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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.
GWD1-Q25:
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.
GWD1-Q26:
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
GWD1-Q27:
According to the passage, the periodic behavior displayed by commuter diatoms under constant laboratory conditions is characterized by which of the following?
A.
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
GWD1-Q28:
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
identify certain environmental factors that limit the effectiveness of the biological clock in the diatom Hantzschia virgata
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