Colony of Epistylis sp. ciliate protozoa covering the surface of a water beetle. Epistylis is a stalked protozoan or peritrichous ciliate that appears as a fluffy growth on the external surface of crustaceans, insects and fish. Epistylis is harmless to the animal unless it grows in such large numbers as to restrict water flow over the gills, causing asphyxiation. Epistylis uses the animal as an attachment substrate, so it can feed on bacteria and particles in the water.