noboru wrote:
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 acquire 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.
Which of the following, if true, most helps to explain the surprising finding?
(A) Tidal currents do not themselves dislodge barnacles from the shells of horseshoe crabs.
(B) Barnacles most readily attach themselves to horseshoe crabs in areas 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 their shells and grow new ones several times a year.
GMATNinja,
GMATNinja,
ChiranjeevSingh,
AnishPassiHi,
I was confused b/w option D and E and chose D eventually. My reasoning for selecting D and rejecting E is as follows:
Option E (rejected): Horseshoe Crabs (HCs) which are growing shed their shells several times a year. But this phenomenon is applicable to both types of HCs - ones in strong-tide areas and others in weak-tide areas. Further, HCs in strong-tide areas also burrow, so they also shouldn't have significant barnacles population (In fact, they should have less barnacles population as they are burrowing and shedding shells). On the other hand, HCs in weak tide areas on have one counter-measure against barnacles (i.e. shedding shells). Therefore, option E doesn't help us resolve the paradox.
Option D (selected): Although, this option does not specficially talk about juvenile HCs and rather presents a general observation on HCs, it is applicable to juvenile HCs also. If excessive barnacles grow on juvenile HCs, latter would struggle to find food. If the host (HC) is weak and not getting food, the parasite (barnacles) wouldn't be able to sustain on the host for long. Therefore, barnacles keep their population growth in check on juvenile HCs in weak-tide areas. Therefore, D explains the paradox.