59. In order to withstand tidal currents,juvenile horseshoe crabs frequently burrow in the sand. Such burrowing discourages barnacles from clinging to their shells. When fully grown, however,the crabs can readily withstand tidal currents without burrowing, and thus they aquire substantial populations of barnacles. Surprisingly,in areas where tidal currents are very weak,juvenile horseshoe crabs are found not to have significant barnacle populations, even though they seldom burrow.
A. Tidal currents do not themselves dislodge barnacles from the shells of horseshoe crabs.
B. Barnacles most readily attach themselves to horseshoe crabs in aeras where tidal currents are weakest.
C. The strength of the tidal currents in a given location varies widely over the course of a day.
D. A very large barnacle population can significantly decrease the ability of a horseshoe crab to find food.
E. Until they are fully grown,horseshoe crabs shed sheir shells and grow new several times a year.
upstair is great . the concise mainline is that juvenile (to withstand,burrow--> not barnacle) and grown(enable to withstand,not burrow-->barnacles ) exist,but some juvenile in weak current area (not burrow,not barnacle). why ?
谢谢上面解释， 但是又有疑问，根据E的说法，那些fully grown的horseshoe crabs 的壳不是一样会 have significant barnacle populations吗？
-- by 会员 yuanxiaoning (2011/8/26 15:52:18)
When fully grown, however,the crabs can readily withstand tidal currents without burrowing, and thus they aquire substantial populations of barnacles. E. Until they are fully grown,horseshoe crabs shed sheir shells and grow new several times a year.
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发表于 2011-8-20 16:49:23
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