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 sur-
face 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 move-
ment. However, when diatoms are
observed under constant conditions
in a laboratory, they still display
periodic behavior, continuing to bur-
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
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
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 envi-
ronment to keep time, environmental
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
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 some-
times 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.


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

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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