Long-polluted waterways near Chicago are part of a system of natural and manmade barriers keeping invasive crustaceans at bay, a recent study suggests.By K.R. CallawayThrough the murky waters of the Illinois River, scud have been moving steadily toward Lake Michigan. Scientists suggest the tiny crustaceans, an invasive species known as Apocorophium lacustre, pose a big threat to the lake’s ecosystem—but so far, pollution has hindered their spread.

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