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DanaJ Site Admin
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Clams Post Wed Sep 01, 2010 2:30 am
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  • Lap #[LAPCOUNT] ([LAPTIME])
    Source: Beat The GMAT Practice Questions

    In their relationship to the general ecology of the reefs, the Mollusca as a group of animals play a highly significant role. Because of the nature of their shells, mollusc remains may be found among the limestone debris of a reef dating back to its very earliest stages of evolution in the geological past, and may, therefore, be considered as having aided in its construction. Yet, as boring organisms, in both living and dead coral, certain species of molluscs rank among the most destructive agents to be found on the reefs. Mollusc eggs are laid in tens of millions, and the floating larval stages form a very important part of the zooplankton that sustains life in the waters over the reefs. In their vast numbers, as herbivores and carnivores, the molluscs are both prey and predator on the reefs.

    Most molluscs obtain their oxygen directly from the seawater by means of gills in the mantle cavity. Molluscs with two valves, or shells, hinged together are referred to as bivalves. In bivalves the gills have become so developed that in many species they also act as the food-catching apparatus. Because most bivalves are filterfeeding animals, sieving minute organisms from the surrounding seawater, they have no differentiated head region and no radula (tonguelike organ).

    It is in the method of feeding that the clams of the family Tridacnidae display their remarkable adaption to their environment. In the course of their evolution, the various members have become structurally modified to best enable them to develop in their particular habitat. These clams normally live with the animal lying on the hinge side of the shell, and the edges of the shell valves pointing upwards. The result of this mode of living is that the internal organs in their relationship to the mantle and shell differ from those of all other bivalve molluscs. The mantle tissue, by reason of the animal’s way of life, is capable of considerable expansion, and is exposed to the direct rays of the sun to the greatest possible extent. This is undoubtedly associated with the most unusual feature of all displayed by these remarkable molluscs: Within this mantle tissue are millions of tiny zooxanthellae, closely resembling the symbiotic algae found in the corals and the alcyonarians. It has been definitely established that these zooxanthellae form a considerable part of the diet of the Tridacnas, and the modifications found in these molluscs indicate that they are not only specialized for harboring these minute algae, but that they also deliberately “farm” them. This must surely be one of the most fascinating examples of symbiosis to be found in nature. The whole ecology of the clams, bound exclusively to warm, shallow waters of tropical seas, appears linked to an increase in the efficiency of that symbiosis, which gives food and protection to the algae and very considerable additional nutrient from its symbionts to the clams.

    3. An appropriate title for this passage would be:

    (A) Filter Feeding in Bivalve Molluscs
    (B) Plankton in Coral Reef Ecology
    (C) Prey and Predator in the Tropical Seas
    (D) Role of Molluscs in the Reef Ecosystem
    (E) Molluscs and Coelenterates of the Coral Reef



    Last edited by DanaJ on Thu Sep 02, 2010 12:47 pm; edited 1 time in total

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    niksworth GMAT Destroyer! Default Avatar
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    Post Wed Sep 01, 2010 7:05 am
    The answer is D.

    I remember this one from your video lecture on RC shown a few days ago. Smile

    deeyah Just gettin' started! Default Avatar
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    Post Wed Sep 01, 2010 7:19 am
    "In their relationship to the general ecology of the reefs, the Mollusca as a group of animals play a highly significant role."

    I would go with option D

    DanaJ Site Admin
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    Post Wed Sep 01, 2010 2:27 pm
    @niksworth: well glad you saw that! Hope you liked it - and maybe you could share with me some other ideas for improvement/future sessions?

    So the answer is indeed D here.

    A is incorrect because filterfeeding in bivalves is only discussed in the second (relatively short) paragraph, which makes this unsuited as a title of a huge, detailed passage like this one.

    B is also incorrect because plankton is only mentioned once, in the very end of paragraph 1: ..and the floating larval stages form a very important part of the zooplankton that sustains life in the waters...

    C is wrong because there's really not much talk about prey and predator or the tropical seas themselves. The molluscs don't really "prey" on anything, they sit in a place and filter water! Plus, "tropical seas" are only mentioned in the very end of the third paragraph.

    D is correct because it encapsulates much of what's said in paragraph 1 (molluscs are both constructive and destructive forces in the reef), paragraph 2 (molluscs filter water and minute organisms) and 3 (molluscs farm certain small creatures).

    E is wrong because there's no mentioning of Coelenterates. If you can believe it, I learned about Coelenterates in 6th grade - my biology teacher was kindda crazy and had us learn scientific terms for animals and plants! Euglena Viridis, Paramoecium Caudatum... Still remember these! Smile

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    Post Sat Nov 06, 2010 1:21 pm
    DanaJ wrote:
    E is wrong because there's no mentioning of Coelenterates. If you can believe it, I learned about Coelenterates in 6th grade - my biology teacher was kindda crazy and had us learn scientific terms for animals and plants! Euglena Viridis, Paramoecium Caudatum... Still remember these! Smile
    OMG that is crazy. This is an extremely boring passage for me. I was somehow trying to keep up with it. Its good to have extra information like these to keep you engrossed. Smile

    thp510 Rising GMAT Star Default Avatar
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    Post Sun Nov 07, 2010 11:18 am
    DanaJ wrote:
    E is wrong because there's no mentioning of Coelenterates. If you can believe it, I learned about Coelenterates in 6th grade - my biology teacher was kindda crazy and had us learn scientific terms for animals and plants! Euglena Viridis, Paramoecium Caudatum... Still remember these! Smile
    DanaJ

    I think your ability to retain past knowledge or have some type of photographic memory is a SIGNIFICANT enabler for scoring well on tests. I don't even remember my 6th grade science teacher's name! How does anyone beat the gmat if they can't even remember what they had for breakfast today Sad

    DanaJ Site Admin
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    Post Sun Nov 07, 2010 11:20 am
    hey you don't really need to remember stuff like this to score well in the GMAT. With RC passages, everything you need to answer a question appears in the text! No outside knowledge required (except maybe super easy stuff like profit = revenues - expenses)!

    mundasingh123 GMAT Titan Default Avatar
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    Post Tue Nov 09, 2010 5:19 am
    DanaJ wrote:
    @niksworth: well glad you saw that! Hope you liked it - and maybe you could share with me some other ideas for improvement/future sessions?

    So the answer is indeed D here.


    C is wrong because there's really not much talk about prey and predator or the tropical seas themselves. The molluscs don't really "prey" on anything, they sit in a place and filter water! Plus, "tropical seas" are only mentioned in the very end of the third paragraph.

    Smile
    Hi DanaJ,can you elaborate on why is C wrong?The entire last para has been devoted to it.The clams of the family Tridacnidae harbour and farm algae in their mantle tissue .The algae in return provides food for the clams.The entire last para has been dedicated to the symbiosis between the clams and the algae.This has also been referred to as the prey and predator relationship in the earlier para.
    The author emphasizes the dependence of the ecology of the clams on this symbiotic relationship between the algae and th the clams.There is no mention of the the importance of the clams as far as the reef eco system is concerned

    DanaJ Site Admin
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    Post Tue Nov 09, 2010 5:43 am
    Yes, it's very true that the entire last paragraph is about the symbiosis between the clams and the small creatures. However, I do not feel that "symbiosis" and "prey and predator" are synonyms. As far as I know, symbiosis is a process which is about one creature helping the other. The "prey and predator" relationship is very different in the sense that one animal preys on the other, which gets nothing in return.

    mundasingh123 GMAT Titan Default Avatar
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    Post Tue Nov 09, 2010 6:30 am
    DanaJ wrote:
    Yes, it's very true that the entire last paragraph is about the symbiosis between the clams and the small creatures. However, I do not feel that "symbiosis" and "prey and predator" are synonyms. As far as I know, symbiosis is a process which is about one creature helping the other. The "prey and predator" relationship is very different in the sense that one animal preys on the other, which gets nothing in return.
    hi dana the last para is the largest para of the passage and it has no mention of the importance of clams in the ecosystem.The clams are not doing anything beneficial for the eco sytem, are they?

    DanaJ Site Admin
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    Post Tue Nov 09, 2010 8:36 am
    Yes, that's very true, there is no mention of "ecosystem" in the last paragraph. However, you do see a series of other words which should tip you off: "environment," "particular habitat," "the whole ecology of the clams." These words all signal that indeed, we are discussing a part of an ecosystem.

    mundasingh123 GMAT Titan Default Avatar
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    Post Tue Nov 09, 2010 9:59 am
    DanaJ wrote:
    Yes, that's very true, there is no mention of "ecosystem" in the last paragraph. However, you do see a series of other words which should tip you off: "environment," "particular habitat," "the whole ecology of the clams." These words all signal that indeed, we are discussing a part of an ecosystem.
    i apologize for being so persistent but this is partly because i am weak at RCs and i thought i was good at pointing out inferences.So,DanaJ, what i am so concerned abt is that i dont see any role played by clams that benefits the ecosytem .I want to clarify whether the dependency of the algae on the clams for their food (clams "farm" them) is what is meant by the importance of clams to the ecosystem.

    DanaJ Site Admin
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    Post Tue Nov 09, 2010 4:10 pm
    It's OK, don't worry!

    The fact is, you don't have anything in D about "benefits." D is strictly about the role of the molluscs in the ecosystem, which is basically the same as their interaction with the creatures that are a part of that ecosystem. The molluscs do not necessarily benefit the ecosystem in any way, they just exist and interact with the world around them. The symbiosis with the algae is just that.

    Abhishek009 GMAT Destroyer!
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    Post Fri Nov 19, 2010 8:21 am
    No doubt I will go with D.

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    Post Tue Nov 23, 2010 6:12 pm
    Ya agree .. the answer should be "d"

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