Field work research on population dynamic of snails from the regions of Belo Horizonte and Lagoa Santa give much information about interactions among two or more species of mollusks: Pomacea haustrum, Biomphalaria glabrata, B. tenagophila, B. straminea and Melanoides tuberculata. Data ranging from two years to several decades ago suggest that the Pampulha reservoir is like a cemetery of B. glabrata and B. straminea, species that coexist for more than 14 years in a small part of a stream, whereas only B. glabrata lives in all the streams of the basin. In the last ten to twenty years B. tenagophila has coexisted with P. haustrum and M. tuberculata in the Serra Verde ponds and in the Pampulha dam. However these species have not settled in any of the brooks, except temporarily. The data suggest that the kind of biotope and the habitat conditions are decisive factors for the permanence of each species in its preferencial biotope. B. glabrata, natural from streams and riverheads, quickly disappears from the reservoirs and ponds where it coexists with other species for a short time, independently of the competitive process. Competition needs to be better studied, since in Central America and Caribean islands this kind of study has favored the biological control of planorbid species.

Observation about conhabitation among B. glabrata and B. tenagophila revealed a greater vulnerability of B. tenagophila population during the process of competition when its density was severaly decreased in 12 trials, moderate in 2 trials. It was higher than B. glabrata in only one trial. Some snail water chemical parameters analysed such as pH, alkalinity, conductivity and oxygen dissolve, an the viability rate of batch of eggs didn't give subsidy to explain the competition mechanism. The newly-born survival, in the situation of cohabitation, was low for both species. This reveals the existence of intra and interspecific competitive interacition. The fertility rate reduction of B. tenagophila during the cohabitation was considered as a cause of its exlusion. One of the factors that seems to have influenced the fertility rate was a possible wrong crossing.

Em presença de extratos de seis macrófitas a Biomphalaria tenagophila apresentou maior atração por quatro delas, na seguinte ordem: Nasturtium pumilum > Polygonum acre > Commelina sp. = Echinocloa crusgalli. Para o perifíton destas macrófitas a B. tenagophila foi atraída, sem mostrar preferência entre eles. Na discussão, tal comportarríento é avaliado sob o ponto de vista da co-evolução entre moluscos e plantas.

In presence of extracts of six flowering plants the Biomphalaria tenagophila was more attracted to four them in the following sequence: Nasturtium pumilum > Polygonum acre > Commelina sp. = Echinochloa crusgalli. The periphyton of these flowering plants attracted in the same way the B. tenagophila but without no preference for either of them. Reporting the results that behavior may be evaluated as a co-evolution between snail and plants.

"Híbridos" de Biomphalaria tenagophila provenientes dos cruzamentos de linhagens albinas de Belo Horizonte (BH) ou de Joinvile (SC), com melânicos de Cabo Frio (CF), do Taim (Ta) ou de Curitiba (PR), submetidos à infecção pela cepa do Schistosoma mansoni de Belo Horizonte (=LE), apresentaram os seguintes resultados: em F1, os "híbridos" TaSC, PRSC e CFTa exibiram taxas de 4,5%, 12,5% e 11,2% de suscetibilidade; em F2, todos os "híbridos" foram negativos e em F3, um exemplar albino, filho de (CFBH) ². TaBH se infectou com a LE. Dentre os controles, a B. glabrata apresentou taxas de 66,7 a 93,6% de suscetibilidade à LE e a B. tenagophila de Joinvile exibiu taxas de infecção de 17,1 e 33,3% pela cepa SJ; e os "híbridos" BHTa e BHCF, taxas de 6,0 a 53,8% também pela cepa SJ. Houve grande influência da linhagem materna nas taxas de suscetibilidade. Devido ao fato de descendentes do cruzamento de linhagens refratárias a LE (CF, Ta e BH), terem se infectado, é recomendado o uso de "híbridos" para a detecção de gens de suscetibilidade em tais linhagens. São ainda discutidas, a necessidade do uso de maior número de miracídios nos testes de infecção e a falta de relação entre a freqüência de contatos parasitas-hospedeiros e as taxas de infectividade. Considerando que estas dependem de características genéticas preexistentes na população, a cepa LE seria uma variedade genética (ou raça) distinta da cepa SJ, dotada de pouca aptidão em infectar as diversas populações de B. tenagophila, exceto a de Joinvile (SC).

Biomphalaria tenagophila "hybrids" originating from the crossing of albino strains from Belo Horizonte (BH) or from Joinvile (SC with melanic specimens from Cabo Frio (CF), Taim (Ta) or from Curitiba (PR), submitted to infection with Schistosoma mansoni (LE strain, Belo Horizonte, MG), presented the following results: in F1, TaSC, PRSC and CFTa "hybrids" showed susceptibility rates of 4.5%, 12.5%, and 11.2%, respectively; in F2, all "hybrids" were shown negative, and in F3, only one albino specimen (CFBH)². TaBH offspring was infected with LE strain. As far as controls were concerned, B. glabrata showed susceptibility rates of 66.7% and 93.6% to LE strain; B. tenagophila from Joinvile presented 17.1% and 33.3% to SJ strain, and BHTa and BHCF hybrids showed 6.0% to 53.8% to SJ strain too. It was noted a great influence of mother strain on susceptibility rates. Since the offsprings originating from crossing of unsusceptible strains to LE (CF, Ta and BH) were infected, the use of hybrids to defect susceptibility gens in such strains is discussed. Further, the necessity of the use of a greater number of miracidia in infection tests, and the lack of correlation between the frequency of host-parasite contacts and infectivity rates are discussed. Considering the fact that those infectivity rates are dependent on genetic characteristics, previously existing in the population, and not on the frequency of contacts, LE strain could be a genetic variety (or race) different from SJ strain, endowed with little ability to infect different populations of B. tenagophila, except for that from Joinvile (SC).