Recently biologists have been interested in a tide-associated periodic behavior displayed by the diatom Hantzschia virgata, a microscopic golden-brown alga that inhabits that portion of a shoreline washed by tides (the intertidal zone). Diatoms of this species, sometimes called “commuter” diatoms, remain burrowed in the sand during high tide, and emerge on the sand surface during the daytime low tide. Just before the sand is inundated by the rising tide, the diatoms burrow again. Some scientists hypothesize that commuter diatoms know that it is low tide because they sense an environmental change, such as an alteration in temperature or a change in pressure caused by tidal movement. However, when diatoms are observed under constant conditions in a laboratory, they still display periodic behavior, continuing to burrow on schedule for several weeks. This indicates that commuter diatoms, rather than relying on environmental cues to keep time, possess an internal pacemaker or biological clock that enables them to anticipate periodic changes in the environment. A commuter diatom has an unusually accurate biological clock, a consequence of the unrelenting environmental pressures to which it is subjected; any diatoms that do not burrow before the tide arrives are washed away. This is not to suggest that the period of this biological clock is immutably fixed. Biologists have concluded that even though a diatom does not rely on the environment to keep time, environmental factors—including changes in the tide’s hydrostatic pressure, salinity, mechanical agitation, and temperature—can alter the period of its biological clock according to changes in the tidal cycle. In short, the relation between an organism’s biological clock and its environment is similar to that between a wristwatch and its owner: the owner cannot make the watch run faster or slower, but can reset the hands. However, this relation is complicated in intertidal dwellers such as commuter diatoms by the fact that these organisms are exposed to the solar-day cycle as well as to the tidal cycle, and sometimes display both solar-day and tidal periods in a single behavior. Commuter diatoms, for example, emerge only during those low tides that occur during the day. 1 C 1. The passage suggests which of the following about the accuracy of the commuter diatom’s biological clock? (A) The accuracy of the commuter diatom’s biological clock varies according to changes in the tidal cycle. (B) The unusual accuracy that characterizes the commuter diatom’s biological clock is rare among intertidal species. (C) The commuter diatom’s biological clock is likely to be more accurate than the biological clock of a species that is subject to less intense environmental pressures. (D) The commuter diatom’s biological clock tends to be more accurate than the biological clocks of most other species because of the consistency of the tidal cycle. (E) The accuracy of the commuter diatom’s biological clock tends to fluctuate when the diatom is observed under variable laboratory conditions. 2 B 2. The author of the passage compares the relationship between an organism’s biological clock and its environment to the relation between a wristwatch and its owner most probably in order to (A) point out a fundamental difference between the function of biological clocks in organisms and the use of mechanical clocks by humans (B) illustrate the way in which the period of an organism’s biological clock can be altered by environmental factors (C) suggest that there are important similarities between the biological clock in organisms such as the commuter diatom and the biological clock in humans (D) support an argument regarding the methods used by certain organisms to counteract the influence of the environment on their biological clocks (E) question the accuracy of the biological clock in organisms such as the commuter diatom 3 B 3. According to the passage, the periodic behavior displayed by commuter diatoms under constant laboratory conditions is characterized by which of the following? (A) Greater unpredictability than the corresponding behavior under natural conditions (B) A consistent periodic schedule in the short term (C) No difference over the long term from the corresponding behavior under natural conditions (D) Initial variability caused by the constant conditions of the laboratory (E) Greater sensitivity to environmental factors than is the case under natural conditions 4 D 4. The primary purpose of the passage is to (A) dispute the influence of environmental factors on the tide-associated behavioral rhythms displayed by the diatom Hantzschia virgata (B) describe how certain tide-associated behavioral rhythms displayed by the diatom Hantzschia virgata have changed over time (C) compare tide-associated behavioral rhythms to solar-day behavioral rhythms in the diatom Hantzschia virgata (D) examine how certain biological and environmental influences affect the tide-associated behavioral rhythms displayed by the diatom Hantzschia virgata (E) identify certain environmental factors that limit the effectiveness of the biological clock in the diatom Hantzschia virgata 5 A 5. According to the passage, each of the following is characteristic of the tide-associated periodic behavior of commuter diatoms EXCEPT: (A) It is triggered when the diatoms are inundated by the tide (B) It is correlated with the rise and fall of the tide (C) It adjusts to changes in the tidal cycle (D) It is influenced by the solar-day cycle (E) It is regulated by an innate time-keeping mechanism 6 E 6. Which of the following can be inferred from the passage about the effect of the solar-day cycle on the tide-associated periodic behavior displayed by commuter diatoms? (A) The solar-day cycle makes this behavior less advantageous to the commuter diatoms at certain times of the year. (B) The solar-day cycle makes this behavior somewhat more erratic. (C) The solar-day cycle makes this behavior less important to the survival of the commuter diatoms. (D) Because of the solar-day cycle, this behavior is forced to proceed at a more rapid pace at certain times during the 24-hour day. (E) Because of the solar-day cycle, this behavior is not expressed at certain times during the 24-hour day. JOURNAL ARTICLE Time, Tide and the Living Clocks of Marine Organisms John D. Palmer American Scientist Vol. 84, No. 6 (NOVEMBER-DECEMBER 1996), pp. 570-578 Published by: Sigma Xi, The Scientific Research Honor Society Stable URL: https://www.jstor.org/stable/29775786 Page Count: 9 SCIENTIFIC AMERICAN February 1975 Volume 232, Issue 2 Biological Clocks of the Tidal Zone Endogenous clocks set to the rhythm of the solar day are known throughout the biological world. Many organisms that live along the shore also have a clock set to the rhythm of the lunar day By John D. Palmer

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Recently biologists have been interested in a tide-associated periodic

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ThatDudeKnows

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11 Jul 2022, 11:02

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Namangupta1997

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IN question 3, I was torn between B and C. I finally chose C as the passage says " they still display periodic behavior, continuing to burrow on schedule for several weeks". The word several made me go for C. I now think C is incorrect because of the extreme language such as " no change" and "long periods". We would have to assume a lot of stuff if we consider the "no change" situation. Is my understanding correct?

Furthermore, I want to ask how can we analyze the phrase "several weeks"? There is little context to precisely place this period of time in a "short term" or "long term" category. Or is this analysis not needed at all?

Namangupta1997

I think the big take-away here is developing the skill of playing answer choices off of each other. Why did you get rid of B (while not getting rid of C)?

I can't make an argument for how we can get rid of B and keep C, but I can make an argument for how we can get rid of C and keep B. That is, if C is correct, B is DEFinitely correct...but they can't both be, and the only way we can eliminate one without eliminating the other is to get rid of C. Make sense?

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11 Jul 2022, 11:13

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Namangupta1997

I hear what you are saying! Thanks for the insight.

To answer your question, I got rid rid of B as "short term" threw me off. I couldn't corelate "several weeks" to "short term". So by POE, only C was left.

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Q3 - Answer: (B)
Explanation: The passage states: "When tide-associated organisms such as the diatom Hantzschia virgata are kept in constant laboratory conditions, they continue to display their periodic behavior." It then goes on to explain how the period lengthens over time, but the immediate implication of "continue to display their periodic behavior" is that there's a consistent rhythm in the short term, even without external cues. The subsequent discussion about the period lengthening implies that the consistency holds for a shorter duration before drifting.

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