Abstract
Many species of marine Monogenea occur at low densities and prevalences of infection. Selection to ensure cross-fertilization may be particularly important for rare species. A result of such selection may be an "Allee-effect': elimination of parasites occurring in very small numbers on a host individual. At least in some cases, a reduced "tail' in negative binomial distributions, used to postulate mortalities of heavily infected host individuals, can also be explained by assuming that an "Allee-effect' has occurred. This considerably reduces the value of frequency distributions for postulating parasite-induced mortalities. Only if other direct or strong circumstantial evidence is available, for instance direct observation of death due to heavy infections or pathological evidence, can parasite-induced mortality be assumed.