Complexity in Parasite Life Cycles: Population Biology of Cestodes in Fish

Abstract

1. This paper examines the population biology of two related species of bothriocephalid cestodes, parasites of teleostean fish, which live in sympatry. One species, Bothriocephalus barbatus, needs in its life cycle only one obligatory intermediate host, a copepod. The second species, B. gregarius, needs the same intermediate host first, but also utilizes a non-obligatory paratenic host, which is a gobiid fish. In the case of B. gregaring, definitive hosts can be infested via the intermediate host or the paratenic host. A simple mathematical model was built to investigate the effect of the addition of a paratenic host into the life cycle of a parasite. 2. Results of the simulation clearly demonstrated that the maintenance or absence of infectivity of cestode larvae in the paratenic host could explain the observed levels of infection in both definitive hosts. 3. Acquisition of paratenic hosts has two advantages: the recovery of lost infective stages in a previously non-suitable intermediate host, and an increase in the time of infection during which the definitive host could be infected as the result of eating copepods in its planctonophagous juvenile existence and by eating gobies in its predaceous older stages. 4. Using the basic transmission rate as a measure of fitness, we also investigated the possibility of maturation of B. gregarius in the paratenic host, e.g. the acquisition of a new definitive host by the parasite. Basic transmission rates and numerical simulations suggest that there is no benefit for the parasite in evolving towards this strategy