In order to disentangle the contribution of host and parasite biology to host specificity, we compared the structure and population dynamics of the Gyrodactylus (von Nordmann, 1832) flatworm community living on sympatric three-spined Gasterosteus aculeatus L. and nine-spined Pungitius pungitius (L.) stickleback. Between April 2002 and March 2003, a small lowland creek was sampled monthly. Species identity of about 75% of the worms per host was determined with a genetic nuclear marker (ITS1). Each stickleback species hosted a characteristic gill- and fin-parasitic Gyrodactylus: G. arcuatus Bychowsky, 1933 and G. gasterostei Gläser, 1974 respectively infecting the three-spined stickleback, with G. rarus Wegener, 1910 and G. pungitii Malmberg, 1964 infecting the nine-spined stickleback. Host size and seasonal dynamics were strong determinants of parasite abundance. A strong interaction between host and parasite species determined infection levels and affected three levels of parasite organisation: community structure, population structure and topographical specialisation. Community and population structure were shaped by asymmetric cross-infections, resulting in a net transmission of the Gyrodactylus species typical of the nine-spined stickleback towards the three-spined stickleback. Host density was not a major determinant of parasite exchange. Aggregation and topographical specialisation of the Gyrodactylus species of the three-spined stickleback were more pronounced than that of the nine-spined stickleback.
Helminthological examination of the snaggletooth shark, Hemipristis elongata (Klunzinger) (Carcharhiniformes: Hemigaleidae), from Moreton Bay, Queensland, Australia, yielded a phyllobothriid genus and species previously unknown to science. Hemipristicola gunterae gen. n., sp. n. is described here, and is placed in the subfamily Phyllobothriinae Braun, 1900. Of the other phyllobothriid genera, the new genus most closely resembles Paraorygmatobothrium in that both genera possess bothridia with a single loculus and apical sucker, post-vaginal testes and lateral vitellarium. Hemipristicola, however, differs from Paraorygmatobothrium in the morphology of the proximal bothridial surface microthrix, possessing serrate gladiate spinitriches with marginal serrations restricted to the distal half of the blade, and in the possession of a more extensive uterus, extending anteriorly from the anterior margin of the ovary to well past the level of the cirrus-sac. The new genus also differs from Paraorygmatobothrium by possessing testes that are more than one layer deep. Hemipristicola gunterae further differs from Paraorygmatobothrium species found in hemigaleid sharks in possessing vitelline follicles arranged in two lateral bands that are restricted to the lateral margins of the proglottid and not possessing a cephalic peduncle. Bayesian inference analysis of partial 28S rDNA data shows that H. gunterae forms a sister taxon to species of Paraorygmatobothrium. These two genera were resolved with high posterior probability support in the analysis. Hemipristicola gunterae is only the second phyllobothriid species to be described from Hemipristis elongata from Australian waters, and the fourth from the Australian hemigaleids.