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Recent research has questioned the long-standing view of pearly mussels as exclusively suspension feeders (animals that strain suspended particles from water) that subsist on phytoplankton（mostly algae). Early studies of mussel feeding were based on analyses of gut contents, a method that has three weaknesses. First, material in mucus-bound gut contents is difficult to identify and quantify. Second, material found in the gut may pass undigested out of the mussel, not contributing to its nutrition. Finally, examination of gut contents offers limited insight into the mechanisms and behaviors by which mussels acquire food. Modern studies suggest that pearly mussels feed on more than just algae and may use other means than suspension feeding. Pedal feeding (sweeping up edible material with a muscular structure called the foot) has been observed in juvenile pearly mussels.

Besides the phytoplankton pearly mussels capture from the water column, their guts also contain small animals, protozoans, and detritus (nonliving particulate organic material). Recent studies show that mussels can capture and assimilate bacteria as well, a potentially important source of food in many fresh waters. Another potential source of food for mussels is dissolved organic matter. Early studies showing that pearly mussels could take up simple organic compounds were largely discounted because such labile (unstable) compounds are rarely abundant in nature. Nevertheless, recent work on other bivalves suggests that dissolved organic matter may be a significant source of nutrition.

Of this complex mix of materials that pearly mussels acquire, what is actually required and assimilated? Stable-isotope analyses of mussels taken from nature and of captive-reared mussels are beginning to offer some insight into this question. Nichols and Garling showed that pearly mussels in a small river were omnivorous, subsisting mainly on particles less than 28 micrometers in diameter, including algae, detritus, and bacteria. Bacterially derived carbon was apparently the primary source of soft-tissue carbon. However, bacteria alone cannot support mussel growth, because they lack the necessary long-chain fatty acids and sterols and are deficient in some amino acids. Bacteria may supplement other food resources, provide growth factors, or be the primary food in habitats such as headwater streams, where phytoplankton is scarce. Juvenile mussels have been most successfully reared in the laboratory on diets containing algae high in polyunsaturated fatty acids. Thus, it appears that the pearly mussel diet in nature may consist of algae, bacteria, detritus, and small animals and that at least some algae and bacteria may be required as a source of essential biochemicals.

The primary purpose of the passage is to

(A) summarize the present state of knowledge about what pearly mussels eat. (B) contrast two mechanisms by which pearly mussels have been shown to feed. (C) explain why evidence used to support a long-standing view about pearly mussels is flawed. (D) suggest that a particular source of nutrients is more essential to pearly mussels than was previously thought. (E) question whether research findings about laboratory-reared pearly mussels can be extrapolated to pearly mussels in natural habitats.

正确答案： C 耗时：

该题平均耗时：4分28秒，平均正确率：41 %，难度系数：4 。该题由网友lgw20423提供。更多GRE题目请点击上传

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当前版本由去没人的岛更新于2023-07-14 11:08:13 感谢由去没人的岛对此题目的解答所做出的贡献。

第一题：主旨题。文章第一段首句提到最近的研究对这种观点提出了质疑；第二句提到之所以有这种质疑，主要有三方面的原因。所以文章就是对于“珍珠贻贝是以浮游植物为食的唯一悬浮喂食者”这一观点有缺陷提出了解释。所以答案选C。