Unlike other assemblages of class Arachnida, many representative groups of the Acari are not predators. Some are exclusively phytophagous, while others include external and internal parasites of both the vertebrates and invertebrates (Krantz, 1970).
Molluscans are an interesting group of invertebrates, that are parasitized by a variety of organisms, ranging from viruses to arthropods. Accordingly, a number of pathological changes induced by the foreign organisms inside the body of the molluscan hosts, ranged from extremely severe to minor alternations. Now, it is well documented that the larvae of some parasites reduces the fecundity of the host, makes a host's cellular reactions and find passive pathologic alternations (Malek & Cheng, 1974).
The majority of water mites parasitize on the water organisms plants or animals. Members of the genus Unionicola have parasitic associations with molluscan or poriferan hosts. The present species is found to be parasitic on the molluscan host Anodonta rubens. The effect of this parasite on the host have not been investigated before. So, the present study is a trial to investigate the incidence of the parasite and its pathogen effects on the host. The present part of the study aims to demonstrate the relationship between the host and the parasite, showing the incidence of parasite and the pathological changes in the gill tissues of the host.