Anguillicola australiensis (Johnston et Mawson, 1940) is widespread and common in Anguilla reinhardtii Steidachner in rivers and dams of eastern Queensland, Australia, having been found in nine out of ten localities. Overall prevalence was 50% and maximum local prevalence reached 77.7%. The parasite never attained high levels of abundance and maximum adult abundance never exceeded 3.22 or intensity 10. Adults were overdispersed throughout the eel populations and abundance was unrelated to eel or swimbladder size. The greater part of the adult population was composed of immature parasites. The occurrence of larvae in the swimbladder wall was erratic and unrelated to the size of the adult population. Larvae were never abundant and the great majority were damaged by a host response. It appears that parasites either pass through the swimbladder wall rapidly and moult to adults or if delayed are destroyed. There was no indication that a paratenic host was involved in the life cycle. There was no evidence that adult parasites had any local pathogenic effects on their hosts. The population biology of Anguillicola australiensis in its natural host Anguilla reinhardtii appears to be far more similar to those of other Pacific species of Anguillicola in Anguilla japonica in China and Japan than to A. crassus in Anguilla anguilla in Europe or Japan. This latter host-parasite combination appears to be the exception not the rule. It is suggested that the lack of pathogenicity of A. australiensis may reflect a long period of host-parasite co-evolution and/or lower transmission rates resulting in lower parasite population densities.
The sanguinicolids Paracardicoloides yamagutii Martin, 1974 and Plethorchis acanthus Martin, 1975 were obtained from their definitive hosts, Anguilla reinhardtii Steindachner and Mugil cephalus Linnaeus (respectively) in the tributaries of the Brisbane River, Queensland, Australia. Two putative sanguinicolid cercariae were collected from a hydrobiid gastropod, Posticobia brazieri Smith, in the same waters. The two cercariae differ markedly in size and the form of their sporocysts. Both putative cercariae develop in the digestive gland of Po. brazieri. The ITS2 rDNA region from these sanguinicolids and a Clinostomum species (utilised as an outgroup due to the close morphological similarities between the cercarial stages of the Clinostomidae and the Sanguinicolidae) were sequenced and aligned. Comparison of the ITS2 sequences showed one cercaria to be that of P. yamagutii. This is the first sanguinicolid life history determined by a molecular method. P. yamagutii is the fourth sanguinicolid known to utilise a freshwater hydrobiid gastropod as its intermediate host. ITS2 rDNA is effective in distinguishing sanguinicolids at the species level.