Host immunity Potential hosts may possess one of two types of immunity to attached glochidia. Acquired immunity occurs when a suitable host has been previously parasitized, and has built up a temporary immunity. In both natural and acquired immunity, encysted glochidia are killed. Acquired immunity may be lost if no subsequent reinfestation occurs within a certain time period, and the fish may become susceptible to parasitization again. However, the amount of time needed to lose acquired immunity is not precisely known.
Host specificity varies greatly among unionoids.
Some mussels, usually widespread and abundant ones, are able to parasitize many species of fishes. Others, usually localized and rare, may use only a few fish species. Either way, fishes must be present at the right time and the right place to serve as hosts. This is particularly true of migratory fishes. Because this symbiosis cannot be bypassed, plans to conserve and manage rare and endangered mussels must include management of the host as well.