第一部分 阅读理解（共20小题，满分60分）
A
The fiddler crab (蟹) is a living clock. It indicates(=shows) the time of day by the colour of its skin, which is dark by day and pale by night. The crab’s changing colour follows a regular twenty—four hour plan that exactly matches the daily rhythm (节奏) of the sun.
Does the crab actually keep time, or does its skin simply answer to the sun’s rays, changing colour according to the amount of light strikes it? To find out, biologists kept crabs in a dark room for two months. Even without daylight, the crab’s skin colour continued to change exactly on time.
This characteristic (特性) probably developed gradually in answer to the daily rising and setting of the sun, to help protect the crab from sunlight and enemies. After millions of years it has become completely regulated (受控制) inside the living body of the crab.
The biologists noticed that once each day the colour of the fiddler crab is especially dark, and that each day this happens 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 (潮水). The crab’s period of greatest darkening is exactly the time of low tide on the beach where it was caught!
1. The fiddler crab is like a clock because it changes colour ______.
