water mite

from the GNU version of the Collaborative International Dictionary of English.

• (Zoöl.) Any of numerous species of aquatic mites belonging to Hydrachna and allied genera of the family Hydrachnidæ, usually having the legs fringed and adapted for swimming. They are often red or red and black in color, and while young are parasites of fresh-water insects and mussels. Called also water tick, and water spider.