Mutual exclusion of congeneric monogenean species in a space-limited habitat

Adults of the monogenean genus Protopolystoma infecting Xenopus species occ ur in an extremely space-limited habitat, the urinary bladder. Xenopus witt ei, from a population in Rwanda naturally infected with Protopolystoma fiss ilis and Protopolystoma simplicis, were exposed to reinfection in captivity (for 1-3 months post-capture) and then monitored in the laboratory for up to 5 months in transmission-free conditions. The two parasites co-occurred in individual bladders less frequently than expected if they were dispersed randomly. Distribution of bladder infections was significantly nonindepend ent (n = 157) and gravid worms of both species were never found in the same host. This pattern might be explained by interference competition between the parasites or by genetic differences in susceptibility within the host s pecies, which is of allopolyploid origin. Other distributional data for sym patric polystomatid species pairs, including P. fissilis and P. ramulosus, show concurrent infections at frequencies consistent with random distributi ons (i.e. no evidence of interspecific competition or variability in specie s-specific susceptibility of the hosts). Interference between P. fissilis a nd P. simplicis (assuming host genetic factors are not involved) may theref ore result from a mechanism specific to this species pair. Observations on infection turnover in captive hosts suggest that loss of adult worms may be related to the arrival of juveniles (of either species) in the urinary bla dder. Ectopic infection of the host urinary ducts by adult and subadult P. fissilis was observed in some single-species infestations and may be densit y related. However, the use of an ectopic-site 'refugium' has never been ob served in concurrent polystomatid infections.