Comparative spermatology of freshwater mites (Hydrachnidia, Acari)

The ultrastructure of sperm cells of representatives of all superfamilies of Hydrachnidia except Stygothrombioidea is described. The sperm are aflagellate cells with magnitudes reaching 1.3 μm up to 6 μm. They are mostly oval cells, but some show an irregular shape. All investigated mites have an acrosomal complex which is composed of an acrosomal vacuole alone. An acrosomal filament is absent. This character together with a prominent field of granules, likely glycogen, may be regarded as synapomorphic of the groups forming a monophylum Hydrachnidia. All Hydrachnidia except Hydrovolzia and the Eylaoidea possess a rather large acrosomal vacuole to which a thin nuclear process attaches. This arrangement supports the taxon Euhydrachnidia. Further details shown in the fine structure of the sperm cells demonstrate the potential of these characters for the development of a better understanding of the systematic relationships within the Hydrachnidia. However, this needs further studies of more species, which should include Stygothrombioidea and observations of spermatogenesis.