A new weapon against the invasive zebra mussel, still wreaking havoc in the Great Lakes, could soon be deployed: microcapsules of a chlorinated salt. Chlorination is the widespread method for attacking zebra mussels. The problem is that the mollusks can detect chlorine and seal their valves for up to three weeks, necessitating prolonged dosing, which poisons the water. Instead University of Cambridge researchers took potassium chloride, a salt especially toxic to zebra mussels but at low doses harmless to most organisms, and wrapped it in 105-micron-wide capsules made of vegetable oil and other ingredients. These match the size of particles that zebra mussels filter from the water. Aquarium findings reported in the February 1 Environmental Science & Technology show that the capsules killed 60 percent of zebra mussels with one dose while leaving native mussels apparently unscathed.