OFFICIAL17 What is the lecture mainly about?

[00:00.00]Narrator: Listen to part of a lecture in a biology class.
[00:06.12]Female Professor: Okay, now I want to talk about an animal that has a fascinating set of defense mechanisms, [00:13.45]and that’s the octopus...one of the unusual creatures that live in the sea. [00:18.47]The octopus is prey to many species, including humans, so how does it escape its predators?
[00:26.15]Well, let me back up here a second…[00:28.47]anyone ever hear of um Proteus?[00:32.79]Proteus was a god in Greek mythology who could change form. [00:38.11]He could make himself look like a lion, or a stone, or a tree, anything he wanted, and he could go through a whole series of changes very quickly.
[00:47.78]Well, the octopus is the real-world version of Proteus; [00:52.79]just like Proteus, the octopus can go through all kinds of incredible transformations... [00:58.38]And it does this in three ways: By changing color, by changing its texture, and by changing its size and shape.[01:16.28]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. [01:28.10]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?
[01:41.77]Male Student: Do we know how that works, I mean, how they change colors?[01:45.80]Female Professor: Well, we know that the reaction that takes place is not chemical in nature. [01:50.47]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.
[02:02.28]Chromatophores consist of tiny sacs filled with colored dye. [02:09.12]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. [02:20.92]Each one of these sacs is controlled by muscles. [02:24.57]If the muscles are relaxed, the sac shrinks, and all you see is a little white point. [02:30.86]But if the muscles contract, then the sac expands, and you can see the colors. [02:36.29]And by expanding different combinations of these color sacs to different degrees, the octopus can create all sorts of colors. Ah yes, Elizabeth?[02:47.84]Female Student: And just with various combinations of those five colors, they can recreate any color in their environment?[02:56.39]Female Professor: 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.
[03:07.48]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. [03:19.91]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.[03:30.73]So that’s how octopuses mimic colors.
[03:33.71]But they don’t just mimic the colors in their environment; they can also mimic the texture of objects in their environment. [03:41.15]They have these little projections on their skin that allow them to resemble various textures. [03:47.13]The projections are called papillae.[03:50.70]If the octopus wants to have a rough texture, it raises the papillae; [03:57.71]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. [04:06.66]So the octopus has the ability to mimic both the color and the texture of its environment, [04:13.91]and it's truly amazing how well it can blend in with its surroundings. [04:18.38]You can easily swim within a few feet of an octopus and never see it.
[04:23.44]Male Student: I read that they often hide from predators by squirting out a cloud of ink or something like that?
[04:31.70]Female Professor: Yes, the octopus can release a cloud of ink if it feels threatened, ah [04:37.21]but it doesn't hide behind it, as is generally believed. [04:41.76]Um the ink cloud is... it serves to distract a predator while the octopus makes its escape. [04:49.10]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.
[05:03.28]The muscular system of the octopus enables it to be very flexible, to assume all sorts of shapes and postures. [05:11.61]So it can contract into the shape of a little round stone, and sit perfectly still on the sea floor... [05:18.88]or it can nestle up in the middle of a plant and take the shape of one of the leaves--[05:24.13]even Proteus would be impressed, I think.