Abstract. Host-parasite associations are assumed to be ecologically specialized, tightly coevolved systems driven by mutual modification in which host switching is a rare phenomenon. Ecological fitting, however, increases the probability of host switching, creating incongruences between host and parasite phylogenies, when (1) specialization on a particular host resource is a shared characteristic of distantly related parasites, and (2) the resource being tracked by the parasite is widespread among many host species. We investigated the effect of ecological fitting on structuring the platyhelminth communities of anurans from a temperate forest and grassland in the United States and tropical dry and wet forests in Mexico and Costa Rica. The six communities all exhibit similar structure in terms of the genera and families inhabiting the frogs. Parasite species richness is highly correlated with the amount of time a host spends in association with aquatic habitats, a conservative aspect of both parasite and host natural history, and determined in a proximal sense by host mobility and diet breadth. The pattern of parasite genera and families within host genera across the regions examined is consistent with the prediction that ecological fitting by phylogenetically conservative species, coupled with historical accidents of speciation and dispersal, should be evidenced as a nestedsubset structure; the shared requirement for aquatic habitats of tadpoles provides a baseline assemblage to which other parasite taxa are added as a function of adult host association with aquatic habitats. We conclude that parasite communities are structured by both ecological fitting and coevolution (mutual modification), the relative influences of which are expected to vary among different communities and associations.

Widespread reports of malformed amphibians are of growing conservation concern. Although accounts of mass malformations (>5%) in North American amphibian populations date back to the 1940s, they are often poorly documented and are rarely explained. We reviewed available information for nine historical accounts from California, Colorado, Idaho, Mississippi, Montana, Ohio, and Texas reported between 1946 and1988. We then asked the following questions: (1) Which of these cases were associated with Ribeiroia (Trematoda: Digenea) infection? (2) Are malformations still occurring at these sites? And (3) if so, have the frequency or types of abnormalities changed? Each site was resurveyed between 1999 and 2002, and original voucher specimens were redescribed and examined for trematode infection. Direct identification and classification by discriminant function analysis indicated that historical malformations at six of eight sites were associated with infection by Ribeiroia, dating back as far as 1946. Malformations recorded historically at these sites were consistent with the documented effects of Ribeiroia infection, including extra limbs, cutaneous fusion, and bony triangles.Of the six sites that still supported amphibians upon resurvey, three continued to support severe limb malformations at frequencies of 7-50% in one or more species. Although no pesticides were detected, amphibians from each of these sites were infected with Ribeiroia metacercariae. Taken together, these results suggest that Ribeiroia infection has historically been an important cause of mass malformations in amphibians. We conclude that although parasite-induced malformations are not a new phenomenon, there is qualitative evidence suggesting that their prevalence has increased recently, and we highlight the need for long-term research to evaluate the impacts of malformations on amphibian population viability.

The independent effects of host age and size on parasite abundance are examined in yellow perch from Garner Lake in east-central Alberta. Older, larger perch tend to have more species of parasites combined with larger parasite infrapopulations. Sexual size dimorphism in perch generates 2 different size classes within each age class >2+ yr, thus allowing the opportunity to separate the effects of age and size on the recruitment of parasites by perch. The abundances of Diplostomum adamsi and Bothriocephalus sp. both increase with host age, whereas the increased abundances of Crepidostomum isostomum and Raphidascaris acus in larger fish are, in part, suggestive of size-specific feeding behaviors of the host. A combination of parasites with size- and age-dependent infrapopulations results in a size-dependent increase in infracommunity size. A comparison of variance-to-mean ratios within age classes to overall variance-to-mean ratios shows that the effects of host age and size on parasite recruitment do contribute to parasite aggregation.

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Taenia crassiceps cysticerci form large infrapopulations that persist in the tissues of their rodent hosts. Early infrapopulation growth appears inhibited and is followed by rapid increases that appear not to be controlled by the host immune response. This investigation was undertaken to examine the infrapopulation growth dynamics of a normally developing strain (WFU) of T. crassiceps during a 60-day primary intraperitoneal (i.p.) infection. Three, 6, 9, 14, 28, and 60 days after i.p. inoculation of 5 cysticerci, mice were killed, and the numbers of larvae, developmental stage, and buds per larva were recorded. Larval infrapopulation abundance increased exponentially beginning on day 6 postinoculation (PI), indicating an initial lag in reproduction. A stage-structured exponential growth model, assuming no mortality, fits the larval infrapopulation dynamics in terms of the numbers of larvae in reproductive and nonreproductive stages, indicating that cysticerci evade or suppress (or both) host immune mechanisms that are parasite restrictive after the first week of infection.