Trematodes of the family Schistosomatidae are considered venous system specialists whose sister group is the vascular system generalists (Spirorchidae) of turtles. Colonization of homeotherms by vascular trematodes required precision egg laying near the conduit for egg passage to the external environment and avoidance of pathogenesis that might result in the premature death of the host. Evolution of dioecy from the hermaphroditic condition may have proceeded through androdioecy in which hermaphrodites were specialized for precision egg placement in the vascular system and larger adults became functional males. The evolution of nuclear genes suppressing female function along with cytoplasmic genes suppressing male function could then have resulted in the origin of dioecious, dimorphic populations. Schistosomes compensated for the reduction in potential reproductive partners by (1) increased overdispersion in the vertebrate host, (2) reduced egg hatching time in the external environment, (3) formation of permanent pairs mimicking the hermaphroditic condition, (4) increased longevity in the definitive host, and (5) increased fecundity. Colonization of the venous system was necessitated by (1) evolutionary radiation into terrestrial vertebrates and (2) the increased immunopathology associated with the high, constant body temperature of homeothermic vertebrates. The immune response to spirorchid and schistosome eggs appears to be qualitatively similar in their respective hosts. The arterial dwelling spirorchids release eggs in the direction of blood flow, resulting in a wide dissemination of eggs within the host. The lower body temperature of poikilotherms accompanied by the seasonal nature of the immune response in these hosts would result in a quantitatively reduced pathogenesis. Hosts that did succumb to the infection would most likely die in water, where eggs could be released by predation, scavengers, or decomposition and develop successfully. Colonization of the venous system by schistosomes would require precision egg placement because eggs are released against blood flow. Eggs are sequestered within the portal system of homeotherms, thus restricting egg dispersal and resulting pathogenesis to less sensitive organs. A significant number of eggs may escape into the external environment before a heavily infected host is incapacitated by, or dies from, the infection.

One of the major threats to biodiversity involves biological invasions with direct consequences on the stability of ecosystems. In this context, the role of parasites is not negligible as it may enhance the success of invaders. The red-eared slider, Trachemys scripta elegans, has been globally considered among the worst invasive species. Since its introduction through the pet trade, T. s. elegans is now widespread and represents a threat for indigenous species. Because T. s. elegans coexists with Emys orbicularis and Mauremys leprosa in Europe, it has been suggested it may compete with the native turtle species and transmit pathogens. We examined parasite transfer from American captive to the two native species that co-exist in artificial pools of a Turtle Farm in France. As model parasite species we used platyhelminth worms of the family Polystomatidae (Monogenea) because polystomes have been described from American turtles in their native range. Phylogenetic relationships among polystomes parasitizing chelonian host species that are geographically widespread show patterns of diversification more complex than expected. Using DNA barcoding to identify species from adult and/or polystome eggs, several cases of host switching from exotic to indigenous individuals were illustrated, corroborating that parasite transmission is important when considering the pet trade and in reintroduction programmes to reinforce wild populations of indigenous species.

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Over 11 000 gastropods from Jasper National Park, Alberta, were examined (1976–1977) for larvae of Parelaphostrongylus odocoilei. Seven species of gastropods were infected: Deroceras laeve (5.3%), Zonitoides nitidus (1.5%), Euconulus fulvus (1.3%), Discus shimeki (1.2%), Zonitoides arboreus (0.8%), Vitrina limpida (0.7%), and Discus cronkhitei (0.6%). Examination of deer feces collected throughout the year (1974–1980) for first-stage larvae of P. odocoilei indicated maximum numbers were passed in March and April. All deer became infected as fawns in autumn apparently by accidently ingesting infected gastropods.

Hapalotrema mistroides (Monticelli, 1896) Stiles and Hassall, 1908 and Hapalotrema synorchis Luhman, 1935 are redescribed using specimens from the loggerhead turtle. Caretta caretta (L.). The redescription of H. mistroides is based on specimens collected by Looss and described as Hapalotrema constrictum Leared, 1862 and renamed Hapalotrema loossi Price, 1934, which is now considered a junior synonym of H. mistroides. Hapalotrema orientalis Takeuti, 1942 is considered a junior synonym of Hapalotrema synorchis. The redescriptions are consistent with the originals but provide an unreported range of variation while adding new information about the reproductive systems. Reexamination of Hapalotrema dorsopora Dailey, Fast, and Balazs, 1993 confirms the presence of a reproductive system typical for the genus and the absence of a dorsal genital pore; therefore, H. dorsopora is reduced to a junior synonym of Hapalotrema mehrai Rao, 1976. Four valid species of Hapalotrema are recognized: H. mistroides. H. synorchis, H. postorchis, and H. mehrai, and a key to the species is provided.