Mysid crustaceans are subject to infestation by a variety of organisms, but little attention has been paid to the biological interaction between the hosts and their parasites. Recent studies have revealed higher diversity of parasitism in mysids than was thought previously. The present paper briefly reviews recent studies on interactions between mysids and their parasites.

Ciliates are common epibionts on mysids. Some peritrich ciliates have close relationships with intertidal species of Archaeomysis in the northwestern Pacific. The host-specificity, attachment site preference, and geographical cline of the ciliates are remarkable along the coasts of Japan. In Ishikari Bay, northern Japan, the prevalence of the ciliates is maintained at high levels throughout the year, indicating that the ciliates are capable of re-attaching on the fresh exoskeleton of mysids immediately after the molt of the hosts takes place. Eggs/embryos within the female marsupium are free from these epibionts.

Infection of some ellobiopsids adversely impacts on host crustaceans including mysids by inducing a reduction in development and reproduction, castration, and/or intersex. Some crustacean parasites found within the host marsupium such as nicothoid copepods and epicaridean isopods greatly influence host population dynamics, because of their relatively high incidence and devouring of host eggs/embryos. In the Seto Inland Sea, western Japan, these two parasites occur on Siriella okadai and seem to compete severely over the habitat (marsupium) and foods (eggs/embryos) of the host mysid.