaves--even Proteus would be impressed, I think.
旁白：听下面一段生物学课上的演讲。
教授：好的，现在我想讲一讲一种特别的动物，它有一套令人惊叹的防御机制。它就是章鱼，一种生活在大海之中的不凡之物。章鱼的天敌包括人类在内的很多物种的捕食者，所以它是如何躲避捕食者的呢？好的，我先讲些背景知识吧！有谁知道普罗透斯吗？
普罗透斯来自古希腊神话中的一位可以变形的海神。使自己看上去像一头狮子或者石头、树木，任何他想要的，并且他能够在很短的时间之内完成变形。嗯，章鱼是真正存在的现世版的普罗透斯。和他一样，章鱼也能完成各种不可思议的变形。同时，它可以用三种方式变形: 改变颜色、改变纹理、改变大小和形状。
对我而言，最有趣的当属改变颜色这项了。它没有变色时的一般肤色，既可能是红的，棕色的，甚至灰色的，体表点缀有许多黑色的斑点。但是，当它试图和环境混同为一个颜色以趋避天敌时它马上就能变成周围景物的颜色：大海、岩石、珊瑚，什么都行。查尔斯，你有什么问题？
学生：我们知道这种变化的原理吗？我的意思是，章鱼是怎么做到的？
教授：嗯，我们肯定都知道，章鱼变色本质上肯定不是依靠化学反应。这种颜色变化是由章鱼皮肤组织中两种不同类型的细胞完成的，主要是体表的叫做“色素体”的颜色细胞。
“色素体”由许多细小的、装有染色剂的色袋构成。在章鱼皮肤表面，每一平方厘米中大概有两百个这样的色袋，根据种类的不同，它们可以组合成多达五种不同的颜色。每一个色袋都由肌肉控制。如果肌肉在放松状态下，色袋就缩小，你仔细观察的话，就会发现这些色袋看上去像小白点一样。但是，如果肌肉一旦紧缩，色袋就会膨胀，你就会看到不同的颜色。通过不同程度扩张这些色袋不同的组合。章鱼可以创造各种颜色。怎么了，维泽博？
学生：就是通过这五种颜色的不同组合，章鱼们就能变成和周围环境一样的颜色吗？
教授：嗯，利用这五种颜色，它们无疑可以变出很多很多的颜色出来，但是，你说得对可能它们并不能模仿它们周围每一种颜色。所以就有了第二种细胞。在“色素体”细胞之下有一个细胞层，它们负责反射环境中的各种光，这些细胞帮助章鱼与周围的颜色形成精确的匹配。彩色囊中的颜色用颜色加以补充。这反映在环境中，这就是为什么他们能够模仿如此精确的颜色。
这就是章鱼如何模仿颜色。不过，它们不只是模仿它们生活环境中出现的颜色；它们还能模仿它们生活环境中出现物品的纹理。它们的皮肤上有非常小的影射器，帮助它们模仿各种纹理。这些影射器名为“触头”
如果章鱼想要纹理粗糙一些，它就启动“触头”，如果它想要纹理光滑一些，它就把身上的“触头”变得平整。这样它看上去就会很光滑，能够和海底的沙地混淆起来。总之，章鱼有能力模仿它生活环境的颜色和质地，它这种熟练地与四周混同的隐蔽能力的确令人惊叹。哪怕你游到距离章鱼几尺远的地方，你都可能看不到它。
学生：我听说章鱼经常喷出墨云之类的东西来躲避它的天敌。
教授：是的，章鱼确实能够在感到威胁存在的时候，释放墨云，但是它并不躲在墨云的后面像公众想象的那样。嗯，墨云的作用是…它主要是吸引天敌的注意力，以便章鱼能够在这时逃开。嗯，现在该谈谈第三种变形以隐藏于环境之中的方式了，即改变它们的形状和尺寸，至少，它们的表观尺寸。章鱼的肌肉系统使得它们身体的柔韧性很强，能够承担各种形状和姿态的张力。所以它能够将自身体积缩小到身边的一块小石头的大小，并且安稳地躺在海底。拜这种能力所赐，它也能够蜗居于植物中，变成植物叶子的形状。我想，普罗透斯看到章鱼的本事，也会吃惊的。
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