An Experimental Study of the Population Dynamics of an Ectoparasitic Digenean, Transversotrema patialense: The Cercarial and Adult Stages

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Abstract
The biological components of the population dynamics of the cercarial and adult stages of the ectoparasitic digenean T. patialense are examined within an experimental framework where temperature and dark-light regimes remained constant. The survival characteristics of the larval and adult parasites are age-dependent. At 24.degree. C the maximum life span of the cercariae is approximately 44 h while that of the adult is close to 10 wk. The factors influencing cercarial production by the molluscan intermediate host Melanoides tuberculata are discussed. The size of the snails and their nutritional status are suggested to be important determinants of larval parasite production. The temporal constancy of cercarial output by snails of unknown infection origin is discussed in relation to single and multiple miracidial infections. Multiple infections probably do not lead to greaterly increased larval production. Losses from the free-living cercarial populations are due to 3 processes; natural mortalities, infection of the final fish host and predation by the fish. Infection of the fish host, Brachydanio rerio, is essentially a chance process depending on random contacts between host and cercariae. The rate of infection is thus directly proportional to the density of larvae in an aquatic habitat, although the degree of proportionality is shown to be dependent on the age of the cercariae. Predation may be a very important component of the dynamics of the larval parasite population. The functional response of the fish predator to prey density is sigmoid in form. The survival characteristics of the adult parasite on the fish host remain constant in form, in parasite populations subject to temporally overlapping infections. This observation is taken to suggest that host generated immunological mechanisms are not important in the dynamics at the parasite population levels used in the experiments. The regulatory influences in the complete life cycle of T. patialense are discussed in general terms. The existence of a number of density dependent processes in the life cycle is suggested, including predation, infection of the snail host, adult parasite survival and fecundity and mortality of infected intermediate and final hosts. Deterministic immigration-death models, incorporating population age structure are developed to describe the dynamics of both cercarial and adult parasite populations. Stochastic influences are discussed in relation to immigration and mortality.