hydrachnid

from The Collaborative International Dictionary of English v.0.48

Hydrachnid \Hy*drach"nid\, n. [Hydr- + arachnid.] (Zool.)
   An aquatic mite of the genus {Hydrachna}. The hydrachnids,
   while young, are parasitic on fresh-water mussels.
   [1913 Webster]