The crustaceans known as harpacticoids are very widespread in marine sediments, where they feed on microorganisms by ingesting the sediment particles to which the microorganisms adhere. Heavy metals, such as those found in industrial pollution, readily adhere to sediment particles. Harpacticoids are poisoned by heavy metals but are unaffected by most other pollutants. Therefore the concentration of harpacticoids in an area is a good indication of whether that marine environment contains heavy metals.