GMAT Critical Reasoning (CR) | Re: Populations of a shrimp species

Populations of a shrimp species at eleven different Indonesian coral reefs show substantial genetic differences from one reef to another. This is surprising because the area’s strong ocean currents probably carry baby shrimp between the different reefs, which would allow the populations to interbreed and become genetically indistinguishable.

Which one of the following, if true, most helps to explain the substantial genetic differences among the shrimp populations?
y
(A) The genetic differences