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Q20 to 23 recently biologists have been interested in

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----------------------------------------------------------------------------------------------------------- - Q20 to 23: Recently biologists have been interested in a tide-associated periodic behavior displayed by Line the diatom Hantzschia virgata , a (5) 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 (10) burrowed in the sand during high tide, and emerge on the sand sur- face during the daytime low tide. Just before the sand is inundated by the rising tide, the diatoms burrow (15) again. Some scientists hypothesize that commuter diatoms know that it is low tide because they sense an environmental change, such as an 22
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alteration in temperature or a change (20) in pressure caused by tidal move- ment. However, when diatoms are observed under constant conditions in a laboratory, they still display periodic behavior, continuing to bur- (25) row on schedule for several weeks. This indicates that commuter diatoms, rather than relying on environmental cues to keep time, possess an inter- nal pacemaker or biological clock (30) that enables them to anticipate peri- odic changes in the environment. A commuter diatom has an unusu- ally accurate biological clock, a consequence of the unrelenting (35) 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 (40) period of this biological clock is immutably fixed. Biologists have concluded that even though a diatom does not rely on the envi- ronment to keep time, environmental (45) factors—including changes in the tide’s hydrostatic pressure, salin- ity, mechanical agitation, and temperature—can alter the period of its biological clock according to (50) 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 (55) 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 (60) exposed to the solar-day cycle as well as to the tidal cycle, and some- times display both solar-day and tidal periods in a single behavior. Commuter diatoms, for example, 23
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(65) emerge only during those low tides that occur during the day. --------------------------------------------------------------------------------
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