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An uncommon snail-trematode interaction pattern was found for the Pseudosuccinea columella - Fasciola hepaticamodel under laboratory conditions. Week-old juveniles from two isolates showed very high rates of infection (90.0% and 93.3%) after exposure to five miracidia. They also showed differences in their life history traits when compared to a control group. However, they did not exhibit the increase in size and reduction/cessation in host fecundity which is usually observed in most of the other related snail-trematode systems. In contrast, infected juvenile P. columella showed increased egg laying after the onset of cercarial emission and there was no effect on growth. A third isolate of P. columella was found to be refractory to miracidial infection. This constitutes the first report of non-susceptibility to F. hepatica in a lymnaeid species which is usually susceptible. These non-susceptible snails exhibited lower fecundity and survival compared to non-exposed susceptible snails under identical laboratory conditions. The differences observed in terms of life span and reproduction could result from the cost of resistance for individuals from this isolate.

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The effect of exposing the lymnaeid snail Fossaria cubensis to the trematode Fasciola hepatica on the snail population's life-history traits was studied under laboratory conditions. Exposed individuals showed a lower survival rate than control snails, although from week 7 onward a slower decrease of this parameter in relation to the control group was observed. There were higher values of fecundity rate for the controls compared to the exposed group except during weeks 9, 10, 11 and 12, which was the time that followed the period when almost all of the infected snails died. Both the intrinsic and finite rates of natural increase were significantly higher for the control group, but exposed snails still attained a lower mean generation time. Age-specific trade-offs were found, mainly for the weekly increase in size versus the number of eggs per mass, the weekly increase in size versus the number of viable eggs per mass, the number of masses versus the hatching probability and the number of eggs versus the hatching probability. All these negative associations were significant for juveniles of both control and exposed snails and not for adults; however, exposed young individuals exhibited much higher values of the correlation coefficient than control animals.

Growth, survival and reproduction in the family Lymnaedae (Mollusca: Prosobranchia) have been widely studied. Snails of this taxon are known to be hermaphrodites and to reproduce both by selffertilization (selfing) and outcrossing. The proportion of selfing in natural populations of lymnaeid snails is to our knowledge unknown, but presumably more important in small populations and in stable environments.Pseudosuccinea columella (Say 1817) is in importance the second intermediate host of the trematode Fasciola hepatica Linnaeus, 1758 in Cuba (Mauri 1981). As this parasite is the causal agent of fasciolosis, a disease that affects domestic animals as well as people, the study of its hosts' biology may help the control of its transmission.Several studies have been published on the demography of lymnaeids (Vergani 1955, DeWitt & Sloan 1958, Malek & Chrosciechowski 1964, Martinez & Miranda 1968, Morales et al. 1983, 1985). Regarding Cuban populations, some studies compare the biology of P. columella with that of Fossaria cubensis, the most important intermediate host of F. hepatica, (Ferrer et al. 1988(Ferrer et al. , 1989) and other works emphasize on the biology and demography of F. cubensis (Yong et al. 1996). However, most of the previous papers based their results on groups of snails, where competition for available resources takes place. groups, sometimes deviations of expected results have been observed as populations grow older and their size is reduced due to the death of some snails. The present research aims to report on the life-history traits of P. columella under the effect of complete isolation, fertilization being accomplished only by selfing; compared to snails raised together with a conspecific partner, allowed to both crossand self-fertilization and where intra-specific communication takes place. MATERIALS AND METHODSThe parental snails for the study were collected from a temporary pond in La Palma municipality, Pinar del Rio Province, located west of the island of Cuba. The specimens were removed from their habitat with a hand sieve, sampling the bottom as well as the vegetation present on the shores, then taken to the laboratory on wet filter paper inside plastic cases. Upon arrival to the laboratory all snails (60 in total) were placed in a petri dish (PYREX® USA, 88 ml of water volume) for 15 days with a pool of algae of the genera Lyngbya, Leptolyngbya, Phormidium and Schimidleinema as the food source. After seven days a number of egg masses was removed and placed into another similar dish, then 40 newly hatched snails were randomly selected for the life tables experiment. Twenty newly-hatched individuals (isolated snails) were raised in complete isolation (one snail per dish) and the other 20 (paired snails) were grown in pairs (two snails per dish).The age zero of the experiment was considered as the week when all 40 newly hatched snails emerged from their egg masses. All mollusks were raised using the petri dishes and pool of algae mentioned above as container and food source respectively ...

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