These holotrich ciliates invade the host gonad causing atrophy of the gonad and castration in heavily infected sea stars. Infection is most prevalent in males and can reduce the proportion of males in a population. The reproductive potential in affected populations is decreased due to reduced male gamete production and the size structure of the population is altered. In addition, infections of O. stellarum were associated with a failure of coelomocytes to aggregate (clump) in A. forbesi (Childs 1970, Taylor and Bang 1978) and A. vulgaris (Bang 1982).