TOEFL Listening: ETS-TOEFL听力机经 - GYS7M7W8632OA10J

Lecture: Symbiotic Relationship: Narrator: Listen to part of a lecture in a marine biology class. Professor: Let's take a few minutes to review the last reading assignment. And that was about close relationships between organisms belonging to different species. When an organism is dependent upon another organism for survival, we call symbiosis. Symbiosis means living together. And there are various types of symbiotic relationships. Cayne, can you tell us the three types? Cayne: Ok, so there are parasitism, commensalism and um, mutualism. Professor: Can you give examples? Cayne: Ok, parasitism. Well, in large fish or marine mammals like seals or dolphins, many of them have parasitic worms, worms that live in the intestines and still absorb nutrients from the host animals' body. Professor: So in parasitism, one animal, the parasite, benefits while the other is disadvantage. Okay. How about commensalism? Cayne: I guess the relationship between the remora fish and a shark would be an example. The remora, a sort of attaches itself to the shark, it gets a free ride, and it can feed on food that's left over after the shark feeds. So the remora benefits while the shark, well, it really isn't harmed, and it doesn't help either. Professor: Ok, good! Question? Female Student: But I was thinking they be considered parasites. I mean, the sharks movement is slowed down at least a little by the remora, right? Professor: That may be true, but probably not enough to really have any harmful effect on the shark. Okay. And how are we going to talk more about it when we talk about the formation of coral reefs? So let's say that for later. Now, this classification categorizing symbiosis into three types. It isn't perfect. The categories overlap a bit, as evidenced by Jone's comment about the sharks. And since it is not always clear to us humans, what kind of advantages or disadvantages animals are actually getting, if any. The appropriate category choice isn't always clear. Now, I should point out that some of symbiosis relationships don't seem to fit any of the three existing categories. For example, the relationship between amphipods and sea butterflies, both small marine animals, but about the same size. And here you can see amphipods that sort out and is now capturing a sea butterfly. The amphipod will then take the sea butterfly, put it on its back, hold it there, and then carry it around. Now I should point out that this relationship is temporary. At some point, the sea butterfly is set free, and later the amphipod will find and capture another one. But this is a highly unusual behavior in a symbiotic relationship. Can you think of any advantages or disadvantages for either creature? Female Student: Well, you said sharks aren't slow down all that much by remora fish. How about amphipods? Are they slowed down significantly by the sea butterflies? If so, that would be a disadvantage. Professor: Good. And indeed they are. Studies show that amphipods with sea butterflies on their backs can move only half as quickly. So there will seem to be a disadvantage for the amphipods, it would probably be more difficult to hunt for food. Cayne: And the sea butterfly probably can't feed at all. I mean, if it's being held on the amphipod's back, right? Professor: You're right. But I can't imagine a type of symbiosis that simply disadvantages both organisms. Cayne: Well, the amphipod does seek out the butterfly. So there must be some advantages for the amphipod. Is it for protection? Somehow the butterfly protects it? Professor: Exactly, very good. And it was interesting. The hypothesis researchers started with what the predators of the amphipods were responding to a visual cue. They saw the amphipod and sea butterfly together and turned away. However, this turned out not to be the case. In a lab experiment. These predatory fish were presented with two kinds of food pellets. One contains just fish meat, and one contains both fish meat and chemicals from sea butterflies. It turns out that the predators rejected only the food pellets containing the sea butterfly chemicals. But getting back to our classification system, this type of symbiotic relationship between the amphipod and the sea butterfly is something new. When you think carefully about it, none of these three existing categories can really be applied in a situation. This may be a special case, a type of symbiosis with both organisms are disadvantaged, and yet one is ultimately protected from its predators by actively kidnapping the other.