单选题 .  The fiddler crab is a living clock. It indicates the time of day by the color of its skin, which is dark by day and pale by night. The crab's changing skin color follows a regular twenty-four-hour cycle that exactly matches the daily rhythm of the sun.
    Does the crab actually keep time, or does its skin simply respond to the sun's rays, changing color according to the amount of light that strikes it? To find out, biologists kept crabs in a dark room for two months. Even without daylight the crab's skin color continued to change precisely on schedule.
    This characteristic probably evolved in response to the rhythm of the sun, to help protect the crab from sunlight and enemies. After millions of years it has become completely regulated inside the living body of the crab.
    The biologists noticed that once each day the color of the fiddler crab is especially dark, and that each day this occurs fifty minutes later than on the day before. From this they discovered that each crab follows not only the rhythm of the sun but also that of the tides. The crab's period of greatest darkening is precisely the time of low tide on the beach where it was caught!1.  The fiddler crab is like a clock because it changes color ______.