A war is raging up and down the Mississippi River Basin over an imported mollusk-eating species of Asian carp, the black carp. Fish farmers say the voracious meter-long fish, which was imported from China in the 1970s by Arkansas aquaculturists, has become an indispensable tool to control fish parasites that threaten their livelihood. But fisheries and mollusk experts worry that it could decimate populations of native mussels, clams, and snails if it escapes into the wild.