During the day in Lake Constance, the zooplankton D. hyalina departs for the depths where food is scarce and the water cold. D. galeata remains near the warm surface where food is abundant. Even though D. galeata grows and reproduces much faster, its population is often outnumbered by D. hyalina.
Which of the following, if true, would help resolve the apparent paradox presented above?
(A) The number of species of zooplankton living at the bottom of the lake is twice that of species living at the surface.
(B) Predators of zooplankton, such as whitefish and perch, live and feed near the surface of the lake during the day.
(C) In order to make the most of scarce food resources, D. hyalina matures more slowly than D. galeata.
(D) D. galeata clusters under vegetation during the hottest part of the day to avoid the Sun’s rays.
(E) D. galeata produces twice as many offspring per individual in any given period of time as does D. hyalina.
The paragraph presents an apparent paradox: the zooplankton that spends the day in less hospitable conditions often outnumbers the one that stays in more hospitable conditions.
The presence of predators of zooplankton feeding near the surface during the day would suggest that D. galeata is consumed at a higher rate than D. hyalina: this would explain why D. hyalina is often more numerous, so Choice B is correct. Choices C and E are incorrect because although they help to explain why the two zooplankton reproduce at different rates, they do not help to resolve the apparent paradox. Choices A and D are incorrect because nothing is said in the paragraph to show the relevance of the presence of other species of zooplankton, nor of the habit of clustering under vegetation, to the relative population size of the two species.