COMPLEXITY IN PARASITE LIFE-CYCLES - POPULATION BIOLOGY OF CESTODES IN FISH

1. This paper examines the population biology of two related species o f bothriocephalid cestodes, parasites of teleostean fish, which live i n sympatry. One species, Bothriocephalus barbatus, needs in its life c ycle only one obligatory intermediate host, a copepod. The second spec ies, B. gregarius, needs the same intermediate host first, but also ut ilizes a non-obligatory paratenic host, which is a gobiid fish, In the case of B. gregarius, definitive hosts can be infested via the interm ediate host or the paratenic host, A simple mathematical model was bui lt to investigate the effect of the addition of a paratenic host into the life cycle of a parasite. 2. Results of the simulation clearly dem onstrated that the maintenance or absence of infectivity of cestode la rvae in the paratenic host could explain the observed levels of infect ion in both definitive hosts. 3. Acquisition of paratenic hosts has tw o advantages: the recovery of lost infective stages in a previously no n-suitable intermediate host, and an increase in the time of infection during which the definitive host could be infected as the result of e ating copepods in its planctonophagous juvenile existence and by eatin g gobies in its predaceous older stages. 4. Using the basic transmissi on rate as a measure of fitness, we also investigated the possibility of maturation of B. gregarius in the paratenic host, e.g. the acquisit ion of a new definitive host by the parasite. Basic transmission rates and numerical simulations suggest that there is no benefit for the pa rasite in evolving towards this strategy.